[PATCH v5 0/6] Use introsort for qsort

[PATCH v5 0/6] Use introsort for qsort

[Adhemerval Zanella]

The main motivation to use introsort is to make it fully AS-Safe and
AC-Safe, with a limited stack size requirement, to remove the use
of malloc (which troublesome since it seems some program do longjmp
from the callback comparison program), and keep worst-case scenario
bounded to O(n*ln(n)) (instead of potentially quadradic as for the
quicksort).

The implementation does not aim to be the state-of-the-art sort
algorithm, rather I used a well understood introsort (used on
libstdc++, for instance) and leverage the current quicksort
implementation along with a heapsort one from Linux kernel.

Performance-wise, the introsort does fall short compare to
mergesort [1].  I have not added the benchmark because I see that
this should not be focus of this patch. I have added some simple
optimization, also used by Linux kernel heapsort.

Changes from v4
- Use enum for swap function selection.
- Simplify is_aligned.
- Renamed insertsort.

[1] https://sourceware.org/pipermail/libc-alpha/2021-September/130698.html

Adhemerval Zanella (6):
  stdlib: Optimization qsort{_r} swap implementation
  stdlib: Move insertion sort out qsort
  stdlib: qsort: Move some macros to inline function
  stdlib: Implement introsort with qsort (BZ 19305)
  stdlib: Remove use of mergesort on qsort (BZ 21719)
  stdlib: Add more qsort{_r} coverage

 include/stdlib.h    |   2 -
 manual/argp.texi    |   2 +-
 manual/locale.texi  |   3 +-
 manual/search.texi  |   7 +-
 stdlib/Makefile     |   3 +-
 stdlib/msort.c      | 309 ----------------------------------------
 stdlib/qsort.c      | 337 +++++++++++++++++++++++++++++++++-----------
 stdlib/tst-qsort3.c | 298 +++++++++++++++++++++++++++++++++++++++
 8 files changed, 562 insertions(+), 399 deletions(-)
 delete mode 100644 stdlib/msort.c
 create mode 100644 stdlib/tst-qsort3.c

-- 
2.34.1

[PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Adhemerval Zanella]

It optimizes take in consideration both the most common elements are
either 32 or 64 bit in size and inputs are aligned to the word
boundary.  This is similar to the optimization done on lib/sort.c
from Linux.

This patchs adds an optimized swap operation on qsort based in previous
msort one.  Instead of byte operation, three variants are provided:

  1. Using uint32_t loads and stores.
  2. Using uint64_t loads and stores.
  3. Generic one with a temporary buffer and memcpy/mempcpy.

The 1. and 2. options are selected only if base pointer is aligned
to required type.

Checked on x86_64-linux-gnu.
---
 stdlib/qsort.c | 109 +++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 91 insertions(+), 18 deletions(-)

diff --git a/stdlib/qsort.c b/stdlib/qsort.c
index 728a0ed370..bf3326e4b9 100644
--- a/stdlib/qsort.c
+++ b/stdlib/qsort.c
@@ -23,20 +23,85 @@
 #include <limits.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stdbool.h>
 
-/* Byte-wise swap two items of size SIZE. */
-#define SWAP(a, b, size)						      \
-  do									      \
-    {									      \
-      size_t __size = (size);						      \
-      char *__a = (a), *__b = (b);					      \
-      do								      \
-	{								      \
-	  char __tmp = *__a;						      \
-	  *__a++ = *__b;						      \
-	  *__b++ = __tmp;						      \
-	} while (--__size > 0);						      \
-    } while (0)
+/* Swap SIZE bytes between addresses A and B.  These helpers are provided
+   along the generic one as an optimization.  */
+
+enum swap_type_t
+  {
+    SWAP_WORDS_64,
+    SWAP_WORDS_32,
+    SWAP_BYTES
+  };
+
+/* Returns true if elements can be copied using word loads and stores.
+   The SIZE and BASE must be a multiple of the ALIGN.  */
+__attribute_const__ __always_inline static bool
+is_aligned (const void *base, size_t size, unsigned char align)
+{
+  return (((uintptr_t) base | size) & (align - 1)) == 0;
+}
+
+static void
+swap_words_64 (void * restrict a, void * restrict b, size_t n)
+{
+  do
+   {
+     n -= 8;
+     uint64_t t = *(uint64_t *)(a + n);
+     *(uint64_t *)(a + n) = *(uint64_t *)(b + n);
+     *(uint64_t *)(b + n) = t;
+   } while (n);
+}
+
+static void
+swap_words_32 (void * restrict a, void * restrict b, size_t n)
+{
+  do
+   {
+     n -= 4;
+     uint32_t t = *(uint32_t *)(a + n);
+     *(uint32_t *)(a + n) = *(uint32_t *)(b + n);
+     *(uint32_t *)(b + n) = t;
+   } while (n);
+}
+
+static void
+swap_bytes (void * restrict a, void * restrict b, size_t n)
+{
+  /* Use multiple small memcpys with constant size to enable inlining
+     on most targets.  */
+  enum { SWAP_GENERIC_SIZE = 32 };
+  unsigned char tmp[SWAP_GENERIC_SIZE];
+  while (n > SWAP_GENERIC_SIZE)
+    {
+      memcpy (tmp, a, SWAP_GENERIC_SIZE);
+      a = __mempcpy (a, b, SWAP_GENERIC_SIZE);
+      b = __mempcpy (b, tmp, SWAP_GENERIC_SIZE);
+      n -= SWAP_GENERIC_SIZE;
+    }
+  while (n > 0)
+    {
+      unsigned char t = ((unsigned char *)a)[--n];
+      ((unsigned char *)a)[n] = ((unsigned char *)b)[n];
+      ((unsigned char *)b)[n] = t;
+    }
+}
+
+/* Replace the indirect call with a serie of if statements.  It should help
+   the branch predictor.  */
+static void
+do_swap (void * restrict a, void * restrict b, size_t size,
+	 enum swap_type_t swap_func)
+{
+  if (swap_func == SWAP_WORDS_64)
+    swap_words_64 (a, b, size);
+  else if (swap_func == SWAP_WORDS_32)
+    swap_words_32 (a, b, size);
+  else
+    swap_bytes (a, b, size);
+}
 
 /* Discontinue quicksort algorithm when partition gets below this size.
    This particular magic number was chosen to work best on a Sun 4/260. */
@@ -96,6 +161,14 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
     /* Avoid lossage with unsigned arithmetic below.  */
     return;
 
+  enum swap_type_t swap_type;
+  if (is_aligned (pbase, size, 8))
+    swap_type = SWAP_WORDS_64;
+  else if (is_aligned (pbase, size, 4))
+    swap_type = SWAP_WORDS_32;
+  else
+    swap_type = SWAP_BYTES;
+
   if (total_elems > MAX_THRESH)
     {
       char *lo = base_ptr;
@@ -119,13 +192,13 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
 	  char *mid = lo + size * ((hi - lo) / size >> 1);
 
 	  if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
-	    SWAP (mid, lo, size);
+	    do_swap (mid, lo, size, swap_type);
 	  if ((*cmp) ((void *) hi, (void *) mid, arg) < 0)
-	    SWAP (mid, hi, size);
+	    do_swap (mid, hi, size, swap_type);
 	  else
 	    goto jump_over;
 	  if ((*cmp) ((void *) mid, (void *) lo, arg) < 0)
-	    SWAP (mid, lo, size);
+	    do_swap (mid, lo, size, swap_type);
 	jump_over:;
 
 	  left_ptr  = lo + size;
@@ -144,7 +217,7 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
 
 	      if (left_ptr < right_ptr)
 		{
-		  SWAP (left_ptr, right_ptr, size);
+		  do_swap (left_ptr, right_ptr, size, swap_type);
 		  if (mid == left_ptr)
 		    mid = right_ptr;
 		  else if (mid == right_ptr)
@@ -216,7 +289,7 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
         tmp_ptr = run_ptr;
 
     if (tmp_ptr != base_ptr)
-      SWAP (tmp_ptr, base_ptr, size);
+      do_swap (tmp_ptr, base_ptr, size, swap_type);
 
     /* Insertion sort, running from left-hand-side up to right-hand-side.  */
 
-- 
2.34.1

Re: [PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Paul Eggert]

On 2023-07-13 06:25, Adhemerval Zanella via Libc-alpha wrote:

> +is_aligned (const void *base, size_t size, unsigned char align)

ALIGN should be of type size_t, not unsigned char. Although this 
shouldn't change the machine code it's better documentation and more 
robust to future changes.


> +  if (is_aligned (pbase, size, 8))

Change "8" to "alignof (uint64_t)".

> +    swap_type = SWAP_WORDS_64;
> +  else if (is_aligned (pbase, size, 4))

Likewise, change "4" to "alignof (uint32_t)".

Or is the problem that alignof didn't work for you? If so, perhaps 
__alignof__. Still, alignof is used elsewhere in glibc and is in C23 so 
it's better to use here if it works.

Re: [PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Paul Eggert]

On 2023-07-13 15:17, Paul Eggert wrote:

>> +  if (is_aligned (pbase, size, 8))
> 
> Change "8" to "alignof (uint64_t)".


Oops, scratch that. The size must be a multiple of sizeof (uint64_t) and 
the alignment must be a multiple of alignof (uint64_t). Since sizeof 
(uint64_t) must be a multiple of alignof (uint64_t) what you wrote was 
safe (but confusing), whereas what I wrote was not.

To help avoid confusion by future readers, I suggest rewriting the 
comments and parameters for is_aligned to be something like the 
following. This shouldn't change the machine code.

   /* If this function returns true, elements can be safely copied using
      word loads and stores.  Otherwise, it might not be safe.
      BASE (as an integer) must be a multiple of the word alignment.
      TOTAL_SIZE must be a multiple of WORDSIZE.
      Since WORDSIZE must be a multiple of the word alignment,
      and WORDSIZE is a power of two on all supported platforms,
      this function for speed merely checks that BASE and SIZE
      are both multiples of the word size.  */
   __attribute_const__ __always_inline static bool
   is_aligned (const void *base, size_t size, size_t wordsize)
   {
     return (((uintptr_t) base | size) & (wordsize - 1)) == 0;
   }

Re: [PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Adhemerval Zanella Netto]

On 13/07/23 22:01, Paul Eggert wrote:
> On 2023-07-13 15:17, Paul Eggert wrote:
> 
>>> +  if (is_aligned (pbase, size, 8))
>>
>> Change "8" to "alignof (uint64_t)".
> 
> 
> Oops, scratch that. The size must be a multiple of sizeof (uint64_t) and the alignment must be a multiple of alignof (uint64_t). Since sizeof (uint64_t) must be a multiple of alignof (uint64_t) what you wrote was safe (but confusing), whereas what I wrote was not.
> 
> To help avoid confusion by future readers, I suggest rewriting the comments and parameters for is_aligned to be something like the following. This shouldn't change the machine code.
> 
>   /* If this function returns true, elements can be safely copied using
>      word loads and stores.  Otherwise, it might not be safe.
>      BASE (as an integer) must be a multiple of the word alignment.
>      TOTAL_SIZE must be a multiple of WORDSIZE.

I think you meant SIZE here instead of TOTAL_SIZE.

>      Since WORDSIZE must be a multiple of the word alignment,
>      and WORDSIZE is a power of two on all supported platforms,
>      this function for speed merely checks that BASE and SIZE
>      are both multiples of the word size.  */
>   __attribute_const__ __always_inline static bool
>   is_aligned (const void *base, size_t size, size_t wordsize)
>   {
>     return (((uintptr_t) base | size) & (wordsize - 1)) == 0;
>   }
> 

Alright, I changed this version.

Re: [PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Alexander Monakov]

On Thu, 13 Jul 2023, Adhemerval Zanella via Libc-alpha wrote:

> +/* Returns true if elements can be copied using word loads and stores.
> +   The SIZE and BASE must be a multiple of the ALIGN.  */
> +__attribute_const__ __always_inline static bool

Kernel folks sure love their double underscores, but plain
'static inline bool' should work just as well here.

I'd recommend to properly credit Linux lib/sort.c since obviously this
commit originated as a direct copy of the code therein, not just being
inspired by what's there.

> +is_aligned (const void *base, size_t size, unsigned char align)
> +{
> +  return (((uintptr_t) base | size) & (align - 1)) == 0;
> +}
> +
> +static void
> +swap_words_64 (void * restrict a, void * restrict b, size_t n)
> +{
> +  do
> +   {
> +     n -= 8;
> +     uint64_t t = *(uint64_t *)(a + n);
> +     *(uint64_t *)(a + n) = *(uint64_t *)(b + n);
> +     *(uint64_t *)(b + n) = t;
> +   } while (n);
> +}

The undefined behavior resulting from attempt to access via an 'uint64_t *'
something of a different type is known to cause miscompilation in practice:
https://www.spec.org/cpu2017/Docs/benchmarks/605.mcf_s.html#portability

I see that the new string routines do it properly via 

  typedef unsigned long int __attribute__ ((__may_alias__)) op_t;

Would be nice to avoid new instances of such UB here.

Alexander

Re: [PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Paul Eggert]

On 2023-07-17 09:57, Alexander Monakov wrote:
> I'd recommend to properly credit Linux lib/sort.c since obviously this
> commit originated as a direct copy of the code therein, not just being
> inspired by what's there.

What are the licensing issues here? The Linux kernel is GPLv2 only, 
whereas glibc/stdlib/qsort.c is LGPLv2.1+, so we can't simply take 
nontrivial source code from the Linux kernel.

Re: [PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Adhemerval Zanella Netto]

On 17/07/23 14:29, Paul Eggert wrote:
> On 2023-07-17 09:57, Alexander Monakov wrote:
>> I'd recommend to properly credit Linux lib/sort.c since obviously this
>> commit originated as a direct copy of the code therein, not just being
>> inspired by what's there.
> 
> What are the licensing issues here? The Linux kernel is GPLv2 only, whereas glibc/stdlib/qsort.c is LGPLv2.1+, so we can't simply take nontrivial source code from the Linux kernel.

I am not sure in fact, my take was the code as simple enough to have this
concern.  But if it were case, I can reimplement it different to avoid this
issue.

Re: [PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Paul Eggert]

On 2023-07-17 11:07, Adhemerval Zanella Netto wrote:
> I am not sure in fact, my take was the code as simple enough to have this
> concern.

The usual rule of thumb is that ten lines are trivial, and that more 
than that might be a concern. When it's a concern, you can't simply read 
the original code and rewrite it in a different way; you need to 
actually rewrite it from scratch, preferably without looking at the 
original code. It's a pain, admittedly.

Re: [PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Adhemerval Zanella Netto]

On 17/07/23 15:58, Paul Eggert wrote:
> On 2023-07-17 11:07, Adhemerval Zanella Netto wrote:
>> I am not sure in fact, my take was the code as simple enough to have this
>> concern.
> 
> The usual rule of thumb is that ten lines are trivial, and that more than that might be a concern. When it's a concern, you can't simply read the original code and rewrite it in a different way; you need to actually rewrite it from scratch, preferably without looking at the original code. It's a pain, admittedly.

Alright, I don't think it should much of trouble to reimplement a simple heapsort.

Re: [PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Adhemerval Zanella Netto]

On 17/07/23 13:57, Alexander Monakov wrote:
> 
> On Thu, 13 Jul 2023, Adhemerval Zanella via Libc-alpha wrote:
> 
>> +/* Returns true if elements can be copied using word loads and stores.
>> +   The SIZE and BASE must be a multiple of the ALIGN.  */
>> +__attribute_const__ __always_inline static bool
> 
> Kernel folks sure love their double underscores, but plain
> 'static inline bool' should work just as well here.
> 
> I'd recommend to properly credit Linux lib/sort.c since obviously this
> commit originated as a direct copy of the code therein, not just being
> inspired by what's there.

In fact our msort_with_tmp uses the exact same idea, but it is somewhat
convoluted because it adds an specific optimization to 'unsigned long'
and for large values it uses a inplace mempcpy.

> 
>> +is_aligned (const void *base, size_t size, unsigned char align)
>> +{
>> +  return (((uintptr_t) base | size) & (align - 1)) == 0;
>> +}
>> +
>> +static void
>> +swap_words_64 (void * restrict a, void * restrict b, size_t n)
>> +{
>> +  do
>> +   {
>> +     n -= 8;
>> +     uint64_t t = *(uint64_t *)(a + n);
>> +     *(uint64_t *)(a + n) = *(uint64_t *)(b + n);
>> +     *(uint64_t *)(b + n) = t;
>> +   } while (n);
>> +}
> 
> The undefined behavior resulting from attempt to access via an 'uint64_t *'
> something of a different type is known to cause miscompilation in practice:
> https://www.spec.org/cpu2017/Docs/benchmarks/605.mcf_s.html#portability
> 
> I see that the new string routines do it properly via 
> 
>   typedef unsigned long int __attribute__ ((__may_alias__)) op_t;
> 
> Would be nice to avoid new instances of such UB here.

Ack.

Re: [PATCH v5 1/6] stdlib: Optimization qsort{_r} swap implementation

[Adhemerval Zanella Netto]

On 18/07/23 11:01, Adhemerval Zanella Netto wrote:
> 
> 
> On 17/07/23 13:57, Alexander Monakov wrote:
>>
>> On Thu, 13 Jul 2023, Adhemerval Zanella via Libc-alpha wrote:
>>
>>> +/* Returns true if elements can be copied using word loads and stores.
>>> +   The SIZE and BASE must be a multiple of the ALIGN.  */
>>> +__attribute_const__ __always_inline static bool
>>
>> Kernel folks sure love their double underscores, but plain
>> 'static inline bool' should work just as well here.
>>
>> I'd recommend to properly credit Linux lib/sort.c since obviously this
>> commit originated as a direct copy of the code therein, not just being
>> inspired by what's there.
> 
> In fact our msort_with_tmp uses the exact same idea, but it is somewhat
> convoluted because it adds an specific optimization to 'unsigned long'
> and for large values it uses a inplace mempcpy.

And just to be clear about the the license, the optimization is really
a common technique already used in our mergesort implementation, and
Linux implementation is slight different because it takes in consideration
if the architectures support fast unaligned accesses (which glibc does not
consider anymore since a9b3b770f596c), for 64 bit it also handles 64 and 32
bits with differently (which I don't think we should not bother,
compiler will know better how to optimize it); and it does not have
the large buffer optimization.  So I don't think this falls to the idea
this code is copy/pasted.

For the heapsort, I really used Linux implementation which I replaced with
textbook one.

[PATCH v5 2/6] stdlib: Move insertion sort out qsort

[Adhemerval Zanella]

---
 stdlib/qsort.c | 101 ++++++++++++++++++++++++++-----------------------
 1 file changed, 54 insertions(+), 47 deletions(-)

diff --git a/stdlib/qsort.c b/stdlib/qsort.c
index bf3326e4b9..410a8d7731 100644
--- a/stdlib/qsort.c
+++ b/stdlib/qsort.c
@@ -149,6 +149,58 @@ typedef struct
       smaller partition.  This *guarantees* no more than log (total_elems)
       stack size is needed (actually O(1) in this case)!  */
 
+static inline void
+insertion_sort_qsort_partitions (void *const pbase, size_t total_elems,
+				 size_t size, enum swap_type_t swap_func,
+				 __compar_d_fn_t cmp, void *arg)
+{
+  char *base_ptr = (char *) pbase;
+  char *const end_ptr = &base_ptr[size * (total_elems - 1)];
+  char *tmp_ptr = base_ptr;
+#define min(x, y) ((x) < (y) ? (x) : (y))
+  const size_t max_thresh = MAX_THRESH * size;
+  char *thresh = min(end_ptr, base_ptr + max_thresh);
+  char *run_ptr;
+
+  /* Find smallest element in first threshold and place it at the
+     array's beginning.  This is the smallest array element,
+     and the operation speeds up insertion sort's inner loop. */
+
+  for (run_ptr = tmp_ptr + size; run_ptr <= thresh; run_ptr += size)
+    if (cmp (run_ptr, tmp_ptr, arg) < 0)
+      tmp_ptr = run_ptr;
+
+  if (tmp_ptr != base_ptr)
+    do_swap (tmp_ptr, base_ptr, size, swap_func);
+
+  /* Insertion sort, running from left-hand-side up to right-hand-side.  */
+
+  run_ptr = base_ptr + size;
+  while ((run_ptr += size) <= end_ptr)
+    {
+      tmp_ptr = run_ptr - size;
+      while (cmp (run_ptr, tmp_ptr, arg) < 0)
+        tmp_ptr -= size;
+
+      tmp_ptr += size;
+      if (tmp_ptr != run_ptr)
+        {
+          char *trav;
+
+          trav = run_ptr + size;
+          while (--trav >= run_ptr)
+            {
+              char c = *trav;
+              char *hi, *lo;
+
+              for (hi = lo = trav; (lo -= size) >= tmp_ptr; hi = lo)
+                *hi = *lo;
+              *hi = c;
+            }
+        }
+    }
+}
+
 void
 _quicksort (void *const pbase, size_t total_elems, size_t size,
 	    __compar_d_fn_t cmp, void *arg)
@@ -271,51 +323,6 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
      for partitions below MAX_THRESH size. BASE_PTR points to the beginning
      of the array to sort, and END_PTR points at the very last element in
      the array (*not* one beyond it!). */
-
-#define min(x, y) ((x) < (y) ? (x) : (y))
-
-  {
-    char *const end_ptr = &base_ptr[size * (total_elems - 1)];
-    char *tmp_ptr = base_ptr;
-    char *thresh = min(end_ptr, base_ptr + max_thresh);
-    char *run_ptr;
-
-    /* Find smallest element in first threshold and place it at the
-       array's beginning.  This is the smallest array element,
-       and the operation speeds up insertion sort's inner loop. */
-
-    for (run_ptr = tmp_ptr + size; run_ptr <= thresh; run_ptr += size)
-      if ((*cmp) ((void *) run_ptr, (void *) tmp_ptr, arg) < 0)
-        tmp_ptr = run_ptr;
-
-    if (tmp_ptr != base_ptr)
-      do_swap (tmp_ptr, base_ptr, size, swap_type);
-
-    /* Insertion sort, running from left-hand-side up to right-hand-side.  */
-
-    run_ptr = base_ptr + size;
-    while ((run_ptr += size) <= end_ptr)
-      {
-	tmp_ptr = run_ptr - size;
-	while ((*cmp) ((void *) run_ptr, (void *) tmp_ptr, arg) < 0)
-	  tmp_ptr -= size;
-
-	tmp_ptr += size;
-        if (tmp_ptr != run_ptr)
-          {
-            char *trav;
-
-	    trav = run_ptr + size;
-	    while (--trav >= run_ptr)
-              {
-                char c = *trav;
-                char *hi, *lo;
-
-                for (hi = lo = trav; (lo -= size) >= tmp_ptr; hi = lo)
-                  *hi = *lo;
-                *hi = c;
-              }
-          }
-      }
-  }
+  insertion_sort_qsort_partitions (pbase, total_elems, size, swap_type, cmp,
+				   arg);
 }
-- 
2.34.1

[PATCH v5 3/6] stdlib: qsort: Move some macros to inline function

[Adhemerval Zanella]

---
 stdlib/qsort.c | 35 +++++++++++++++++++++++------------
 1 file changed, 23 insertions(+), 12 deletions(-)

diff --git a/stdlib/qsort.c b/stdlib/qsort.c
index 410a8d7731..53013ef281 100644
--- a/stdlib/qsort.c
+++ b/stdlib/qsort.c
@@ -114,15 +114,28 @@ typedef struct
     char *hi;
   } stack_node;
 
-/* The next 4 #defines implement a very fast in-line stack abstraction. */
 /* The stack needs log (total_elements) entries (we could even subtract
    log(MAX_THRESH)).  Since total_elements has type size_t, we get as
    upper bound for log (total_elements):
    bits per byte (CHAR_BIT) * sizeof(size_t).  */
-#define STACK_SIZE	(CHAR_BIT * sizeof (size_t))
-#define PUSH(low, high)	((void) ((top->lo = (low)), (top->hi = (high)), ++top))
-#define	POP(low, high)	((void) (--top, (low = top->lo), (high = top->hi)))
-#define	STACK_NOT_EMPTY	(stack < top)
+enum { STACK_SIZE = CHAR_BIT * sizeof (size_t) };
+
+static inline stack_node *
+push (stack_node *top, char *lo, char *hi)
+{
+  top->lo = lo;
+  top->hi = hi;
+  return ++top;
+}
+
+static inline stack_node *
+pop (stack_node *top, char **lo, char **hi)
+{
+  --top;
+  *lo = top->lo;
+  *hi = top->hi;
+  return top;
+}
 
 
 /* Order size using quicksort.  This implementation incorporates
@@ -226,11 +239,9 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
       char *lo = base_ptr;
       char *hi = &lo[size * (total_elems - 1)];
       stack_node stack[STACK_SIZE];
-      stack_node *top = stack;
-
-      PUSH (NULL, NULL);
+      stack_node *top = stack + 1;
 
-      while (STACK_NOT_EMPTY)
+      while (stack < top)
         {
           char *left_ptr;
           char *right_ptr;
@@ -295,7 +306,7 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
             {
               if ((size_t) (hi - left_ptr) <= max_thresh)
 		/* Ignore both small partitions. */
-                POP (lo, hi);
+                top = pop (top, &lo, &hi);
               else
 		/* Ignore small left partition. */
                 lo = left_ptr;
@@ -306,13 +317,13 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
           else if ((right_ptr - lo) > (hi - left_ptr))
             {
 	      /* Push larger left partition indices. */
-              PUSH (lo, right_ptr);
+              top = push (top, lo, right_ptr);
               lo = left_ptr;
             }
           else
             {
 	      /* Push larger right partition indices. */
-              PUSH (left_ptr, hi);
+              top = push (top, left_ptr, hi);
               hi = right_ptr;
             }
         }
-- 
2.34.1

[PATCH v5 4/6] stdlib: Implement introsort with qsort (BZ 19305)

[Adhemerval Zanella]

This patch adds a introsort implementation on qsort to avoid worse-case
performance of quicksort to O(nlog n).  The heapsort fallback used is a
heapsort based on Linux implementation (commit 22a241ccb2c19962a).  As a
side note the introsort implementation is similar the one used on
libstdc++ for std::sort.

Checked on x86_64-linux-gnu.
---
 stdlib/qsort.c | 92 ++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 86 insertions(+), 6 deletions(-)

diff --git a/stdlib/qsort.c b/stdlib/qsort.c
index 53013ef281..ed0ac53467 100644
--- a/stdlib/qsort.c
+++ b/stdlib/qsort.c
@@ -112,6 +112,7 @@ typedef struct
   {
     char *lo;
     char *hi;
+    size_t depth;
   } stack_node;
 
 /* The stack needs log (total_elements) entries (we could even subtract
@@ -121,22 +122,90 @@ typedef struct
 enum { STACK_SIZE = CHAR_BIT * sizeof (size_t) };
 
 static inline stack_node *
-push (stack_node *top, char *lo, char *hi)
+push (stack_node *top, char *lo, char *hi, size_t depth)
 {
   top->lo = lo;
   top->hi = hi;
+  top->depth = depth;
   return ++top;
 }
 
 static inline stack_node *
-pop (stack_node *top, char **lo, char **hi)
+pop (stack_node *top, char **lo, char **hi, size_t *depth)
 {
   --top;
   *lo = top->lo;
   *hi = top->hi;
+  *depth = top->depth;
   return top;
 }
 
+/* A fast, small, non-recursive O(nlog n) heapsort, adapted from Linux
+   lib/sort.c.  Used on introsort implementation as a fallback routine with
+   worst-case performance of O(nlog n) and worst-case space complexity of
+   O(1).  */
+
+static inline size_t
+parent (size_t i, unsigned int lsbit, size_t size)
+{
+  i -= size;
+  i -= size & -(i & lsbit); /* i.e., 'if (i & lsbit) i -= size;'  */
+  return i / 2; 
+}
+
+static void
+heapsort_r (void *base, void *end, size_t size, enum swap_type_t swap_type,
+	    __compar_d_fn_t cmp, void *arg)
+{
+  size_t num = ((uintptr_t) end - (uintptr_t) base) / size;
+  size_t n = num * size, a = (num/2) * size;
+  /* Used to find parent  */
+  const size_t lsbit = size & -size;
+
+  /* num < 2 || size == 0.  */
+  if (a == 0)
+    return;
+
+  for (;;)
+    {
+      size_t b, c, d;
+
+      if (a != 0)
+	/* Building heap: sift down --a */
+	a -= size;
+      else if (n -= size)
+	/* Sorting: Extract root to --n */
+	do_swap (base, base + n, size, swap_type);
+      else
+	break;
+
+      /* Sift element at "a" down into heap.  This is the "bottom-up" variant,
+	 which significantly reduces calls to cmp_func(): we find the
+	 sift-down path all the way to the leaves (one compare per level),
+	 then backtrack to find where to insert the target element.
+
+	 Because elements tend to sift down close to the leaves, this uses
+	 fewer compares than doing two per level on the way down.  (A bit more
+	 than half as many on average, 3/4 worst-case.).  */
+      for (b = a; c = 2 * b + size, (d = c + size) < n;)
+	b = cmp (base + c, base + d, arg) >= 0 ? c : d;
+      if (d == n)
+	/* Special case last leaf with no sibling.  */
+	b = c;
+
+      /* Now backtrack from "b" to the correct location for "a".  */
+      while (b != a && cmp (base + a, base + b, arg) >= 0)
+	b = parent (b, lsbit, size);
+      /* Where "a" belongs.  */
+      c = b;
+      while (b != a)
+	{
+	  /* Shift it into place.  */
+	  b = parent (b, lsbit, size);
+          do_swap (base + b, base + c, size, swap_type);
+        }
+    }
+}
 
 /* Order size using quicksort.  This implementation incorporates
    four optimizations discussed in Sedgewick:
@@ -222,7 +291,7 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
 
   const size_t max_thresh = MAX_THRESH * size;
 
-  if (total_elems == 0)
+  if (total_elems <= 1)
     /* Avoid lossage with unsigned arithmetic below.  */
     return;
 
@@ -234,6 +303,10 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
   else
     swap_type = SWAP_BYTES;
 
+  /* Maximum depth before quicksort switches to heapsort.  */
+  size_t depth = 2 * (sizeof (size_t) * CHAR_BIT - 1
+		      - __builtin_clzl (total_elems));
+
   if (total_elems > MAX_THRESH)
     {
       char *lo = base_ptr;
@@ -243,6 +316,13 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
 
       while (stack < top)
         {
+          if (depth == 0)
+            {
+              heapsort_r (lo, hi, size, swap_type, cmp, arg);
+              top = pop (top, &lo, &hi, &depth);
+              continue;
+            }
+
           char *left_ptr;
           char *right_ptr;
 
@@ -306,7 +386,7 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
             {
               if ((size_t) (hi - left_ptr) <= max_thresh)
 		/* Ignore both small partitions. */
-                top = pop (top, &lo, &hi);
+                top = pop (top, &lo, &hi, &depth);
               else
 		/* Ignore small left partition. */
                 lo = left_ptr;
@@ -317,13 +397,13 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
           else if ((right_ptr - lo) > (hi - left_ptr))
             {
 	      /* Push larger left partition indices. */
-              top = push (top, lo, right_ptr);
+              top = push (top, lo, right_ptr, depth - 1);
               lo = left_ptr;
             }
           else
             {
 	      /* Push larger right partition indices. */
-              top = push (top, left_ptr, hi);
+              top = push (top, left_ptr, hi, depth - 1);
               hi = right_ptr;
             }
         }
-- 
2.34.1

[PATCH v5 5/6] stdlib: Remove use of mergesort on qsort (BZ 21719)

[Adhemerval Zanella]

This patch removes the mergesort optimization on qsort implementation
and uses the introsort instead.  The mergesort implementation has some
issues:

  - It is as-safe only for certain types sizes (if total size is less
    than 1 KB with large element sizes also forcing memory allocation)
    which contradicts the function documentation.  Although not required
    by the C standard, it is preferable and doable to have an O(1) space
    implementation.

  - The malloc for certain element size and element number adds
    arbitrary latency (might even be worse if malloc is interposed).

  - To avoid trigger swap from memory allocation the implementation
    relies on system information that might be virtualized (for instance
    VMs with overcommit memory) which might lead to potentially use of
    swap even if system advertise more memory than actually has.  The
    check also have the downside of issuing syscalls where none is
    expected (although only once per execution).

  - The mergesort is suboptimal on an already sorted array (BZ#21719).

The introsort implementation is already optimized to use constant extra
space (due to the limit of total number of elements from maximum VM
size) and thus can be used to avoid the malloc usage issues.

Resulting performance is slower due the usage of qsort, specially in the
worst-case scenario (partialy or sorted arrays) and due the fact
mergesort uses a slight improved swap operations.

This change also renders the BZ#21719 fix unrequired (since it is meant
to fix the sorted input performance degradation for mergesort).  The
manual is also updated to indicate the function is now async-cancel
safe.

Checked on x86_64-linux-gnu.
---
 include/stdlib.h   |   2 -
 manual/argp.texi   |   2 +-
 manual/locale.texi |   3 +-
 manual/search.texi |   7 +-
 stdlib/Makefile    |   2 -
 stdlib/msort.c     | 309 ---------------------------------------------
 stdlib/qsort.c     |  14 +-
 7 files changed, 16 insertions(+), 323 deletions(-)
 delete mode 100644 stdlib/msort.c

diff --git a/include/stdlib.h b/include/stdlib.h
index 7deb8193d7..7b421f1bb3 100644
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -147,8 +147,6 @@ extern int __posix_openpt (int __oflag) attribute_hidden;
 extern int __add_to_environ (const char *name, const char *value,
 			     const char *combines, int replace)
      attribute_hidden;
-extern void _quicksort (void *const pbase, size_t total_elems,
-			size_t size, __compar_d_fn_t cmp, void *arg);
 
 extern int __on_exit (void (*__func) (int __status, void *__arg), void *__arg);
 
diff --git a/manual/argp.texi b/manual/argp.texi
index 0023441812..b77ad68285 100644
--- a/manual/argp.texi
+++ b/manual/argp.texi
@@ -735,7 +735,7 @@ for options, bad phase of the moon, etc.
 @c  hol_set_group ok
 @c   hol_find_entry ok
 @c  hol_sort @mtslocale @acucorrupt
-@c   qsort dup @acucorrupt
+@c   qsort dup
 @c    hol_entry_qcmp @mtslocale
 @c     hol_entry_cmp @mtslocale
 @c      group_cmp ok
diff --git a/manual/locale.texi b/manual/locale.texi
index 720e0ca952..f6afa5dc44 100644
--- a/manual/locale.texi
+++ b/manual/locale.texi
@@ -253,7 +253,7 @@ The symbols in this section are defined in the header file @file{locale.h}.
 @c    calculate_head_size ok
 @c    __munmap ok
 @c    compute_hashval ok
-@c    qsort dup @acucorrupt
+@c    qsort dup
 @c     rangecmp ok
 @c    malloc @ascuheap @acsmem
 @c    strdup @ascuheap @acsmem
@@ -275,7 +275,6 @@ The symbols in this section are defined in the header file @file{locale.h}.
 @c      realloc @ascuheap @acsmem
 @c     realloc @ascuheap @acsmem
 @c     fclose @ascuheap @asulock @acsmem @acsfd @aculock
-@c     qsort @ascuheap @acsmem
 @c      alias_compare dup
 @c    libc_lock_unlock @aculock
 @c   _nl_explode_name @ascuheap @acsmem
diff --git a/manual/search.texi b/manual/search.texi
index 5691bf2f2b..a550858478 100644
--- a/manual/search.texi
+++ b/manual/search.texi
@@ -159,7 +159,7 @@ To sort an array using an arbitrary comparison function, use the
 
 @deftypefun void qsort (void *@var{array}, size_t @var{count}, size_t @var{size}, comparison_fn_t @var{compare})
 @standards{ISO, stdlib.h}
-@safety{@prelim{}@mtsafe{}@assafe{}@acunsafe{@acucorrupt{}}}
+@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
 The @code{qsort} function sorts the array @var{array}.  The array
 contains @var{count} elements, each of which is of size @var{size}.
 
@@ -199,9 +199,8 @@ Functions}):
 The @code{qsort} function derives its name from the fact that it was
 originally implemented using the ``quick sort'' algorithm.
 
-The implementation of @code{qsort} in this library might not be an
-in-place sort and might thereby use an extra amount of memory to store
-the array.
+The implementation of @code{qsort} in this library is an in-place sort
+and uses a constant extra space (allocated on the stack).
 @end deftypefun
 
 @node Search/Sort Example
diff --git a/stdlib/Makefile b/stdlib/Makefile
index 25e42a77e7..095518eef4 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -96,7 +96,6 @@ routines := \
   mbtowc \
   mrand48 \
   mrand48_r \
-  msort \
   nrand48 \
   nrand48_r \
   old_atexit  \
@@ -380,7 +379,6 @@ generated += \
   # generated
 
 CFLAGS-bsearch.c += $(uses-callbacks)
-CFLAGS-msort.c += $(uses-callbacks)
 CFLAGS-qsort.c += $(uses-callbacks)
 CFLAGS-system.c += -fexceptions
 CFLAGS-system.os = -fomit-frame-pointer
diff --git a/stdlib/msort.c b/stdlib/msort.c
deleted file mode 100644
index bbaa5e9f82..0000000000
--- a/stdlib/msort.c
+++ /dev/null
@@ -1,309 +0,0 @@
-/* An alternative to qsort, with an identical interface.
-   This file is part of the GNU C Library.
-   Copyright (C) 1992-2023 Free Software Foundation, Inc.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library; if not, see
-   <https://www.gnu.org/licenses/>.  */
-
-#include <alloca.h>
-#include <stdint.h>
-#include <stdlib.h>
-#include <string.h>
-#include <unistd.h>
-#include <memcopy.h>
-#include <errno.h>
-#include <atomic.h>
-
-struct msort_param
-{
-  size_t s;
-  size_t var;
-  __compar_d_fn_t cmp;
-  void *arg;
-  char *t;
-};
-static void msort_with_tmp (const struct msort_param *p, void *b, size_t n);
-
-static void
-msort_with_tmp (const struct msort_param *p, void *b, size_t n)
-{
-  char *b1, *b2;
-  size_t n1, n2;
-
-  if (n <= 1)
-    return;
-
-  n1 = n / 2;
-  n2 = n - n1;
-  b1 = b;
-  b2 = (char *) b + (n1 * p->s);
-
-  msort_with_tmp (p, b1, n1);
-  msort_with_tmp (p, b2, n2);
-
-  char *tmp = p->t;
-  const size_t s = p->s;
-  __compar_d_fn_t cmp = p->cmp;
-  void *arg = p->arg;
-  switch (p->var)
-    {
-    case 0:
-      while (n1 > 0 && n2 > 0)
-	{
-	  if ((*cmp) (b1, b2, arg) <= 0)
-	    {
-	      *(uint32_t *) tmp = *(uint32_t *) b1;
-	      b1 += sizeof (uint32_t);
-	      --n1;
-	    }
-	  else
-	    {
-	      *(uint32_t *) tmp = *(uint32_t *) b2;
-	      b2 += sizeof (uint32_t);
-	      --n2;
-	    }
-	  tmp += sizeof (uint32_t);
-	}
-      break;
-    case 1:
-      while (n1 > 0 && n2 > 0)
-	{
-	  if ((*cmp) (b1, b2, arg) <= 0)
-	    {
-	      *(uint64_t *) tmp = *(uint64_t *) b1;
-	      b1 += sizeof (uint64_t);
-	      --n1;
-	    }
-	  else
-	    {
-	      *(uint64_t *) tmp = *(uint64_t *) b2;
-	      b2 += sizeof (uint64_t);
-	      --n2;
-	    }
-	  tmp += sizeof (uint64_t);
-	}
-      break;
-    case 2:
-      while (n1 > 0 && n2 > 0)
-	{
-	  unsigned long *tmpl = (unsigned long *) tmp;
-	  unsigned long *bl;
-
-	  tmp += s;
-	  if ((*cmp) (b1, b2, arg) <= 0)
-	    {
-	      bl = (unsigned long *) b1;
-	      b1 += s;
-	      --n1;
-	    }
-	  else
-	    {
-	      bl = (unsigned long *) b2;
-	      b2 += s;
-	      --n2;
-	    }
-	  while (tmpl < (unsigned long *) tmp)
-	    *tmpl++ = *bl++;
-	}
-      break;
-    case 3:
-      while (n1 > 0 && n2 > 0)
-	{
-	  if ((*cmp) (*(const void **) b1, *(const void **) b2, arg) <= 0)
-	    {
-	      *(void **) tmp = *(void **) b1;
-	      b1 += sizeof (void *);
-	      --n1;
-	    }
-	  else
-	    {
-	      *(void **) tmp = *(void **) b2;
-	      b2 += sizeof (void *);
-	      --n2;
-	    }
-	  tmp += sizeof (void *);
-	}
-      break;
-    default:
-      while (n1 > 0 && n2 > 0)
-	{
-	  if ((*cmp) (b1, b2, arg) <= 0)
-	    {
-	      tmp = (char *) __mempcpy (tmp, b1, s);
-	      b1 += s;
-	      --n1;
-	    }
-	  else
-	    {
-	      tmp = (char *) __mempcpy (tmp, b2, s);
-	      b2 += s;
-	      --n2;
-	    }
-	}
-      break;
-    }
-
-  if (n1 > 0)
-    memcpy (tmp, b1, n1 * s);
-  memcpy (b, p->t, (n - n2) * s);
-}
-
-
-void
-__qsort_r (void *b, size_t n, size_t s, __compar_d_fn_t cmp, void *arg)
-{
-  size_t size = n * s;
-  char *tmp = NULL;
-  struct msort_param p;
-
-  /* For large object sizes use indirect sorting.  */
-  if (s > 32)
-    size = 2 * n * sizeof (void *) + s;
-
-  if (size < 1024)
-    /* The temporary array is small, so put it on the stack.  */
-    p.t = __alloca (size);
-  else
-    {
-      /* We should avoid allocating too much memory since this might
-	 have to be backed up by swap space.  */
-      static long int phys_pages;
-      static int pagesize;
-
-      if (pagesize == 0)
-	{
-	  phys_pages = __sysconf (_SC_PHYS_PAGES);
-
-	  if (phys_pages == -1)
-	    /* Error while determining the memory size.  So let's
-	       assume there is enough memory.  Otherwise the
-	       implementer should provide a complete implementation of
-	       the `sysconf' function.  */
-	    phys_pages = (long int) (~0ul >> 1);
-
-	  /* The following determines that we will never use more than
-	     a quarter of the physical memory.  */
-	  phys_pages /= 4;
-
-	  /* Make sure phys_pages is written to memory.  */
-	  atomic_write_barrier ();
-
-	  pagesize = __sysconf (_SC_PAGESIZE);
-	}
-
-      /* Just a comment here.  We cannot compute
-	   phys_pages * pagesize
-	   and compare the needed amount of memory against this value.
-	   The problem is that some systems might have more physical
-	   memory then can be represented with a `size_t' value (when
-	   measured in bytes.  */
-
-      /* If the memory requirements are too high don't allocate memory.  */
-      if (size / pagesize > (size_t) phys_pages)
-	{
-	  _quicksort (b, n, s, cmp, arg);
-	  return;
-	}
-
-      /* It's somewhat large, so malloc it.  */
-      int save = errno;
-      tmp = malloc (size);
-      __set_errno (save);
-      if (tmp == NULL)
-	{
-	  /* Couldn't get space, so use the slower algorithm
-	     that doesn't need a temporary array.  */
-	  _quicksort (b, n, s, cmp, arg);
-	  return;
-	}
-      p.t = tmp;
-    }
-
-  p.s = s;
-  p.var = 4;
-  p.cmp = cmp;
-  p.arg = arg;
-
-  if (s > 32)
-    {
-      /* Indirect sorting.  */
-      char *ip = (char *) b;
-      void **tp = (void **) (p.t + n * sizeof (void *));
-      void **t = tp;
-      void *tmp_storage = (void *) (tp + n);
-
-      while ((void *) t < tmp_storage)
-	{
-	  *t++ = ip;
-	  ip += s;
-	}
-      p.s = sizeof (void *);
-      p.var = 3;
-      msort_with_tmp (&p, p.t + n * sizeof (void *), n);
-
-      /* tp[0] .. tp[n - 1] is now sorted, copy around entries of
-	 the original array.  Knuth vol. 3 (2nd ed.) exercise 5.2-10.  */
-      char *kp;
-      size_t i;
-      for (i = 0, ip = (char *) b; i < n; i++, ip += s)
-	if ((kp = tp[i]) != ip)
-	  {
-	    size_t j = i;
-	    char *jp = ip;
-	    memcpy (tmp_storage, ip, s);
-
-	    do
-	      {
-		size_t k = (kp - (char *) b) / s;
-		tp[j] = jp;
-		memcpy (jp, kp, s);
-		j = k;
-		jp = kp;
-		kp = tp[k];
-	      }
-	    while (kp != ip);
-
-	    tp[j] = jp;
-	    memcpy (jp, tmp_storage, s);
-	  }
-    }
-  else
-    {
-      if ((s & (sizeof (uint32_t) - 1)) == 0
-	  && ((uintptr_t) b) % __alignof__ (uint32_t) == 0)
-	{
-	  if (s == sizeof (uint32_t))
-	    p.var = 0;
-	  else if (s == sizeof (uint64_t)
-		   && ((uintptr_t) b) % __alignof__ (uint64_t) == 0)
-	    p.var = 1;
-	  else if ((s & (sizeof (unsigned long) - 1)) == 0
-		   && ((uintptr_t) b)
-		      % __alignof__ (unsigned long) == 0)
-	    p.var = 2;
-	}
-      msort_with_tmp (&p, b, n);
-    }
-  free (tmp);
-}
-libc_hidden_def (__qsort_r)
-weak_alias (__qsort_r, qsort_r)
-
-
-void
-qsort (void *b, size_t n, size_t s, __compar_fn_t cmp)
-{
-  return __qsort_r (b, n, s, (__compar_d_fn_t) cmp, NULL);
-}
-libc_hidden_def (qsort)
diff --git a/stdlib/qsort.c b/stdlib/qsort.c
index ed0ac53467..0bcbb6a14a 100644
--- a/stdlib/qsort.c
+++ b/stdlib/qsort.c
@@ -19,7 +19,6 @@
    Engineering a sort function; Jon Bentley and M. Douglas McIlroy;
    Software - Practice and Experience; Vol. 23 (11), 1249-1265, 1993.  */
 
-#include <alloca.h>
 #include <limits.h>
 #include <stdlib.h>
 #include <string.h>
@@ -284,8 +283,8 @@ insertion_sort_qsort_partitions (void *const pbase, size_t total_elems,
 }
 
 void
-_quicksort (void *const pbase, size_t total_elems, size_t size,
-	    __compar_d_fn_t cmp, void *arg)
+__qsort_r (void *const pbase, size_t total_elems, size_t size,
+	   __compar_d_fn_t cmp, void *arg)
 {
   char *base_ptr = (char *) pbase;
 
@@ -417,3 +416,12 @@ _quicksort (void *const pbase, size_t total_elems, size_t size,
   insertion_sort_qsort_partitions (pbase, total_elems, size, swap_type, cmp,
 				   arg);
 }
+libc_hidden_def (__qsort_r)
+weak_alias (__qsort_r, qsort_r)
+
+void
+qsort (void *b, size_t n, size_t s, __compar_fn_t cmp)
+{
+  return __qsort_r (b, n, s, (__compar_d_fn_t) cmp, NULL);
+}
+libc_hidden_def (qsort)
-- 
2.34.1

[PATCH v5 6/6] stdlib: Add more qsort{_r} coverage

[Adhemerval Zanella]

This patch adds a qsort and qsort_r (which glibc current lacks
coverage).  The test is done with random input, dfferent internal
types (uint8_t, uint16_t, uint32_t, and uint64_t), and with
different set of element numbers (from 0 to 262144).

Checked on x86_64-linux-gnu.
---
 stdlib/Makefile     |   1 +
 stdlib/tst-qsort3.c | 298 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 299 insertions(+)
 create mode 100644 stdlib/tst-qsort3.c

diff --git a/stdlib/Makefile b/stdlib/Makefile
index 095518eef4..6af606136e 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -214,6 +214,7 @@ tests := \
   tst-on_exit \
   tst-qsort \
   tst-qsort2 \
+  tst-qsort3 \
   tst-quick_exit \
   tst-rand48 \
   tst-rand48-2 \
diff --git a/stdlib/tst-qsort3.c b/stdlib/tst-qsort3.c
new file mode 100644
index 0000000000..6940540289
--- /dev/null
+++ b/stdlib/tst-qsort3.c
@@ -0,0 +1,298 @@
+/* qsort(_r) tests to trigger worst case for quicksort.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <array_length.h>
+#include <errno.h>
+#include <getopt.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <support/check.h>
+#include <support/support.h>
+#include <support/test-driver.h>
+
+typedef enum
+{
+  Sorted,
+  Random,
+  Repeated,
+  Bitonic
+} arraytype_t;
+
+/* Ratio of total of elements which will be repeated.  */
+static const double RepeatedRatio = 0.2;
+
+struct array_t
+{
+  arraytype_t type;
+  const char *name;
+} static const arraytypes[] =
+{
+  { Sorted,       "Sorted" },
+  { Random,       "Random" },
+  { Repeated,     "Repeated" },
+  { Bitonic,      "Bitonic" },
+};
+
+/* Return the index of BASE as interpreted as an array of elements
+   of size SIZE.  */
+static inline void *
+arr (void *base, size_t idx, size_t size)
+{
+  return (void*)((uintptr_t)base + (idx * size));
+}
+
+/* Functions used to check qsort.  */
+static int
+uint8_t_cmp (const void *a, const void *b)
+{
+  uint8_t ia = *(uint8_t*)a;
+  uint8_t ib = *(uint8_t*)b;
+  return (ia > ib) - (ia < ib);
+}
+
+static int
+uint16_t_cmp (const void *a, const void *b)
+{
+  uint16_t ia = *(uint16_t*)a;
+  uint16_t ib = *(uint16_t*)b;
+  return (ia > ib) - (ia < ib);
+}
+
+static int
+uint32_t_cmp (const void *a, const void *b)
+{
+  uint32_t ia = *(uint32_t*)a;
+  uint32_t ib = *(uint32_t*)b;
+  return (ia > ib) - (ia < ib);
+}
+
+static int
+uint64_t_cmp (const void *a, const void *b)
+{
+  uint64_t ia = *(uint64_t*)a;
+  uint64_t ib = *(uint64_t*)b;
+  return (ia > ib) - (ia < ib);
+}
+
+#define LARGE_SIZE 48
+
+static int
+large_cmp (const void *a, const void *b)
+{
+  return memcmp (a, b, LARGE_SIZE);
+}
+
+/* Function used to check qsort_r.  */
+typedef enum
+{
+  UINT8_CMP_T,
+  UINT16_CMP_T,
+  UINT32_CMP_T,
+  UINT64_CMP_T,
+  LARGE_CMP_T
+} type_cmp_t;
+
+static type_cmp_t
+uint_t_cmp_type (size_t sz)
+{
+  switch (sz)
+    {
+      case sizeof (uint8_t):  return UINT8_CMP_T;
+      case sizeof (uint16_t): return UINT16_CMP_T;
+      case sizeof (uint64_t): return UINT64_CMP_T;
+      case sizeof (uint32_t): return UINT32_CMP_T;
+      default:                return LARGE_CMP_T;
+    }
+}
+
+static int
+uint_t_cmp (const void *a, const void *b, void *arg)
+{
+  type_cmp_t type = *(type_cmp_t*) arg;
+  switch (type)
+    {
+    case UINT8_CMP_T:  return uint8_t_cmp (a, b);
+    case UINT32_CMP_T: return uint32_t_cmp (a, b);
+    case UINT16_CMP_T: return uint16_t_cmp (a, b);
+    case UINT64_CMP_T: return uint64_t_cmp (a, b);
+    default:           return large_cmp (a, b);
+    }
+}
+
+static void
+seq (void *elem, size_t type_size, int value)
+{
+  if (type_size == sizeof (uint8_t))
+    *(uint8_t*)elem = value;
+  else if (type_size == sizeof (uint16_t))
+    *(uint16_t*)elem = value;
+  else if (type_size == sizeof (uint32_t))
+    *(uint32_t*)elem = value;
+  else if (type_size == sizeof (uint64_t))
+    *(uint64_t*)elem = value;
+  else
+    memset (elem, value, type_size);
+}
+
+static void *
+create_array (size_t nmemb, size_t type_size, arraytype_t type)
+{
+  size_t size = nmemb * type_size;
+  void *array = xmalloc (size);
+
+  switch (type)
+    {
+    case Sorted:
+      for (size_t i = 0; i < nmemb; i++)
+	seq (arr (array, i, type_size), type_size, i);
+      break;
+
+    case Random:
+      arc4random_buf (array, size);
+      break;
+
+    case Repeated:
+      {
+        arc4random_buf (array, size);
+
+	void *randelem = xmalloc (type_size);
+	arc4random_buf (randelem, type_size);
+
+	/* Repeat REPEATED elements (based on RepeatRatio ratio) in the random
+	   array.  */
+        size_t repeated = (size_t)(nmemb * RepeatedRatio);
+	for (size_t i = 0; i < repeated; i++)
+	  {
+	    size_t pos = arc4random_uniform (nmemb - 1);
+	    memcpy (arr (array, pos, type_size), randelem, type_size);
+	  }
+	free (randelem);
+      }
+      break;
+
+    case Bitonic:
+      {
+	size_t i;
+        for (i = 0; i < nmemb / 2; i++)
+	  seq (arr (array, i, type_size), type_size, i);
+        for (     ; i < nmemb;     i++)
+	  seq (arr (array, i, type_size), type_size, (nmemb - 1) - i);
+      }
+      break;
+    }
+
+  return array;
+}
+
+typedef int (*cmpfunc_t)(const void *, const void *);
+
+/* Check if ARRAY of total NMEMB element of size SIZE is sorted
+   based on CMPFUNC.  */
+static void
+check_array (void *array, size_t nmemb, size_t type_size,
+	     cmpfunc_t cmpfunc)
+{
+  for (size_t i = 1; i < nmemb; i++)
+    {
+      int ret = cmpfunc (arr (array, i,   type_size),
+			 arr (array, i-1, type_size));
+      TEST_VERIFY_EXIT (ret >= 0);
+    }
+}
+
+static void
+check_qsort (size_t nelem, size_t type_size, arraytype_t type,
+	     cmpfunc_t cmpfunc)
+{
+  void *array = create_array (nelem, type_size, type);
+
+  qsort (array, nelem, type_size, cmpfunc);
+
+  check_array (array, nelem, type_size, cmpfunc);
+
+  free (array);
+}
+
+static void
+check_qsort_r (size_t nelem, size_t type_size, arraytype_t type,
+	       cmpfunc_t cmpfunc)
+{
+  void *array = create_array (nelem, type_size, type);
+
+  type_cmp_t typecmp = uint_t_cmp_type (type_size);
+  qsort_r (array, nelem, type_size, uint_t_cmp, &typecmp);
+
+  check_array (array, nelem, type_size, cmpfunc);
+
+  free (array);
+}
+
+static int
+do_test (void)
+{
+  /* Some random sizes.  */
+  const size_t nelems[] = { 0, 1, 7, 20, 32, 100, 256, 1024, 8560, 17384 };
+
+  struct test_t
+    {
+      size_t type_size;
+      cmpfunc_t cmpfunc;
+    }
+  const tests[] =
+    {
+      { sizeof (uint8_t),  uint8_t_cmp },
+      { sizeof (uint16_t), uint16_t_cmp },
+      { sizeof (uint32_t), uint32_t_cmp },
+      { sizeof (uint64_t), uint64_t_cmp },
+      /* Test swap with large elements.  */
+      { LARGE_SIZE,        large_cmp },
+    };
+
+  for (const struct test_t *test = tests; test < array_end (tests); ++test)
+    {
+      if (test_verbose > 0)
+	printf ("info: testing qsort with type_size=%zu\n", test->type_size);
+      for (const struct array_t *arraytype = arraytypes;
+	   arraytype < array_end (arraytypes);
+	   ++arraytype)
+	{
+	  if (test_verbose > 0)
+            printf ("  distribution=%s\n", arraytype->name);
+	  for (const size_t *nelem = nelems;
+	       nelem < array_end (nelems);
+	       ++nelem)
+	    {
+	      if (test_verbose > 0)
+		printf ("  i  nelem=%zu, total size=%zu\n", *nelem,
+			*nelem * test->type_size);
+
+	      check_qsort (*nelem, test->type_size, arraytype->type,
+			   test->cmpfunc);
+	      check_qsort_r (*nelem, test->type_size, arraytype->type,
+			     test->cmpfunc);
+	   }
+	}
+    }
+
+  return 0;
+}
+
+#include <support/test-driver.c>
-- 
2.34.1