[llvm-bugs] [Bug 40359] New: [X86][SSE] Memory fold scalar unary ops with zero register passthrough

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Jan 17 09:09:28 PST 2019


            Bug ID: 40359
           Summary: [X86][SSE] Memory fold scalar unary ops with zero
                    register passthrough
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: andrea.dibiagio at gmail.com, craig.topper at gmail.com,
                    llvm-bugs at lists.llvm.org, llvm-dev at redking.me.uk,
                    spatel+llvm at rotateright.com


We currently do this:

  vmovss (%rdi), %xmm0 # xmm0 = mem[0],zero,zero,zero
  vmovss (%rsi), %xmm1 # xmm1 = mem[0],zero,zero,zero
  vsqrtss %xmm0, %xmm0, %xmm0
  vsqrtss %xmm1, %xmm1, %xmm1
  vaddss %xmm1, %xmm0, %xmm0

but we can reduce register pressure when we have multiple uses of the zero by
doing this instead, and even when we don't reuse the register there's no

  vxorps %xmm1, %xmm1, %xmm1
  vsqrtss (%rdi), %xmm1, %xmm0
  vsqrtss (%rsi), %xmm1, %xmm1
  vaddss %xmm1, %xmm0, %xmm0

This is really about AVX encoded instructions but I can't see any reason not to
do this on older SSE instructions as well.

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