[llvm] [ConstraintElimination] Add support for UCMP/SCMP intrinsics (PR #97974)

[Antonio Frighetto via llvm-commits]

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@@ -1434,6 +1436,33 @@ static bool checkAndReplaceMinMax(MinMaxIntrinsic *MinMax, ConstraintInfo &Info,
   return false;
 }
 
+static bool checkAndReplaceCmp(IntrinsicInst *II, ConstraintInfo &Info,
+                               SmallVectorImpl<Instruction *> &ToRemove) {
+  bool IsSigned = II->getIntrinsicID() == Intrinsic::scmp;
+  Value *LHS = II->getOperand(0);
+  Value *RHS = II->getOperand(1);
+  if (checkCondition(IsSigned ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT, LHS,
+                     RHS, II, Info)
+          .value_or(false)) {
+    II->replaceAllUsesWith(ConstantInt::get(II->getType(), 1));
+    ToRemove.push_back(II);
+    return true;
+  }
+  if (checkCondition(IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT, LHS,
+                     RHS, II, Info)
+          .value_or(false)) {
+    II->replaceAllUsesWith(ConstantInt::getSigned(II->getType(), -1));
+    ToRemove.push_back(II);
+    return true;
+  }
+  if (checkCondition(ICmpInst::ICMP_EQ, LHS, RHS, II, Info).value_or(false)) {
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antoniofrighetto wrote:

I think that, for the first example, it could suffice to check whether the equality holds and change the predicate to unsigned. Compile-time does not look good though: https://llvm-compile-time-tracker.com/compare.php?from=7911fb1a257b3a7014b44b4e7d04ee5c3b73a3e3&to=612eaf91c85d5e4c6e3fdddd46ebe6a41d19dfcb&stat=instructions:u. Perhaps there's a better way to achieve this?

https://github.com/llvm/llvm-project/pull/97974