[llvm] r248634 - [InstCombine] match De Morgan's Law hidden by zext ops (PR22723)

[Sanjay Patel via llvm-commits]

Author: spatel
Date: Fri Sep 25 18:21:38 2015
New Revision: 248634

URL: http://llvm.org/viewvc/llvm-project?rev=248634&view=rev
Log:
[InstCombine] match De Morgan's Law hidden by zext ops (PR22723)

This is a fix for PR22723:
https://llvm.org/bugs/show_bug.cgi?id=22723

My first attempt at this was to change what I thought was the root problem:

xor (zext i1 X to i32), 1 --> zext (xor i1 X, true) to i32

...but we create the opposite pattern in InstCombiner::visitZExt(), so infinite loop!

My next idea was to fix the matchIfNot() implementation in PatternMatch, but that would
mean potentially returning a different size for the match than what was input. I think
this would require all users of m_Not to check the size of the returned match, so I 
abandoned that idea.

I settled on just fixing the exact case presented in the PR. This patch does allow the
2 functions in PR22723 to compile identically (x86):

bool test(bool x, bool y) { return !x | !y; }
bool test(bool x, bool y) { return !x || !y; }
...
andb	%sil, %dil
xorb	$1, %dil
movb	%dil, %al
retq

Differential Revision: http://reviews.llvm.org/D12705


Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
    llvm/trunk/test/Transforms/InstCombine/demorgan-zext.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp?rev=248634&r1=248633&r2=248634&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp Fri Sep 25 18:21:38 2015
@@ -1208,6 +1208,11 @@ static Instruction *matchDeMorgansLaws(B
   auto Opcode = I.getOpcode();
   assert((Opcode == Instruction::And || Opcode == Instruction::Or) &&
          "Trying to match De Morgan's Laws with something other than and/or");
+  // Flip the logic operation.
+  if (Opcode == Instruction::And)
+    Opcode = Instruction::Or;
+  else
+    Opcode = Instruction::And;
 
   Value *Op0 = I.getOperand(0);
   Value *Op1 = I.getOperand(1);
@@ -1215,16 +1220,31 @@ static Instruction *matchDeMorgansLaws(B
   if (Value *Op0NotVal = dyn_castNotVal(Op0))
     if (Value *Op1NotVal = dyn_castNotVal(Op1))
       if (Op0->hasOneUse() && Op1->hasOneUse()) {
-        // Flip the logic operation.
-        if (Opcode == Instruction::And)
-          Opcode = Instruction::Or;
-        else
-          Opcode = Instruction::And;
         Value *LogicOp = Builder->CreateBinOp(Opcode, Op0NotVal, Op1NotVal,
                                               I.getName() + ".demorgan");
         return BinaryOperator::CreateNot(LogicOp);
       }
 
+  // De Morgan's Law in disguise:
+  // (zext(bool A) ^ 1) & (zext(bool B) ^ 1) -> zext(~(A | B))
+  // (zext(bool A) ^ 1) | (zext(bool B) ^ 1) -> zext(~(A & B))
+  Value *A = nullptr;
+  Value *B = nullptr;
+  ConstantInt *C1 = nullptr;
+  if (match(Op0, m_OneUse(m_Xor(m_ZExt(m_Value(A)), m_ConstantInt(C1)))) &&
+      match(Op1, m_OneUse(m_Xor(m_ZExt(m_Value(B)), m_Specific(C1))))) {
+    // TODO: This check could be loosened to handle different type sizes.
+    // Alternatively, we could fix the definition of m_Not to recognize a not
+    // operation hidden by a zext?
+    if (A->getType()->isIntegerTy(1) && B->getType()->isIntegerTy(1) &&
+        C1->isOne()) {
+      Value *LogicOp = Builder->CreateBinOp(Opcode, A, B,
+                                            I.getName() + ".demorgan");
+      Value *Not = Builder->CreateNot(LogicOp);
+      return CastInst::CreateZExtOrBitCast(Not, I.getType());
+    }
+  }
+
   return nullptr;
 }
 

Modified: llvm/trunk/test/Transforms/InstCombine/demorgan-zext.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/demorgan-zext.ll?rev=248634&r1=248633&r2=248634&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/demorgan-zext.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/demorgan-zext.ll Fri Sep 25 18:21:38 2015
@@ -11,12 +11,10 @@ define i32 @demorgan_or(i1 %X, i1 %Y) {
   ret i32 %or
 
 ; CHECK-LABEL: demorgan_or(
-; CHECK-NEXT:  = zext
-; CHECK-NEXT:  = zext
-; CHECK-NEXT:  = xor
-; CHECK-NEXT:  = xor
-; CHECK-NEXT:  = or
-; CHECK-NEXT:  ret
+; CHECK-NEXT:  %[[AND:.*]] = and i1 %X, %Y
+; CHECK-NEXT:  %[[ZEXT:.*]] = zext i1 %[[AND]] to i32
+; CHECK-NEXT:  %[[XOR:.*]] = xor i32 %[[ZEXT]], 1
+; CHECK-NEXT:  ret i32 %[[XOR]]
 }
 
 define i32 @demorgan_and(i1 %X, i1 %Y) {
@@ -28,11 +26,9 @@ define i32 @demorgan_and(i1 %X, i1 %Y) {
   ret i32 %and
 
 ; CHECK-LABEL: demorgan_and(
-; CHECK-NEXT:  = zext
-; CHECK-NEXT:  = zext
-; CHECK-NEXT:  = xor
-; CHECK-NEXT:  = xor
-; CHECK-NEXT:  = and
-; CHECK-NEXT:  ret
+; CHECK-NEXT:  %[[OR:.*]] = or i1 %X, %Y
+; CHECK-NEXT:  %[[ZEXT:.*]] = zext i1 %[[OR]] to i32
+; CHECK-NEXT:  %[[XOR:.*]] = xor i32 %[[ZEXT]], 1
+; CHECK-NEXT:  ret i32 %[[XOR]]
 }