[llvm] [RISCV][GISel] Add support for G_FCMP with F and D extensions. (PR #70624)

[Michael Maitland via llvm-commits]

================
@@ -674,6 +689,123 @@ bool RISCVInstructionSelector::selectSelect(MachineInstr &MI,
   return constrainSelectedInstRegOperands(*Result, TII, TRI, RBI);
 }
 
+// Convert an FCMP predicate to one of the supported F or D instructions.
+static unsigned getFCmpOpcode(CmpInst::Predicate Pred, unsigned Size) {
+  assert((Size == 32 || Size == 64) && "Unsupported size");
+  switch (Pred) {
+  default:
+    llvm_unreachable("Unsupported predicate");
+  case CmpInst::FCMP_OLT:
+    return Size == 32 ? RISCV::FLT_S : RISCV::FLT_D;
+  case CmpInst::FCMP_OLE:
+    return Size == 32 ? RISCV::FLE_S : RISCV::FLE_D;
+  case CmpInst::FCMP_OEQ:
+    return Size == 32 ? RISCV::FEQ_S : RISCV::FEQ_D;
+  }
+}
+
+// Try legalizing an FCMP by swapping or inverting the predicate to one that
+// is supported.
+static bool legalizeFCmpPredicate(Register &LHS, Register &RHS,
+                                  CmpInst::Predicate &Pred, bool &NeedInvert) {
+  auto isLegalFCmpPredicate = [](CmpInst::Predicate Pred) {
+    return Pred == CmpInst::FCMP_OLT || Pred == CmpInst::FCMP_OLE ||
+           Pred == CmpInst::FCMP_OEQ;
+  };
+
+  CmpInst::Predicate InvPred = CmpInst::getSwappedPredicate(Pred);
+  if (isLegalFCmpPredicate(InvPred)) {
+    Pred = InvPred;
+    std::swap(LHS, RHS);
+    return true;
+  }
+
+  InvPred = CmpInst::getInversePredicate(Pred);
+  NeedInvert = true;
+  if (isLegalFCmpPredicate(InvPred)) {
+    Pred = InvPred;
+    return true;
+  }
+  InvPred = CmpInst::getSwappedPredicate(InvPred);
+  if (isLegalFCmpPredicate(InvPred)) {
+    Pred = InvPred;
+    std::swap(LHS, RHS);
+    return true;
+  }
+
+  return false;
+}
+
+// Emit a sequence of instructions to compare LHS and RHS using Pred. Return
+// the result in DstReg.
+// FIXME: Maybe we should expand this earlier.
+bool RISCVInstructionSelector::emitFPCompare(CmpInst::Predicate Pred,
+                                             Register DstReg, Register LHS,
+                                             Register RHS,
+                                             MachineIRBuilder &MIB) const {
+  MachineRegisterInfo &MRI = *MIB.getMRI();
+
+  unsigned Size = MRI.getType(LHS).getSizeInBits();
+  assert((Size == 32 || Size == 64) && "Unexpected size");
+
+  Register TmpReg = DstReg;
+
+  bool NeedInvert = false;
+  // First try swapping operands or inverting.
+  if (legalizeFCmpPredicate(LHS, RHS, Pred, NeedInvert)) {
+    if (NeedInvert)
+      TmpReg = MRI.createVirtualRegister(&RISCV::GPRRegClass);
+    auto Cmp = MIB.buildInstr(getFCmpOpcode(Pred, Size), {TmpReg}, {LHS, RHS});
----------------
michaelmaitland wrote:

`MIB.buildFCmp(getFCmpOpcode(Pred, Size), TmpReg, LHS, RHS)` and below?

https://github.com/llvm/llvm-project/pull/70624