[llvm-bugs] [Bug 31143] New: False dependence breaking fails for ROUNDSSr

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31143

            Bug ID: 31143
           Summary: False dependence breaking fails for ROUNDSSr
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: mkuper at google.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

target triple = "x86_64-pc-linux-gnu"

define void @test(float* nocapture %a, <4 x float>* nocapture %b, i32 %k) {
entry:
  br label %for.body

for.body:
  %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
  %arrayidx = getelementptr inbounds float, float* %a, i64 %indvars.iv
  %v = load float, float* %arrayidx, align 4
  %floor = call float @floorf(float %v)
  %sub = fsub float %floor, %v
  %v1 = insertelement <4 x float> undef, float %sub, i32 0
  %br = shufflevector <4 x float> %v1, <4 x float> undef, <4 x i32> <i32 0, i32
0, i32 0, i32 0>
  store volatile <4 x float> %br, <4 x float>* %b, align 4
  %indvars.iv.next = add i64 %indvars.iv, 1
  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
  %exitcond = icmp eq i32 %lftr.wideiv, %k
  br i1 %exitcond, label %for.end, label %for.body

for.end:
  ret void
}

declare float @floorf(float) nounwind readnone

When compiled with -msse4.1 (to emit a roundss for the floorf) we get:

.LBB0_1:                                # %for.body
                                        # =>This Inner Loop Header: Depth=1
    movss    (%rdi), %xmm0           # xmm0 = mem[0],zero,zero,zero
    roundss    $9, %xmm0, %xmm1
    subss    %xmm0, %xmm1
    shufps    $0, %xmm1, %xmm1        # xmm1 = xmm1[0,0,0,0]
    movups    %xmm1, (%rsi)
    addq    $4, %rdi
    decl    %edx
    jne    .LBB0_1
# BB#2:                                 # %for.end
    retq

The false dependence on %xmm1 is not broken.

As expected, adding the requires xorps makes this loop about 3 times faster.

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