[llvm] [InstCombine] Missed optimization: Fold (sext(a) & c1) == c2 to (a & c3) == trunc(c2) (PR #112646)

[Nikita Popov via llvm-commits]

================
@@ -7716,6 +7716,32 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
   if (Instruction *Res = foldReductionIdiom(I, Builder, DL))
     return Res;
 
+  {
+    Value *A;
+    const APInt *C1, *C2;
+    ICmpInst::Predicate Pred = I.getPredicate();
+    if (ICmpInst::isEquality(Pred)) {
+      // sext(a) & c1 == c2 --> a & c3 == trunc(c2)
+      // sext(a) & c1 != c2 --> a & c3 != trunc(c2)
+      if (match(Op0, m_And(m_SExtLike(m_Value(A)), m_APInt(C1))) &&
+          match(Op1, m_APInt(C2))) {
+        Type *InputTy = A->getType();
+        unsigned InputBitWidth = InputTy->getScalarSizeInBits();
+        // c2 must be non-negative at the bitwidth of a.
+        if (C2->ult(APInt::getOneBitSet(C2->getBitWidth(), InputBitWidth))) {
----------------
nikic wrote:

I think this can be more simply written like this?
```suggestion
        if (C2->getActiveBits() < InputBitWidth) {
```

https://github.com/llvm/llvm-project/pull/112646