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

[Craig Topper via llvm-commits]

================
@@ -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) {
----------------
topperc wrote:

I'm referring to the instruction matching table in files like `RISCVGenGlobalISel.inc` which are created from the same patterns used for SelectionDAG.

For example here is one of the matches for an instruction with an immediate. It does not appear to do any look through.

```
        GIM_CheckOpcode, /*MI*/1, TargetOpcode::G_CONSTANT,                      
        GIM_CheckI64ImmPredicate, /*MI*/1, /*Predicate*/GICXXPred_I64_Predicate_simm12,
```

The code in this patch probably could have been written with a lot of tblgen patterns instead of custom C++ code. I don't see why we should use look through here if we wouldn't have gotten look through using a tblgen pattern.

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