InstCombine: Combine (xor (or %a, %b) (xor %a, %b)) to (add %a, %b)

Correctness proof of the transform using CVC3-

$ cat t.cvc
A, B : BITVECTOR(32);
QUERY BVXOR(A | B, BVXOR(A,B) ) = A & B;

$ cvc3 t.cvc
Valid.

llvm-svn: 215524

Author: kvbhat

llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp

@@ -2508,6 +2508,18 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
       if ((A == C && B == D) || (A == D && B == C))
         return BinaryOperator::CreateXor(A, B);
     }
+    // (A ^ B)^(A | B) -> A & B
+    if (match(Op0I, m_Xor(m_Value(A), m_Value(B))) &&
+        match(Op1I, m_Or(m_Value(C), m_Value(D)))) {
+      if ((A == C && B == D) || (A == D && B == C))
+        return BinaryOperator::CreateAnd(A, B);
+    }
+    // (A | B)^(A ^ B) -> A & B
+    if (match(Op0I, m_Or(m_Value(A), m_Value(B))) &&
+        match(Op1I, m_Xor(m_Value(C), m_Value(D)))) {
+      if ((A == C && B == D) || (A == D && B == C))
+        return BinaryOperator::CreateAnd(A, B);
+    }
     // (A & B) ^ (A ^ B) -> (A | B)
     if (match(Op0I, m_And(m_Value(A), m_Value(B))) &&
         match(Op1I, m_Xor(m_Specific(A), m_Specific(B))))

llvm/test/Transforms/InstCombine/or-xor.ll

@@ -136,3 +136,14 @@ define i32 @test13(i32 %x, i32 %y) {
 ; CHECK-NEXT: %and = and i32 %x, %y
 ; CHECK-NEXT: ret i32 %and
 }
+
+; ((x | y) ^ (x ^ y)) -> (x & y)
+define i32 @test15(i32 %x, i32 %y) {
+  %1 = xor i32 %y, %x
+  %2 = or i32 %y, %x
+  %3 = xor i32 %2, %1
+  ret i32 %3
+; CHECK-LABEL: @test15(
+; CHECK-NEXT: %1 = and i32 %y, %x
+; CHECK-NEXT: ret i32 %1
+}