[llvm] r301923 - [InstCombine] don't use DeMorgan's Law on integer constants
Sanjay Patel via llvm-commits
llvm-commits at lists.llvm.org
Tue May 2 07:31:30 PDT 2017
Author: spatel
Date: Tue May 2 09:31:30 2017
New Revision: 301923
URL: http://llvm.org/viewvc/llvm-project?rev=301923&view=rev
Log:
[InstCombine] don't use DeMorgan's Law on integer constants
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 D32255.
There are 2 or 3 problems with dyn_castNotVal, and I don't think we can
reinstate 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
Modified:
llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
llvm/trunk/test/Transforms/InstCombine/assume2.ll
llvm/trunk/test/Transforms/InstCombine/demorgan.ll
Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp?rev=301923&r1=301922&r2=301923&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp Tue May 2 09:31:30 2017
@@ -2433,29 +2433,32 @@ Instruction *InstCombiner::visitXor(Bina
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),
Modified: llvm/trunk/test/Transforms/InstCombine/assume2.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/assume2.ll?rev=301923&r1=301922&r2=301923&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/assume2.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/assume2.ll Tue May 2 09:31:30 2017
@@ -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
;
Modified: llvm/trunk/test/Transforms/InstCombine/demorgan.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/demorgan.ll?rev=301923&r1=301922&r2=301923&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/demorgan.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/demorgan.ll Tue May 2 09:31:30 2017
@@ -367,12 +367,12 @@ define i8 @demorgan_nor_use2bc(i8 %A, i8
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
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