[llvm-bugs] [Bug 32177] New: Bug in APFloat::mod

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=32177

            Bug ID: 32177
           Summary: Bug in APFloat::mod
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Core LLVM classes
          Assignee: unassignedbugs at nondot.org
          Reporter: sanjoy at playingwithpointers.com
                CC: llvm-bugs at lists.llvm.org

Running @test_a through -instcombine folds %val to 0.0, while at runtime
executing @test_b returns a non-zero value 185438208.000000.  I suspect this is
a bug in APFloat::mod (though I do not speak floating point well enough to be
sure):

define float @test_a() {
entry:
  %val = frem float 0x43cbfcd960000000, 0xc1e2b34a00000000
  ret float %val
}

define float @helper(float %x, float %y) {
  %val = frem float %x, %y
  ret float %val
}

define external float @test_b() {
  %val = call float @helper(float 0x43cbfcd960000000, float 0xc1e2b34a00000000)
  ret float %val
}


Notably, IEEEFloat::mod says: "This is not currently correct in all cases".

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