[llvm] [InstCombine] Fold ((cst << x) & 1) --> x == 0 when cst is odd (PR #79772)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: None (elhewaty)

<details>
<summary>Changes</summary>

- [InstCombine] Add test coverage ((cst << x) & 1) --> x == 0 when cst is odd (NFC)
- [InstCombine] Fold ((cst << x) & 1) to x == 0 when cst is odd
- Fixes: https://github.com/llvm/llvm-project/issues/73384
- Alive2: https://alive2.llvm.org/ce/z/Y8EhNa


---
Full diff: https://github.com/llvm/llvm-project/pull/79772.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp (+5-3) 
- (modified) llvm/test/Transforms/InstCombine/and.ll (+62) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 0620752e3213945..e95cf7eaa940623 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2220,10 +2220,13 @@ Instruction *InstCombinerImpl::visitAnd(BinaryOperator &I) {
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
 
   Value *X, *Y;
-  if (match(Op0, m_OneUse(m_LogicalShift(m_One(), m_Value(X)))) &&
+  const APInt *C;
+  if ((match(Op0, m_OneUse(m_LShr(m_One(), m_Value(X)))) ||
+       (match(Op0, m_OneUse(m_Shl(m_APInt(C), m_Value(X)))) &&
+        ((*C & 1) == 1))) &&
       match(Op1, m_One())) {
-    // (1 << X) & 1 --> zext(X == 0)
     // (1 >> X) & 1 --> zext(X == 0)
+    // (C << X) & 1 --> zext(X == 0)
     Value *IsZero = Builder.CreateICmpEQ(X, ConstantInt::get(Ty, 0));
     return new ZExtInst(IsZero, Ty);
   }
@@ -2246,7 +2249,6 @@ Instruction *InstCombinerImpl::visitAnd(BinaryOperator &I) {
       isKnownToBeAPowerOfTwo(Y, /*OrZero*/ true, /*Depth*/ 0, &I))
     return BinaryOperator::CreateAnd(Builder.CreateNot(X), Y);
 
-  const APInt *C;
   if (match(Op1, m_APInt(C))) {
     const APInt *XorC;
     if (match(Op0, m_OneUse(m_Xor(m_Value(X), m_APInt(XorC))))) {
diff --git a/llvm/test/Transforms/InstCombine/and.ll b/llvm/test/Transforms/InstCombine/and.ll
index 2e37fee07cc47bf..a6833fe9a1b9c6e 100644
--- a/llvm/test/Transforms/InstCombine/and.ll
+++ b/llvm/test/Transforms/InstCombine/and.ll
@@ -7,6 +7,68 @@ declare void @use32(i32)
 
 ; There should be no 'and' instructions left in any test.
 
+define i32 @test_with_1(i32 %x) {
+; CHECK-LABEL: @test_with_1(
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[X:%.*]], 0
+; CHECK-NEXT:    [[AND:%.*]] = zext i1 [[TMP1]] to i32
+; CHECK-NEXT:    ret i32 [[AND]]
+;
+  %shl = shl i32 1, %x
+  %and = and i32 %shl, 1
+  ret i32 %and
+}
+
+define i32 @test_with_3(i32 %x) {
+; CHECK-LABEL: @test_with_3(
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[X:%.*]], 0
+; CHECK-NEXT:    [[AND:%.*]] = zext i1 [[TMP1]] to i32
+; CHECK-NEXT:    ret i32 [[AND]]
+;
+  %shl = shl i32 3, %x
+  %and = and i32 %shl, 1
+  ret i32 %and
+}
+
+define i32 @test_with_5(i32 %x) {
+; CHECK-LABEL: @test_with_5(
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[X:%.*]], 0
+; CHECK-NEXT:    [[AND:%.*]] = zext i1 [[TMP1]] to i32
+; CHECK-NEXT:    ret i32 [[AND]]
+;
+  %shl = shl i32 5, %x
+  %and = and i32 %shl, 1
+  ret i32 %and
+}
+
+define i32 @test_with_neg_5(i32 %x) {
+; CHECK-LABEL: @test_with_neg_5(
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[X:%.*]], 0
+; CHECK-NEXT:    [[AND:%.*]] = zext i1 [[TMP1]] to i32
+; CHECK-NEXT:    ret i32 [[AND]]
+;
+  %shl = shl i32 -5, %x
+  %and = and i32 %shl, 1
+  ret i32 %and
+}
+
+define i32 @test_with_even(i32 %x) {
+; CHECK-LABEL: @test_with_even(
+; CHECK-NEXT:    ret i32 0
+;
+  %shl = shl i32 4, %x
+  %and = and i32 %shl, 1
+  ret i32 %and
+}
+
+define i32 @test_with_neg_even(i32 %x) {
+; CHECK-LABEL: @test_with_neg_even(
+; CHECK-NEXT:    ret i32 0
+;
+  %shl = shl i32 -4, %x
+  %and = and i32 %shl, 1
+  ret i32 %and
+}
+
 define i32 @test1(i32 %A) {
 ; CHECK-LABEL: @test1(
 ; CHECK-NEXT:    ret i32 0

``````````

</details>


https://github.com/llvm/llvm-project/pull/79772