[llvm] [InstCombine] fold icmp of select with invertible shl (PR #147182)

[Nikita Popov via llvm-commits]

================
@@ -5948,6 +5954,29 @@ static Instruction *foldICmpEqualityWithOffset(ICmpInst &I,
   collectOffsetOp(Op1, OffsetOps, /*AllowRecursion=*/true);
 
   auto ApplyOffsetImpl = [&](Value *V, unsigned BinOpc, Value *RHS) -> Value * {
+    switch (BinOpc) {
+    // V = shl nsw X, RHS => X = ashr V, RHS
+    case Instruction::AShr: {
+      const APInt *CV, *CRHS;
+      if (!(match(V, m_APInt(CV)) && match(RHS, m_APInt(CRHS)) &&
+            CV->ashr(*CRHS).shl(*CRHS) == *CV) &&
+          !match(V, m_NSWShl(m_Value(), m_Specific(RHS))))
+        return nullptr;
+      break;
+    }
+    // V = shl nuw X, RHS => X = lshr V, RHS
+    case Instruction::LShr: {
+      const APInt *CV, *CRHS;
+      if (!(match(V, m_APInt(CV)) && match(RHS, m_APInt(CRHS)) &&
+            CV->lshr(*CRHS).shl(*CRHS) == *CV) &&
+          !match(V, m_NUWShl(m_Value(), m_Specific(RHS))))
+        return nullptr;
+      break;
+    }
+    default:
+      break;
+    }
----------------
nikic wrote:

Looks like there is no test coverage for these checks. Is that because any IR failing the precondition would already get folded in a different way?

https://github.com/llvm/llvm-project/pull/147182