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

[via llvm-commits]

================
@@ -7687,6 +7687,30 @@ Instruction *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
   if (Instruction *Res = foldReductionIdiom(I, Builder, DL))
     return Res;
 
+  {
+    Value *A;
+    const APInt *C1, *C2;
+    ICmpInst::Predicate PredEq = ICmpInst::ICMP_EQ;
+    if (I.getPredicate() == PredEq) {
+      // c2 must be positive
+      // 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))) {
+        if (C2->isNonNegative()) {
+          Type *InputTy = A->getType();
+          unsigned TargetBitSize = InputTy->getScalarSizeInBits();
+          // Count the number of 1s in C1 high bits of size TargetBitSize.
+          unsigned Leading1sCount = C1->lshr(C1->getBitWidth() - TargetBitSize).popcount();
+          APInt TruncC1 = C1->trunc(TargetBitSize);
+          if (Leading1sCount > 0) {
+            TruncC1.setBit(TargetBitSize - 1);
+          }
----------------
XChy wrote:

See https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements

```suggestion
          if (Leading1sCount > 0)
            TruncC1.setBit(TargetBitSize - 1);
```

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