Re: [PATCH] Analyze niter for until-wrap condition [PR101145]

[Richard Biener <rguenther@suse.de>]

On Thu, 1 Jul 2021, Jiufu Guo wrote:

> For code like:
> unsigned foo(unsigned val, unsigned start)
> {
>   unsigned cnt = 0;
>   for (unsigned i = start; i > val; ++i)
>     cnt++;
>   return cnt;
> }
> 
> The number of iterations should be about UINT_MAX - start.

For

unsigned foo(unsigned val, unsigned start)
{
  unsigned cnt = 0;
  for (unsigned i = start; i >= val; ++i)
    cnt++;
  return cnt;
}

and val == 0 the loop never terminates.  I don't see anywhere
in the patch that you disregard GE_EXPR and I remember
the code handles GE as well as GT?  From a quick look this is
also not covered by a testcase you add - not exactly sure
how it would materialize in a miscompilation.

> There is function adjust_cond_for_loop_until_wrap which
> handles similar work for const bases.
> Like adjust_cond_for_loop_until_wrap, this patch enhance
> function number_of_iterations_cond/number_of_iterations_lt
> to analyze number of iterations for this kind of loop.
> 
> Bootstrap and regtest pass on powerpc64le, is this ok for trunk?
> 
> gcc/ChangeLog:
> 
> 	PR tree-optimization/101145
> 	* tree-ssa-loop-niter.c
> 	(number_of_iterations_until_wrap): New function.
> 	(number_of_iterations_lt): Invoke above function.
> 	(adjust_cond_for_loop_until_wrap):
> 	Merge to number_of_iterations_until_wrap.
> 	(number_of_iterations_cond): Update invokes for
> 	adjust_cond_for_loop_until_wrap and number_of_iterations_lt.
> 
> gcc/testsuite/ChangeLog:
> 
> 	PR tree-optimization/101145
> 	* gcc.dg/vect/pr101145.c: New test.
> 	* gcc.dg/vect/pr101145.inc: New test.
> 	* gcc.dg/vect/pr101145_1.c: New test.
> 	* gcc.dg/vect/pr101145_2.c: New test.
> 	* gcc.dg/vect/pr101145_3.c: New test.
> ---
>  gcc/testsuite/gcc.dg/vect/pr101145.c   | 187 +++++++++++++++++++++++++
>  gcc/testsuite/gcc.dg/vect/pr101145.inc |  63 +++++++++
>  gcc/testsuite/gcc.dg/vect/pr101145_1.c |  15 ++
>  gcc/testsuite/gcc.dg/vect/pr101145_2.c |  15 ++
>  gcc/testsuite/gcc.dg/vect/pr101145_3.c |  15 ++
>  gcc/tree-ssa-loop-niter.c              | 150 +++++++++++---------
>  6 files changed, 380 insertions(+), 65 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/vect/pr101145.c
>  create mode 100644 gcc/testsuite/gcc.dg/vect/pr101145.inc
>  create mode 100644 gcc/testsuite/gcc.dg/vect/pr101145_1.c
>  create mode 100644 gcc/testsuite/gcc.dg/vect/pr101145_2.c
>  create mode 100644 gcc/testsuite/gcc.dg/vect/pr101145_3.c
> 
> diff --git a/gcc/testsuite/gcc.dg/vect/pr101145.c b/gcc/testsuite/gcc.dg/vect/pr101145.c
> new file mode 100644
> index 00000000000..74031b031cf
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/vect/pr101145.c
> @@ -0,0 +1,187 @@
> +/* { dg-require-effective-target vect_int } */
> +/* { dg-options "-O3 -fdump-tree-vect-details" } */
> +#include <limits.h>
> +
> +unsigned __attribute__ ((noinline))
> +foo (int *__restrict__ a, int *__restrict__ b, unsigned l, unsigned n)
> +{
> +  while (n < ++l)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +unsigned __attribute__ ((noinline))
> +foo_1 (int *__restrict__ a, int *__restrict__ b, unsigned l, unsigned)
> +{
> +  while (UINT_MAX - 64 < ++l)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +unsigned __attribute__ ((noinline))
> +foo_2 (int *__restrict__ a, int *__restrict__ b, unsigned l, unsigned n)
> +{
> +  l = UINT_MAX - 32;
> +  while (n < ++l)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +unsigned __attribute__ ((noinline))
> +foo_3 (int *__restrict__ a, int *__restrict__ b, unsigned l, unsigned n)
> +{
> +  while (n <= ++l)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +unsigned __attribute__ ((noinline))
> +foo_4 (int *__restrict__ a, int *__restrict__ b, unsigned l, unsigned n)
> +{  // infininate 
> +  while (0 <= ++l)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +unsigned __attribute__ ((noinline))
> +foo_5 (int *__restrict__ a, int *__restrict__ b, unsigned l, unsigned n)
> +{
> +  //no loop
> +  l = UINT_MAX;
> +  while (n < ++l)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +unsigned __attribute__ ((noinline))
> +bar (int *__restrict__ a, int *__restrict__ b, unsigned l, unsigned n)
> +{
> +  while (--l < n)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +unsigned __attribute__ ((noinline))
> +bar_1 (int *__restrict__ a, int *__restrict__ b, unsigned l, unsigned)
> +{
> +  while (--l < 64)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +unsigned __attribute__ ((noinline))
> +bar_2 (int *__restrict__ a, int *__restrict__ b, unsigned l, unsigned n)
> +{
> +  l = 32;
> +  while (--l < n)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +
> +int a[3200], b[3200];
> +int fail;
> +
> +int
> +main ()
> +{
> +  unsigned l, n;
> +  unsigned res;
> +  /* l > n*/
> +  n = UINT_MAX - 64;
> +  l = n + 32;
> +  res = foo (a, b, l, n);
> +  if (res != 0)
> +    fail++;
> +
> +  l = n;
> +  res = foo (a, b, l, n);
> +  if (res != 0)
> +    fail++;
> +
> +  l = n - 1;
> +  res = foo (a, b, l, n);
> +  if (res != l + 1)
> +    fail++;
> +  
> +  l = n - 32;
> +  res = foo (a, b, l, n);
> +  if (res != l + 1)
> +    fail++;
> +
> +  l = UINT_MAX;
> +  res = foo (a, b, l, n);
> +  if (res != 0)
> +    fail++;
> +
> +  l = n + 32;
> +  res = foo_1 (a, b, l, n);
> +  if (res != 0)
> +    fail++;
> +
> +  l = n + 32;
> +  res = foo_2 (a, b, l, n);
> +  if (res != 0)
> +    fail++;
> +
> +  l = n;
> +  res = foo_3 (a, b, l, n);
> +  if (res != 0)
> +    fail++;
> +
> +  l = n - 1;
> +  res = foo_3 (a, b, l, n);
> +  if (res != 0)
> +    fail++;
> +
> +  l = n - 2;
> +  res = foo_3 (a, b, l, n);
> +  if (res != l + 1)
> +    fail++;
> +
> +  res = foo_5 (a, b, l, n);
> +  if (res != 0)
> +    fail++;
> +
> +  n = 64;
> +  l = n - 32;
> +  res = bar (a, b, l, n);
> +  res++;
> +  if (res != 0)
> +    fail++;
> +
> +  l = n;
> +  res = bar (a, b, l, n);
> +  res++;
> +  if (res != 0)
> +    fail++;
> +
> +  l = n + 1;
> +  res = bar (a, b, l, n);
> +  res++;
> +  if (res != l)
> +    fail++;
> +
> +  l = 0;
> +  res = bar (a, b, l, n);
> +  res++;
> +  if (res != l)
> +    fail++;
> +
> +  l = 32;
> +  res = bar_1 (a, b, l, n);
> +  res++;
> +  if (res != 0)
> +    fail++;  
> +
> +  res = bar_1 (a, b, l, n);
> +  res++;
> +  if (res != 0)
> +    fail++;  
> +
> +  if (fail)
> +    __builtin_abort ();
> +  return 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 7 "vect" } } */
> diff --git a/gcc/testsuite/gcc.dg/vect/pr101145.inc b/gcc/testsuite/gcc.dg/vect/pr101145.inc
> new file mode 100644
> index 00000000000..6eed3fa8aca
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/vect/pr101145.inc
> @@ -0,0 +1,63 @@
> +TYPE __attribute__ ((noinline))
> +foo_sign (int *__restrict__ a, int *__restrict__ b, TYPE l, TYPE n)
> +{
> +  for (l = L_BASE; n < l; l += C)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +TYPE __attribute__ ((noinline))
> +bar_sign (int *__restrict__ a, int *__restrict__ b, TYPE l, TYPE n)
> +{
> +  for (l = L_BASE_DOWN; l < n; l -= C)
> +    *a++ = *b++ + 1;
> +  return l;
> +}
> +
> +int __attribute__ ((noinline)) neq (int a, int b) { return a != b; }
> +
> +int a[1000], b[1000];
> +int fail;
> +
> +int
> +main ()
> +{
> +  TYPE res;
> +  TYPE l;
> +  TYPE n;
> +  n = N_BASE;
> +  l = n - C;
> +  res = foo_sign (a, b, l, n);
> +  if (res != l)
> +    fail++;
> +
> +  l = n;
> +  res = foo_sign (a, b, l, n);
> +  if (res != l)
> +    fail++;
> +
> +  l = n + C;
> +  res = foo_sign (a, b, l, n);
> +  if (neq ((res - MIN) / C, 0))
> +    fail++;
> +
> +  n = N_BASE_DOWN;
> +  l = n - C;
> +  res = bar_sign (a, b, l, n);
> +  if (neq ((MAX - res) / C, 0))
> +    fail++;
> +
> +  l = n;
> +  res = bar_sign (a, b, l, n);
> +  if (res != l)
> +    fail++;
> +
> +  l = n + C;
> +  res = bar_sign (a, b, l, n);
> +  if (res != l)
> +    fail++;
> +
> +  if (fail)
> +    __builtin_abort ();
> +  return 0;
> +}
> diff --git a/gcc/testsuite/gcc.dg/vect/pr101145_1.c b/gcc/testsuite/gcc.dg/vect/pr101145_1.c
> new file mode 100644
> index 00000000000..94f6b99b893
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/vect/pr101145_1.c
> @@ -0,0 +1,15 @@
> +/* { dg-require-effective-target vect_int } */
> +/* { dg-options "-O3 -fdump-tree-vect-details" } */
> +#define TYPE signed char
> +#define MIN -128
> +#define MAX 127
> +#define N_BASE (MAX - 32)
> +#define N_BASE_DOWN (MIN + 32)
> +
> +#define C 3
> +#define L_BASE l
> +#define L_BASE_DOWN l
> +
> +#include "pr101145.inc"
> +
> +/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 2 "vect" } } */
> diff --git a/gcc/testsuite/gcc.dg/vect/pr101145_2.c b/gcc/testsuite/gcc.dg/vect/pr101145_2.c
> new file mode 100644
> index 00000000000..d3cfc9e01e1
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/vect/pr101145_2.c
> @@ -0,0 +1,15 @@
> +/* { dg-require-effective-target vect_int } */
> +/* { dg-options "-O3 -fdump-tree-vect-details" } */
> +#define TYPE unsigned char
> +#define MIN 0
> +#define MAX 255
> +#define N_BASE (MAX - 32 + 1)
> +#define N_BASE_DOWN (MIN + 32)
> +
> +#define C 2
> +#define L_BASE l
> +#define L_BASE_DOWN l
> +
> +#include "pr101145.inc"
> +
> +/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 2 "vect" } } */
> diff --git a/gcc/testsuite/gcc.dg/vect/pr101145_3.c b/gcc/testsuite/gcc.dg/vect/pr101145_3.c
> new file mode 100644
> index 00000000000..e2cf9b1f7e6
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/vect/pr101145_3.c
> @@ -0,0 +1,15 @@
> +/* { dg-require-effective-target vect_int } */
> +/* { dg-options "-O3 -fdump-tree-vect-details" } */
> +#define TYPE int *
> +#define MIN ((TYPE)0)
> +#define MAX ((TYPE)((long long)-1))
> +#define N_BASE ((TYPE) (-32))
> +#define N_BASE_DOWN (MIN + 32)
> +
> +#define C 1
> +#define L_BASE l
> +#define L_BASE_DOWN l
> +
> +#include "pr101145.inc"
> +
> +/* { dg-final { scan-tree-dump-times "vectorized 1 loops" 2 "vect" } } */
> diff --git a/gcc/tree-ssa-loop-niter.c b/gcc/tree-ssa-loop-niter.c
> index b5add827018..06db6a36ef8 100644
> --- a/gcc/tree-ssa-loop-niter.c
> +++ b/gcc/tree-ssa-loop-niter.c
> @@ -1473,6 +1473,86 @@ assert_loop_rolls_lt (tree type, affine_iv *iv0, affine_iv *iv1,
>      }
>  }
>  
> +/* Determines number of iterations of loop whose ending condition
> +   is IV0 < IV1 which likes:  {base, -C} < n,  or n < {base, C}.
> +   The number of iterations is stored to NITER.  */
> +
> +static bool
> +number_of_iterations_until_wrap (class loop *, tree type, affine_iv *iv0,
> +				 affine_iv *iv1, class tree_niter_desc *niter)
> +{
> +  tree niter_type = unsigned_type_for (type);
> +  tree max, min;
> +
> +  if (POINTER_TYPE_P (type))
> +    {
> +      max = fold_convert (type, TYPE_MAX_VALUE (niter_type));
> +      min = fold_convert (type, TYPE_MIN_VALUE (niter_type));
> +    }
> +  else
> +    {
> +      max = TYPE_MAX_VALUE (type);
> +      min = TYPE_MIN_VALUE (type);
> +    }

It would be cheaper to use wide_int here - wi::min/max_value 
(TYPE_PRECISION (...), TYPE_SIGN (...)) and in the simplification
code below.

> +  tree high = max, low = min, one = build_int_cst (niter_type, 1);
> +  tree step;
> +
> +  /* n < {base, C}. */
> +  if (integer_zerop (iv0->step) && TREE_CODE (iv1->step) == INTEGER_CST
> +      && !tree_int_cst_sign_bit (iv1->step))
> +    {
> +      step = iv1->step;
> +      niter->niter = fold_build2 (MINUS_EXPR, niter_type, max, iv1->base);
> +      if (TREE_CODE (iv1->base) == INTEGER_CST)
> +	low = fold_build2 (MINUS_EXPR, type, iv1->base, one);
> +      else if (TREE_CODE (iv0->base) == INTEGER_CST)
> +	low = iv0->base;
> +    }
> +  /* {base, -C} < n. */
> +  else if (TREE_CODE (iv0->step) == INTEGER_CST
> +	   && tree_int_cst_sign_bit (iv0->step) && integer_zerop (iv1->step))
> +    {
> +      step = fold_build1 (NEGATE_EXPR, TREE_TYPE (iv0->step), iv0->step);
> +      niter->niter = fold_build2 (MINUS_EXPR, niter_type, iv0->base, min);
> +      if (TREE_CODE (iv0->base) == INTEGER_CST)
> +	high = fold_build2 (PLUS_EXPR, type, iv0->base, one);
> +      else if (TREE_CODE (iv1->base) == INTEGER_CST)
> +	high = iv1->base;
> +    }
> +  else
> +    return false;
> +
> +  /* (delta + step - 1) / step */
> +  step = fold_convert (niter_type, step);
> +  niter->niter = fold_convert (niter_type, niter->niter);
> +  niter->niter = fold_build2 (PLUS_EXPR, niter_type, niter->niter, step);
> +  niter->niter = fold_build2 (FLOOR_DIV_EXPR, niter_type, niter->niter, step);
> +
> +  tree m = fold_build2 (MINUS_EXPR, niter_type, high, low);
> +  m = fold_convert (niter_type, m);
> +  mpz_t mstep, tmp, mmax;
> +  mpz_init (mstep);
> +  mpz_init (tmp);
> +  mpz_init (mmax);
> +  wi::to_mpz (wi::to_wide (step), mstep, UNSIGNED);
> +  wi::to_mpz (wi::to_wide (m), mmax, UNSIGNED);
> +  mpz_add (tmp, mmax, mstep);
> +  mpz_sub_ui (tmp, tmp, 1);
> +  mpz_fdiv_q (tmp, tmp, mstep);
> +  niter->max = widest_int::from (wi::from_mpz (niter_type, tmp, false),
> +				 TYPE_SIGN (niter_type));
> +  mpz_clear (mstep);
> +  mpz_clear (tmp);

You forget to clear mmax?  I wonder why you need to involve GMP here
at all since this is just add/sub/divide - you could use
widest_int directly I think.  At least the back-and-forth between
trees, wide-int and mpz looks odd.

Richard.

> +  niter->may_be_zero
> +    = fold_build2 (LE_EXPR, boolean_type_node, iv1->base, iv0->base);
> +
> +  niter->control.no_overflow = false;
> +
> +  return true;
> +}
> +
>  /* Determines number of iterations of loop whose ending condition
>     is IV0 < IV1.  TYPE is the type of the iv.  The number of
>     iterations is stored to NITER.  BNDS bounds the difference
> @@ -1501,6 +1581,11 @@ number_of_iterations_lt (class loop *loop, tree type, affine_iv *iv0,
>        niter->bound = iv0->base;
>      }
>  
> +  /* {base, -C} < n,  or n < {base, C} */
> +  if (tree_int_cst_sign_bit (iv0->step)
> +      || (!integer_zerop (iv1->step) && !tree_int_cst_sign_bit (iv1->step)))
> +    return number_of_iterations_until_wrap (loop, type, iv0, iv1, niter);
> +
>    delta = fold_build2 (MINUS_EXPR, niter_type,
>  		       fold_convert (niter_type, iv1->base),
>  		       fold_convert (niter_type, iv0->base));
> @@ -1665,62 +1750,6 @@ dump_affine_iv (FILE *file, affine_iv *iv)
>      }
>  }
>  
> -/* Given exit condition IV0 CODE IV1 in TYPE, this function adjusts
> -   the condition for loop-until-wrap cases.  For example:
> -     (unsigned){8, -1}_loop < 10        => {0, 1} != 9
> -     10 < (unsigned){0, max - 7}_loop   => {0, 1} != 8
> -   Return true if condition is successfully adjusted.  */
> -
> -static bool
> -adjust_cond_for_loop_until_wrap (tree type, affine_iv *iv0, tree_code *code,
> -				 affine_iv *iv1)
> -{
> -  /* Only support simple cases for the moment.  */
> -  if (TREE_CODE (iv0->base) != INTEGER_CST
> -      || TREE_CODE (iv1->base) != INTEGER_CST)
> -    return false;
> -
> -  tree niter_type = unsigned_type_for (type), high, low;
> -  /* Case: i-- < 10.  */
> -  if (integer_zerop (iv1->step))
> -    {
> -      /* TODO: Should handle case in which abs(step) != 1.  */
> -      if (!integer_minus_onep (iv0->step))
> -	return false;
> -      /* Give up on infinite loop.  */
> -      if (*code == LE_EXPR
> -	  && tree_int_cst_equal (iv1->base, TYPE_MAX_VALUE (type)))
> -	return false;
> -      high = fold_build2 (PLUS_EXPR, niter_type,
> -			  fold_convert (niter_type, iv0->base),
> -			  build_int_cst (niter_type, 1));
> -      low = fold_convert (niter_type, TYPE_MIN_VALUE (type));
> -    }
> -  else if (integer_zerop (iv0->step))
> -    {
> -      /* TODO: Should handle case in which abs(step) != 1.  */
> -      if (!integer_onep (iv1->step))
> -	return false;
> -      /* Give up on infinite loop.  */
> -      if (*code == LE_EXPR
> -	  && tree_int_cst_equal (iv0->base, TYPE_MIN_VALUE (type)))
> -	return false;
> -      high = fold_convert (niter_type, TYPE_MAX_VALUE (type));
> -      low = fold_build2 (MINUS_EXPR, niter_type,
> -			 fold_convert (niter_type, iv1->base),
> -			 build_int_cst (niter_type, 1));
> -    }
> -  else
> -    gcc_unreachable ();
> -
> -  iv0->base = low;
> -  iv0->step = fold_convert (niter_type, integer_one_node);
> -  iv1->base = high;
> -  iv1->step = build_int_cst (niter_type, 0);
> -  *code = NE_EXPR;
> -  return true;
> -}
> -
>  /* Determine the number of iterations according to condition (for staying
>     inside loop) which compares two induction variables using comparison
>     operator CODE.  The induction variable on left side of the comparison
> @@ -1855,15 +1884,6 @@ number_of_iterations_cond (class loop *loop,
>        return true;
>      }
>  
> -  /* Handle special case loops: while (i-- < 10) and while (10 < i++) by
> -     adjusting iv0, iv1 and code.  */
> -  if (code != NE_EXPR
> -      && (tree_int_cst_sign_bit (iv0->step)
> -	  || (!integer_zerop (iv1->step)
> -	      && !tree_int_cst_sign_bit (iv1->step)))
> -      && !adjust_cond_for_loop_until_wrap (type, iv0, &code, iv1))
> -    return false;
> -
>    /* OK, now we know we have a senseful loop.  Handle several cases, depending
>       on what comparison operator is used.  */
>    bound_difference (loop, iv1->base, iv0->base, &bnds);
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)