[llvm-bugs] [Bug 35926] New: [ARM] TableGen doesn't select VFNMSD

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https://bugs.llvm.org/show_bug.cgi?id=35926

            Bug ID: 35926
           Summary: [ARM] TableGen doesn't select VFNMSD
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: GlobalISel
          Assignee: unassignedbugs at nondot.org
          Reporter: diana.picus at linaro.org
                CC: llvm-bugs at lists.llvm.org

Created attachment 19666
  --> https://bugs.llvm.org/attachment.cgi?id=19666&action=edit
Reproducer

When running
llc -mtriple arm-gnueabihf -mattr=+vfp4 vfnmsd.ll
-stop-before=expand-isel-pseudos
we get a VFNMSD instruction. If we add -global-isel, we only get a VNEGD
followed by a VFNMAD.

It seems this only happens because the code for matching the VFNMSD comes after
the one for matching VFNMAD in the match table (if we manually tinker with the
order in ARMGenGlobalISel.inc, then we can successfully select VFNMSD with
-global-isel).

The relevant patterns are in ARMInstrVFP.td:

// (fneg (fma x, (fneg y), z) -> (vfnms z, x, y)
def : Pat<(fneg (f64 (fma DPR:$Dn, (fneg DPR:$Dm), DPR:$Ddin))),
          (VFNMSD DPR:$Ddin, DPR:$Dn, DPR:$Dm)>,
      Requires<[HasVFP4,HasDPVFP]>;

// (fneg (fma x, y, z)) -> (vfnma z, x, y)
def : Pat<(fneg (fma (f64 DPR:$Dn), (f64 DPR:$Dm), (f64 DPR:$Ddin))),
          (VFNMAD DPR:$Ddin, DPR:$Dn, DPR:$Dm)>,
      Requires<[HasVFP4,HasDPVFP]>;

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