[llvm] [InstCombine] Fold (A & B) - (A & ~B) => B - (A ^ B) (PR #79717)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: AtariDreams (AtariDreams)

<details>
<summary>Changes</summary>



---
Full diff: https://github.com/llvm/llvm-project/pull/79717.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp (+18) 
- (modified) llvm/test/Transforms/InstCombine/sub.ll (+26) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 8a00b75a1f7404..aceb178263811c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -2371,6 +2371,24 @@ Instruction *InstCombinerImpl::visitSub(BinaryOperator &I) {
       return BinaryOperator::CreateNeg(Y);
   }
 
+  {
+    Value *A, *B;
+    // (A & ~B) - (A & B) => (A ^ B) - B
+    if (match(Op0, m_c_And(m_Value(A), m_Not(m_Value(B)))) &&
+        match(Op1, m_c_And(m_Specific(A), m_Specific(B)))) {
+      return BinaryOperator::CreateSub(Builder.CreateXor(A, B), B);
+    }
+  }
+
+  {
+    Value *A, *B;
+    // (A & B) - (A & ~B) => B - (A ^ B)
+    if (match(Op1, m_c_And(m_Value(A), m_Not(m_Value(B)))) &&
+        match(Op0, m_c_And(m_Specific(A), m_Specific(B)))) {
+      return BinaryOperator::CreateSub(Builder.CreateXor(A, B), B);
+    }
+  }
+
   // (sub (or A, B) (and A, B)) --> (xor A, B)
   {
     Value *A, *B;
diff --git a/llvm/test/Transforms/InstCombine/sub.ll b/llvm/test/Transforms/InstCombine/sub.ll
index 494cdf62c7975e..65142d4e200633 100644
--- a/llvm/test/Transforms/InstCombine/sub.ll
+++ b/llvm/test/Transforms/InstCombine/sub.ll
@@ -2626,3 +2626,29 @@ define i8 @sub_of_adds_2xc(i8 %x, i8 %y) {
   %r = sub i8 %xc, %yc
   ret i8 %r
 }
+
+define i32 @sub_and_xor(i32 %A, i32 %B) {
+; CHECK-LABEL: @sub_and_xor(
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i32 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[RES:%.*]] = sub i32 [[TMP1]], [[B]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+  %xor = xor i32 %B, -1
+  %and1 = and i32 %xor, %A
+  %and2 = and i32 %B, %A
+  %res = sub nsw i32 %and1, %and2
+  ret i32 %res
+}
+
+define i32 @sub_xor_and(i32 %A, i32 %B) {
+; CHECK-LABEL: @sub_xor_and(
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i32 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[RES:%.*]] = sub i32 [[TMP1]], [[B]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+  %and1 = and i32 %B, %A
+  %xor = xor i32 %B, -1
+  %and2 = and i32 %xor, %A
+  %res = sub nsw i32 %and1, %and2
+  ret i32 %res
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/79717