Re: [PATCH][GCC] Simplification of 1U << (31 - x)

[Jakub Jelinek]

On Thu, Apr 13, 2017 at 01:49:01PM +0200, Richard Biener wrote:
> It is IMHO a valid GIMPLE optimization / canonicalization.

As I said, we can do it as GIMPLE canonicalization, but
we should have code to undo it if beneficial at RTL level.
And the patch has not included that.
> 
>         movabsq $-9223372036854775808, %rax
> 
> so this should then have been generated as 1<<63?

Maybe.  But it seems to be still more expensive than the original code,
though better than movabsq:

__attribute__((noinline, noclone)) unsigned long long int
foo (int x)
{
  asm volatile ("" : : : "memory");
  return 1ULL << (63 - x);
}

__attribute__((noinline, noclone)) unsigned long long int
bar (int x)
{
  asm volatile ("" : : : "memory");
  return (1ULL << 63) >> x;
}

__attribute__((noinline, noclone)) unsigned long long int
baz (int x)
{
  unsigned long long int y = 1;
  asm volatile ("" : "+r" (y) : : "memory");
  return (y << 63) >> x;
}

int
main (int argc, const char **argv)
{
  int i;
  if (argc == 1)
    for (i = 0; i < 1000000000; i++)
      asm volatile ("" : : "r" (foo (13)));
  else if (argc == 2)
    for (i = 0; i < 1000000000; i++)
      asm volatile ("" : : "r" (bar (13)));
  else if (argc == 3)
    for (i = 0; i < 1000000000; i++)
      asm volatile ("" : : "r" (baz (13)));
  return 0;
}

        Jakub