[llvm-bugs] [Bug 44565] New: [X86] Failure to use MOVMSK reduction result for branches

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44565

            Bug ID: 44565
           Summary: [X86] Failure to use MOVMSK reduction result for
                    branches
           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: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

https://godbolt.org/z/GLDirF

If we are selecting between 2 values using a comparison reduction then we use
the all_of/any_of MOVMSK result directly:

define i32 @select(<2 x double> %a0)  {
  %1 = fcmp ogt <2 x double> %a0, zeroinitializer
  %2 = extractelement <2 x i1> %1, i32 0
  %3 = extractelement <2 x i1> %1, i32 1
  %4 = and i1 %2, %3
  %5 = select i1 %4, i32 42, i32 88
  ret i32 %5
}

select: # @select
  vxorpd %xmm1, %xmm1, %xmm1
  vcmpltpd %xmm0, %xmm1, %xmm0
  vmovmskpd %xmm0, %eax
  cmpb $3, %al
  movl $42, %ecx
  movl $88, %eax
  cmovel %ecx, %eax
  retq

But if we decided to create a branch instead then the MOVMSK bits are used
separately, meaning we end up with 2 compares+jmps instead:

define i32 @jmp(<2 x double> %a0)  {
  %1 = fcmp ogt <2 x double> %a0, zeroinitializer
  %2 = extractelement <2 x i1> %1, i32 0
  %3 = extractelement <2 x i1> %1, i32 1
  %4 = and i1 %2, %3
  br i1 %4, label %foo, label %bar
foo:
  ret i32 42
bar:
  ret i32 88
}

jmp: # @jmp
  vxorpd %xmm1, %xmm1, %xmm1
  vcmpltpd %xmm0, %xmm1, %xmm0
  vmovmskpd %xmm0, %eax
  testb $1, %al
  je .LBB1_3
  shrb %al
  je .LBB1_3
  movl $42, %eax
  retq
.LBB1_3: # %bar
  movl $88, %eax
  retq

when I think it could be:

jmp: # @jmp
  vxorpd %xmm1, %xmm1, %xmm1
  vcmpltpd %xmm0, %xmm1, %xmm0
  vmovmskpd %xmm0, %eax
  cmp $3, %al
  jne .LBB1_3
  movl $42, %eax
  retq
.LBB1_3: # %bar
  movl $88, %eax
  retq

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