[llvm] r195496 - X86: Perform integer comparisons at i32 or larger.

Chandler Carruth chandlerc at google.com
Mon Nov 25 22:26:30 PST 2013

On Mon, Nov 25, 2013 at 10:22 PM, Sean Silva <silvas at purdue.edu> wrote:

> On Mon, Nov 25, 2013 at 4:48 PM, Jim Grosbach <grosbach at apple.com> wrote:
>> A few select examples I’m seeing are: 256.bzip2 improves by 7%. 401.bzip2
>> improves by 4.5%. 300.twolf improves by 3%. 186.crafty improves by 4%. The
>> details vary, but this is true for both Ivy Bridge and Haswell in
>> particular.
> Hmm... on second thought, do these programs use lots of i16's? Agner
> reports that on Ivy Bridge and Haswell there is no partial register access
> cost for the i8 low subregs. He doesn't seem to mention anything about
> 16-bit, so I assume that the partial register stall is still there for the
> i16 subregs??? I don't have an Ivy Bridge or Haswell to test on
> unfortunately :(

The primary performance problem Jim and I both looked at here were with i8

However, regardless of partial register stalls, the existence of false
dependencies seems just as problematic...
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