[llvm] [RISCV][GISel] Select G_SELECT (G_ICMP, A, B) (PR #68247)

[Michael Maitland via llvm-commits]

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@@ -376,6 +391,120 @@ bool RISCVInstructionSelector::selectSExtInreg(MachineInstr &MI,
   return true;
 }
 
+/// Returns the RISCVCC::CondCode that corresponds to the CmpInst::Predicate CC.
+/// CC Must be an ICMP Predicate.
+static RISCVCC::CondCode getRISCVCCFromICMP(CmpInst::Predicate CC) {
+  switch (CC) {
+  default:
+    llvm_unreachable("Expected ICMP CmpInst::Predicate.");
+  case CmpInst::Predicate::ICMP_EQ:
+    return RISCVCC::COND_EQ;
+  case CmpInst::Predicate::ICMP_NE:
+    return RISCVCC::COND_NE;
+  case CmpInst::Predicate::ICMP_ULT:
+    return RISCVCC::COND_LTU;
+  case CmpInst::Predicate::ICMP_SLT:
+    return RISCVCC::COND_LT;
+  case CmpInst::Predicate::ICMP_UGE:
+    return RISCVCC::COND_GEU;
+  case CmpInst::Predicate::ICMP_SGE:
+    return RISCVCC::COND_GE;
+  }
+}
+
+void RISCVInstructionSelector::getICMPOperandsForBranch(
+    MachineInstr &MI, MachineIRBuilder &MIB, MachineRegisterInfo &MRI,
+    RISCVCC::CondCode &CC, Register &LHS, Register &RHS) const {
+  assert(MI.getOpcode() == TargetOpcode::G_ICMP);
+  CmpInst::Predicate ICMPCC =
+      static_cast<CmpInst::Predicate>(MI.getOperand(1).getPredicate());
+  LHS = MI.getOperand(2).getReg();
+  RHS = MI.getOperand(3).getReg();
+
+  // Adjust comparisons to use comparison with 0 if possible.
+  MachineInstr *MaybeConstant = MRI.getVRegDef(RHS);
+  if (MaybeConstant && MaybeConstant->getOpcode() == TargetOpcode::G_CONSTANT) {
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michaelmaitland wrote:

Did we come to agreement on whether to use look through or not?

https://github.com/llvm/llvm-project/pull/68247