silly assembly for comparing Doubles

A function like f x = if x > 0.0 then ... else ... compiles to the assembly

        movsd 7(%rbx),%xmm0
        xorpd %xmm1,%xmm1
        ucomisd %xmm1,%xmm0
        seta %al
        movzbl %al,%eax
        cmpq $1,%rax
        jb _c1tb

This seta/movzbl/cmpq/jb is bad, we can just generate ja (and 7.10 did).

The cause is that the code generator produces Cmm like

           switch [0 .. 1] %MO_F_Gt_W64(_s1sv::F64, 0.0 :: W64)::I64 {
               case 0 : goto c1tb;
               case 1 : goto c1tc;
           }

which turns into

      if (%MO_F_Gt_W64(_s1sv::F64,
                       0.0 :: W64) < 1) goto c1tb; else goto c1tc;

and then GHC is stuck. It knows how to turn condition >= 1 into condition, and it knows how to turn condition < 1 into a negated version of condition when possible, but there is no negated version of MO_F_Gt_W64 (it's not MO_F_Le_W64 because of NaNs and signed zeros). It doesn't know how to turn a negated conditional into a conditional with the branches swapped because it doesn't look at that much at once.

Presumably more fallout from #10137 (closed), and maybe can be fixed simultaneously with #10677 (closed).

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (CodeGen)
Test case
Differential revisions
BlockedBy
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Blocking
CC
Operating system
Architecture