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

[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)) {
----------------
Poseydon42 wrote:

It's true in principle, but implementing it this way wouldn't fold specifically `scmp` in situations where equality is implied.
E.g. this test case doesn't fold:
```
define i8 @test(i32 noundef %x, i32 noundef %y) {
  %cond = icmp eq i32 %x, %y
  call void @llvm.assume(i1 %cond)
  %r = call i8 @llvm.scmp.i8.i32(i32 %x, i32 %y)
  ret i8 %r
}
```
I'm not sure why this is the case, but it looks like when constraint elimination state is fed an assumption that `x == y` it adds the two implied inequalities to its unsigned table, but not to the signed one. That's also why the same test above will fold nicely if you replace `scmp` with `ucmp`. Unless I'm missing something and this implication isn't necessarily correct I'm happy to change my code as per your suggestion and fix the contraint elimination pass in as a whole in a separate PR.

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