[PATCH] D28744: [X86][AVX] Remove "OptForSize" condition from some memory foldings.

[Craig Topper via Phabricator via llvm-commits]

craig.topper added inline comments.


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Comment at: test/CodeGen/X86/avx-arith.ll:353
 ; CHECK:       ## BB#0:
-; CHECK-NEXT:    vmovss {{.*#+}} xmm0 = mem[0],zero,zero,zero
-; CHECK-NEXT:    vsqrtss %xmm0, %xmm0, %xmm0
+; CHECK-NEXT:    vsqrtss (%rax), %xmm0, %xmm0
 ; CHECK-NEXT:    retq
----------------
aymanmus wrote:
> craig.topper wrote:
> > This now has a register read dependency on xmm0 which I believe is what the OptForSize was originally protecting against.  I know work has gone into ExeDepFix for UndefRegClearance. Do we believe that is sufficient to allow this folding now?
> The OptForSize was incorrectly copied from the SSE version multiclass, where the memory form instructions performed a partial register updates (optimization guide states that partial updates come with a penalty and thus should be avoided).
> I'm afraid I didn't understand how the memory folding adds read dependency on xmm0 in this case, the read dependency was already there.
Yes there was a read of xmm0 on the dart but it was from the movss instruction which wrote all bits. After this patch the xmm0 read is dependent on an unknown instruction.


https://reviews.llvm.org/D28744