llvm dev - Nov 2012 - [LLVMdev] load widening no more?

Hi,

Here is a simple test on which (I swear!) clang used to perform load
widening.
Not any more. What's happened?

% cat lw.cc
struct S {
  int a;
  unsigned char x, y, z, w;
};

int foo(S *s) {
  if (s->x == 0 && s->y == 0) return 0;
  return 1;
}

% clang -O2 -S -o -  lw.cc | grep '(%'
        cmpb    $0, 4(%rdi)
        cmpb    $0, 5(%rdi)
% gcc -O2 -S -o -  lw.cc | grep '(%'
        cmpw    $0, 4(%rdi)
%

--kcc
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On Thu, Nov 22, 2012 at 8:44 PM, Kostya Serebryany <kcc at google.com>
wrote:
> Hi,
>
> Here is a simple test on which (I swear!) clang used to perform load
> widening.
> Not any more. What's happened?
clang isn't setting the alignment on the load of "s->x"
aggressively
enough for the LLVM optimizers to kick in.

Granted, even if you do fix that, we still generate completely
ridiculous code, but that's a separate issue.

-Eli