Transform (x >> cst) != 0 to x >= (1 << cst) and (x >> cst) == 0 to x < (1 << cst)

[Jakub Jelinek]

On Tue, Oct 03, 2017 at 12:54:39PM -0700, Prathamesh Kulkarni wrote:
> Hi,
> This follow-up patch implements the patterns mentioned in $subject.
> Bootstrap+test in progress on x86_64-unknown-linux-gnu and aarch64-linux-gnu.
> OK to commit if passes ?
> 
> Thanks,
> Prathamesh

> 2017-10-03  Prathamesh Kulkarni  <prathamesh.kulkarni@linaro.org>
> 
> 	* match.pd ((X >> CST) == 0 -> X < (1 << CST)): New pattern.
> 	((X >> CST) != 0 -> X >= (1 << CST)): Likewise.
> 
> testsuite/
> 	* gcc.dg/tree-ssa/cmpdiv.c: Add test-cases f3 and f4.

Why this way and not the other way around?
E.g. i?86/x86_64 and various other targets have shift instructions which
set condition codes, so (X >> 51) == 0 is certainly smaller
smaller and I believe cheaper than the latter.
Try:
void foo (void);

void
bar (unsigned long long x)
{
  if ((x >> 51) == 0)
    foo ();
}

void
baz (unsigned long long x)
{
  if (x < (1LL << 51))
    foo ();
}
with -Os on x86_64, the first function is 4 insns, 12 bytes,
the second one 5 insns, 21 bytes.

I wonder if this kind of instruction selection stuff shouldn't be
done in target.pd instead, with input from the target.

	Jakub