Re: [PATCH 6/7] stdlib: Optimization qsort{_r} swap implementation

[Paul Eggert <eggert@cs.ucla.edu>]

Adhemerval Zanella wrote:
> +static inline bool
> +check_alignment (const void *base, size_t align)
> +{
> +  return _STRING_ARCH_unaligned || ((uintptr_t)base % (align - 1)) == 0;
> +}

Surely the '(align - 1)' was supposed to be 'align'. Has this been tested on an 
architecture that does not allow unaligned access?

> +static inline void
> +swap_generic (void *a, void *b, size_t size)

Why is this inline? It's used only as a function pointer, and the other 
functions so used are not declared inline.

> +static inline swap_t
> +select_swap_func (const void *base, size_t size)
> +{
> +  if (size == 4 && check_alignment (base, 4))
> +    return swap_u32;
> +  else if (size == 8 && check_alignment (base, 8))
> +    return swap_u64;
> +  return swap_generic;
> +}

The conditions aren't portable enough. Use something like this instead for 
swap_u32, and similarly for swap_u64.

   if (size == sizeof (uint32_t) && check_alignment (base, alignof (uint32_t)))
     return swap_u32;

> +static void
> +swap_u32 (void *a, void *b, size_t size)

The pointer arguments should be declared 'void *restrict'. This can help GCC 
generate better code. Similarly for the other swap functions.

> +  uint32_t tmp = *(uint32_t*) a;
> +  *(uint32_t*) a = *(uint32_t*) b;
> +  *(uint32_t*) b = tmp;

It's nicer to avoid casts when possible, as is the case here and elsewhere. This 
is because casts are too powerful in C. Something like this, say:

     uint32_t *ua = a, *ub = b, tmp = *ua;
     *ua = *ub, *ub = tmp;

> +  unsigned char tmp[128];

Why 128? A comment seems called for.

> +static inline void
> +swap_generic (void *a, void *b, size_t size)
> +{
> +  unsigned char tmp[128];
> +  do
> +    {
> +      size_t s = size > sizeof (tmp) ? sizeof (tmp) : size;
> +      memcpy (tmp, a, s);
> +      a = __mempcpy (a, b, s);
> +      b = __mempcpy (b, tmp, s);
> +      size -= s;
> +    }
> +  while (size > 0);
> +}

On my platform (GCC 7.2.1 20170915 (Red Hat 7.2.1-2) x86-64) this inlined the 
memcpy but not the mempcpy calls. How about something like this instead? It 
should let the compiler do a better job of block-move-style operations in the 
loop. If mempcpy is inlined for you, feel free to substitute it for two of the 
loop's calls to memcpy.

   static void
   swap_generic (void *restrict va, void *restrict vb, size_t size)
   {
     char *a = va, *b = vb;
     enum { n = 128 }; /* Why 128?  */
     unsigned char tmp[n];
     while (size >= n)
       {
	memcpy (tmp, a, n);
	memcpy (a, b, n);
	memcpy (b, tmp, n);
	a += n;
	b += n;
	size -= n;
       }
     memcpy (tmp, a, size);
     memcpy (a, b, size);
     memcpy (b, tmp, size);
   }