[LLVMbugs] [Bug 22024] Unnecessary vxorps instructions

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22024

Quentin Colombet <qcolombet at apple.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |qcolombet at apple.com
         Resolution|---                         |WONTFIX

--- Comment #4 from Quentin Colombet <qcolombet at apple.com> ---
As far as I know, xorps A,A,A are good for performance because it tells that A
is zeroed. In this particular case, it even helps to break a scheduling
dependency. Indeed, the out of order engine would see two independent
computation:
     vcvtsi2ssq    %rcx, %xmm0, %xmm0
    vmovss    %xmm0, (%rsi)
and
    vcvtsi2ssq    %rdx, %xmm0, %xmm0
    vmovss    %xmm0, (%rdi)

Instead of one big computation:
    vcvtsi2ssq    %rcx, %xmm0, %xmm0
    vmovss    %xmm0, (%rsi)
    vcvtsi2ssq    %rdx, %xmm0, %xmm0    # <-- we do not know that the xmm0
given as input is independent of the previous convert.
    vmovss    %xmm0, (%rdi)

If you actually see a performance problem, feel free to reopen.

Thanks.

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