[InstCombine] fold (icmp eq/ne (and (shl -1, X), Y), 0) -> (icmp eq/ne (lshr Y, X), 0)

Proofs: https://alive2.llvm.org/ce/z/oSRGBt

Closes #84691

Author: goldsteinn

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -1962,6 +1962,17 @@ Instruction *InstCombinerImpl::foldICmpAndConstant(ICmpInst &Cmp,
     return BinaryOperator::CreateAnd(TruncY, X);
   }
 
+  // (icmp eq/ne (and (shl -1, X), Y), 0)
+  //    -> (icmp eq/ne (lshr Y, X), 0)
+  // We could technically handle any C == 0 or (C < 0 && isOdd(C)) but it seems
+  // highly unlikely the non-zero case will ever show up in code.
+  if (C.isZero() &&
+      match(And, m_OneUse(m_c_And(m_OneUse(m_Shl(m_AllOnes(), m_Value(X))),
+                                  m_Value(Y))))) {
+    Value *LShr = Builder.CreateLShr(Y, X);
+    return new ICmpInst(Pred, LShr, Constant::getNullValue(LShr->getType()));
+  }
+
   return nullptr;
 }
 

llvm/test/Transforms/InstCombine/icmp-and-shift.ll

@@ -523,8 +523,7 @@ define i1 @slt_and_shl_one(i8 %x, i8 %y) {
 
 define i1 @fold_eq_lhs(i8 %x, i8 %y) {
 ; CHECK-LABEL: @fold_eq_lhs(
-; CHECK-NEXT:    [[SHL:%.*]] = shl nsw i8 -1, [[X:%.*]]
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[SHL]], [[Y:%.*]]
+; CHECK-NEXT:    [[AND:%.*]] = lshr i8 [[Y:%.*]], [[X:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[AND]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
@@ -565,8 +564,7 @@ define i1 @fold_eq_lhs_fail_multiuse_shl(i8 %x, i8 %y) {
 define i1 @fold_ne_rhs(i8 %x, i8 %yy) {
 ; CHECK-LABEL: @fold_ne_rhs(
 ; CHECK-NEXT:    [[Y:%.*]] = xor i8 [[YY:%.*]], 123
-; CHECK-NEXT:    [[SHL:%.*]] = shl nsw i8 -1, [[X:%.*]]
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[Y]], [[SHL]]
+; CHECK-NEXT:    [[AND:%.*]] = lshr i8 [[Y]], [[X:%.*]]
 ; CHECK-NEXT:    [[R:%.*]] = icmp ne i8 [[AND]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;