[llvm] [InstCombine] Fold (1 << a) to (a + 1) and (2 >> a) to (2 - a) when a is in [0,1] (PR #207108)

[Aayush Shrivastava via llvm-commits]

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@@ -1406,6 +1418,19 @@ Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
   if (match(Op0, m_One()))
     return new ZExtInst(Builder.CreateIsNull(Op1), Ty);
 
+  // 2 >> X --> 2 - X if 0 <= X <= 1.
+  if (match(Op0, m_SpecificInt(2)) &&
+      computeKnownBits(Op1, &I).countMaxActiveBits() <= 1) {
+    auto *Sub = BinaryOperator::CreateSub(ConstantInt::get(Ty, 2), Op1);
+    Sub->setHasNoUnsignedWrap();
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iamaayushrivastava wrote:

Done. Thanks!

https://github.com/llvm/llvm-project/pull/207108