InstCombine: Simplify (A ^ B) or/and (A ^ B ^ C)

While we can already transform A | (A ^ B) into A | B, things get bad
once we have (A ^ B) | (A ^ B ^ Cst) because reassociation will morph
this into (A ^ B) | ((A ^ Cst) ^ B).  Our existing patterns fail once
this happens.

To fix this, we add a new pattern which looks through the tree of xor
binary operators to see that, in fact, there exists a redundant xor
operation.

What follows bellow is a correctness proof of the transform using CVC3.

$ cat t.cvc
A, B, C : BITVECTOR(64);

QUERY BVXOR(A, B) | BVXOR(BVXOR(B, C), A) = BVXOR(A, B) | C;
QUERY BVXOR(BVXOR(A, C), B) | BVXOR(A, B) = BVXOR(A, B) | C;

QUERY BVXOR(A, B) & BVXOR(BVXOR(B, C), A) = BVXOR(A, B) & ~C;
QUERY BVXOR(BVXOR(A, C), B) & BVXOR(A, B) = BVXOR(A, B) & ~C;

$ cvc3 < t.cvc
Valid.
Valid.
Valid.
Valid.

llvm-svn: 214342

1 files changed, 24 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 718cf29f184..51564088e67 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -1283,6 +1283,18 @@ Instruction *InstCombiner::visitAnd(BinaryOperator &I) {
     if (match(Op1, m_Or(m_Not(m_Specific(Op0)), m_Value(A))) ||
         match(Op1, m_Or(m_Value(A), m_Not(m_Specific(Op0)))))
       return BinaryOperator::CreateAnd(A, Op0);
+
+    // (A ^ B) & ((B ^ C) ^ A) -> (A ^ B) & ~C
+    if (match(Op0, m_Xor(m_Value(A), m_Value(B))))
+      if (match(Op1, m_Xor(m_Xor(m_Specific(B), m_Value(C)), m_Specific(A))))
+        if (Op1->hasOneUse() || cast<BinaryOperator>(Op1)->hasOneUse())
+          return BinaryOperator::CreateAnd(Op0, Builder->CreateNot(C));
+
+    // ((A ^ C) ^ B) & (B ^ A) -> (B ^ A) & ~C
+    if (match(Op0, m_Xor(m_Xor(m_Value(A), m_Value(C)), m_Value(B))))
+      if (match(Op1, m_Xor(m_Specific(B), m_Specific(A))))
+        if (Op0->hasOneUse() || cast<BinaryOperator>(Op0)->hasOneUse())
+          return BinaryOperator::CreateAnd(Op1, Builder->CreateNot(C));
   }
 
   if (ICmpInst *RHS = dyn_cast<ICmpInst>(Op1))
@@ -2080,6 +2092,18 @@ Instruction *InstCombiner::visitOr(BinaryOperator &I) {
     }
   }
 
+  // (A ^ B) | ((B ^ C) ^ A) -> (A ^ B) | C
+  if (match(Op0, m_Xor(m_Value(A), m_Value(B))))
+    if (match(Op1, m_Xor(m_Xor(m_Specific(B), m_Value(C)), m_Specific(A))))
+      if (Op1->hasOneUse() || cast<BinaryOperator>(Op1)->hasOneUse())
+        return BinaryOperator::CreateOr(Op0, C);
+
+  // ((A ^ C) ^ B) | (B ^ A) -> (B ^ A) | C
+  if (match(Op0, m_Xor(m_Xor(m_Value(A), m_Value(C)), m_Value(B))))
+    if (match(Op1, m_Xor(m_Specific(B), m_Specific(A))))
+      if (Op0->hasOneUse() || cast<BinaryOperator>(Op0)->hasOneUse())
+        return BinaryOperator::CreateOr(Op1, C);
+
   // (X >> Z) | (Y >> Z)  -> (X|Y) >> Z  for all shifts.
   if (BinaryOperator *SI1 = dyn_cast<BinaryOperator>(Op1)) {
     if (BinaryOperator *SI0 = dyn_cast<BinaryOperator>(Op0))