[InstCombine] don't use DeMorgan's Law on integer constants (2nd try)

This was originally checked in here:
https://reviews.llvm.org/rL301923

And reverted here:
https://reviews.llvm.org/rL301924

Because there's a clang test that would fail after this. I fixed/removed the
offending CHECK lines in:
https://reviews.llvm.org/rL301928

So let's try this again. Original commit message:

This is the fold that causes the infinite loop in BoringSSL
(https://github.com/google/boringssl/blob/master/crypto/cipher/e_rc2.c)
when we fix instcombine demanded bits to prefer 'not' ops as in https://reviews.llvm.org/D32255.

There are 2 or 3 problems with dyn_castNotVal, and I don't think we can
reinstate https://reviews.llvm.org/D32255 until dyn_castNotVal is completely eliminated.

1. As shown here, it transforms 'not' into random xor. This transform is harmful to SCEV and codegen because 'not' can often be folded while random xor cannot.
2. It does not transform vector constants. This is actually a good thing, but if you don't believe the above argument, then we shouldn't have excluded vectors.
3. It tries to avoid transforming not(not(X)). That's nice, but it doesn't match the greedy nature of instcombine. If we DeMorganize a pattern that has an extra 'not' in it: ~(~(~X) & Y) --> (~X | ~Y)

  That's just another case of DeMorgan, so we should trust that we'll fold that pattern too: (~X | ~ Y) --> ~(X & Y)

Differential Revision: https://reviews.llvm.org/D32665

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301929 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 41ae37e..c7092bf 100644 (file)
--- a/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2433,29 +2433,32 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) {
   if (Value *V = SimplifyBSwap(I))
     return replaceInstUsesWith(I, V);
 
+  // Apply DeMorgan's Law for 'nand' / 'nor' logic with an inverted operand.
+  Value *X, *Y;
+
+  // We must eliminate the and/or (one-use) for these transforms to not increase
+  // the instruction count.
+  // ~(~X & Y) --> (X | ~Y)
+  // ~(Y & ~X) --> (X | ~Y)
+  if (match(&I, m_Not(m_OneUse(m_c_And(m_Not(m_Value(X)), m_Value(Y)))))) {
+    Value *NotY = Builder->CreateNot(Y, Y->getName() + ".not");
+    return BinaryOperator::CreateOr(X, NotY);
+  }
+  // ~(~X | Y) --> (X & ~Y)
+  // ~(Y | ~X) --> (X & ~Y)
+  if (match(&I, m_Not(m_OneUse(m_c_Or(m_Not(m_Value(X)), m_Value(Y)))))) {
+    Value *NotY = Builder->CreateNot(Y, Y->getName() + ".not");
+    return BinaryOperator::CreateAnd(X, NotY);
+  }
+
   // Is this a 'not' (~) fed by a binary operator?
   BinaryOperator *NotOp;
   if (match(&I, m_Not(m_BinOp(NotOp)))) {
     if (NotOp->getOpcode() == Instruction::And ||
         NotOp->getOpcode() == Instruction::Or) {
-      // We must eliminate the and/or for this transform to not increase the
-      // instruction count.
-      if (NotOp->hasOneUse()) {
-        // ~(~X & Y) --> (X | ~Y) - De Morgan's Law
-        // ~(~X | Y) === (X & ~Y) - De Morgan's Law
-        if (dyn_castNotVal(NotOp->getOperand(1)))
-          NotOp->swapOperands();
-        if (Value *Op0NotVal = dyn_castNotVal(NotOp->getOperand(0))) {
-          Value *NotY = Builder->CreateNot(
-              NotOp->getOperand(1), NotOp->getOperand(1)->getName() + ".not");
-          if (NotOp->getOpcode() == Instruction::And)
-            return BinaryOperator::CreateOr(Op0NotVal, NotY);
-          return BinaryOperator::CreateAnd(Op0NotVal, NotY);
-        }
-      }
-
-      // ~(X & Y) --> (~X | ~Y) - De Morgan's Law
-      // ~(X | Y) === (~X & ~Y) - De Morgan's Law
+      // Apply DeMorgan's Law when inverts are free:
+      // ~(X & Y) --> (~X | ~Y)
+      // ~(X | Y) --> (~X & ~Y)
       if (IsFreeToInvert(NotOp->getOperand(0),
                          NotOp->getOperand(0)->hasOneUse()) &&
           IsFreeToInvert(NotOp->getOperand(1),

diff --git a/test/Transforms/InstCombine/assume2.ll b/test/Transforms/InstCombine/assume2.ll
index e8fbc04..8dc8831 100644 (file)
--- a/test/Transforms/InstCombine/assume2.ll
+++ b/test/Transforms/InstCombine/assume2.ll
@@ -21,8 +21,8 @@ define i32 @test1(i32 %a) #0 {
 
 define i32 @test2(i32 %a) #0 {
 ; CHECK-LABEL: @test2(
-; CHECK-NEXT:    [[A_NOT:%.*]] = or i32 [[A:%.*]], -16
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[A_NOT]], -6
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[A:%.*]], 15
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[AND]], 10
 ; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP]])
 ; CHECK-NEXT:    ret i32 2
 ;
@@ -50,8 +50,8 @@ define i32 @test3(i32 %a) #0 {
 
 define i32 @test4(i32 %a) #0 {
 ; CHECK-LABEL: @test4(
-; CHECK-NEXT:    [[A_NOT:%.*]] = and i32 [[A:%.*]], 15
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[A_NOT]], 10
+; CHECK-NEXT:    [[V:%.*]] = or i32 [[A:%.*]], -16
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[V]], -6
 ; CHECK-NEXT:    tail call void @llvm.assume(i1 [[CMP]])
 ; CHECK-NEXT:    ret i32 2
 ;

diff --git a/test/Transforms/InstCombine/demorgan.ll b/test/Transforms/InstCombine/demorgan.ll
index fc4af3c..26c2270 100644 (file)
--- a/test/Transforms/InstCombine/demorgan.ll
+++ b/test/Transforms/InstCombine/demorgan.ll
@@ -367,12 +367,12 @@ define i8 @demorgan_nor_use2bc(i8 %A, i8 %B) {
   ret i8 %r2
 }
 
-; FIXME: Do not apply DeMorgan's Law to constants. We prefer 'not' ops.
+; Do not apply DeMorgan's Law to constants. We prefer 'not' ops.
 
 define i32 @demorganize_constant1(i32 %a) {
 ; CHECK-LABEL: @demorganize_constant1(
-; CHECK-NEXT:    [[A_NOT:%.*]] = or i32 %a, -16
-; CHECK-NEXT:    [[AND1:%.*]] = xor i32 [[A_NOT]], 15
+; CHECK-NEXT:    [[AND:%.*]] = and i32 %a, 15
+; CHECK-NEXT:    [[AND1:%.*]] = xor i32 [[AND]], -1
 ; CHECK-NEXT:    ret i32 [[AND1]]
 ;
   %and = and i32 %a, 15
@@ -380,12 +380,12 @@ define i32 @demorganize_constant1(i32 %a) {
   ret i32 %and1
 }
 
-; FIXME: Do not apply DeMorgan's Law to constants. We prefer 'not' ops.
+; Do not apply DeMorgan's Law to constants. We prefer 'not' ops.
 
 define i32 @demorganize_constant2(i32 %a) {
 ; CHECK-LABEL: @demorganize_constant2(
-; CHECK-NEXT:    [[A_NOT:%.*]] = and i32 %a, -16
-; CHECK-NEXT:    [[AND1:%.*]] = xor i32 [[A_NOT]], -16
+; CHECK-NEXT:    [[AND:%.*]] = or i32 %a, 15
+; CHECK-NEXT:    [[AND1:%.*]] = xor i32 [[AND]], -1
 ; CHECK-NEXT:    ret i32 [[AND1]]
 ;
   %and = or i32 %a, 15