[llvm-bugs] [Bug 25959] New: [x86, SSE] suboptimal register allocation for scalar minnum/maxnum

via llvm-bugs llvm-bugs at lists.llvm.org
Mon Dec 28 13:35:03 PST 2015


            Bug ID: 25959
           Summary: [x86, SSE] suboptimal register allocation for scalar
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: spatel+llvm at rotateright.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Follow-on from bug 24475: the scalar codegen could be almost identical to the
vector codegen (just change 'ps' ops to 'ss' ops), but different register
allocation causes us to need more register copies.

$ cat fmax.ll
declare <4 x float> @llvm.maxnum.v4f32(<4 x float>, <4 x float>)
declare float @llvm.maxnum.f32(float, float)

define float @maxnum_f32(float %x, float %y) {
  %call = tail call float @llvm.maxnum.f32(float %x, float %y)
  ret float %call

define <4 x float> @maxnum_v4f32(<4 x float> %x, <4 x float> %y) {
  %call = tail call <4 x float> @llvm.maxnum.v4f32(<4 x float> %x, <4 x float>
  ret <4 x float> %call

$ ./llc fmax.ll -o -
    movaps    %xmm0, %xmm2    <--- could have copied %xmm1 instead
    cmpunordss    %xmm2, %xmm2
    movaps    %xmm2, %xmm3    <--- extra move
    andps    %xmm1, %xmm3
    maxss    %xmm0, %xmm1
    andnps    %xmm1, %xmm2
    orps    %xmm3, %xmm2
    movaps    %xmm2, %xmm0    <--- extra move

    movaps    %xmm1, %xmm2
    maxps    %xmm0, %xmm2
    cmpunordps    %xmm0, %xmm0
    andps    %xmm0, %xmm1
    andnps    %xmm2, %xmm0
    orps    %xmm1, %xmm0

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