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

[Matt Arsenault via llvm-commits]

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@@ -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,
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arsenm wrote:

This kind of stuff should go in LegalizerHelper. We should be able to add the cond code to the legalizer queries for G_ICMP/G_FCMP and replicate the DAG's setCondCodeAction 

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