[llvm-bugs] [Bug 43088] New: LLVM is unable to combine multiple equivalent UCOMISDrr instructions

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43088

            Bug ID: 43088
           Summary: LLVM is unable to combine multiple equivalent
                    UCOMISDrr instructions
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: simonas+llvm.org at kazlauskas.me
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

Given code like this (keep in mind that this applies to most code examples
where there are multiple `fcmp` instructions with the same arguments but
differing comparison code):

define i8 @test(double, double) unnamed_addr #0 {
start:
    %2 = fcmp ole double %0, %1
    %3 = fcmp oge double %0, %1
    %spec.select1.i = select i1 %3, i8 1, i8 2
    %not..i = xor i1 %3, true
    %spec.select.i = sext i1 %not..i to i8
    %_0.0.i = select i1 %2, i8 %spec.select.i, i8 %spec.select1.i
    ret i8 %_0.0.i
}

which returns -1, 0, 1, or 2 depending on whether the comparison between the
two arguments is less-than, equal, greater-than or either-is-nan. On x86 this
information is made available by a single execution of the `UCOMISD`
instruction.

However this IR ends up being built into something like:

bb.0.start:
  successors: %bb.1(0x40000000), %bb.2(0x40000000); %bb.1(50.00%),
%bb.2(50.00%)
  liveins: $xmm0, $xmm1
  %1:fr64 = COPY $xmm1
  %0:fr64 = COPY $xmm0
  UCOMISDrr %0:fr64, %1:fr64, implicit-def $eflags
  %2:gr8 = SETAEr implicit $eflags
  %3:gr8 = MOV8ri 2
  %4:gr8 = SUB8rr %3:gr8(tied-def 0), %2:gr8, implicit-def dead $eflags
  %5:gr8 = DEC8r %2:gr8(tied-def 0), implicit-def dead $eflags
  UCOMISDrr %1:fr64, %0:fr64, implicit-def $eflags
  JAE_1 %bb.2, implicit $eflags

bb.1.start:
; predecessors: %bb.0
  successors: %bb.2(0x80000000); %bb.2(100.00%)


bb.2.start:
; predecessors: %bb.0, %bb.1

  %6:gr8 = PHI %4:gr8, %bb.1, %5:gr8, %bb.0
  $al = COPY %6:gr8
  RET 0, $al

and the duplicate `UCOMISDrr` end up never being removed.

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