RPCSX/llvm
1
0
Fork 0
mirror of https://github.com/RPCSX/llvm.git synced 2024-12-04 10:04:33 +00:00
Code Issues Actions Packages Projects Releases Wiki Activity

[InstCombine] don't use DeMorgan's Law on integer constants

Browse Source 
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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301923 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjay Patel 2017-05-02 14:31:30 +00:00
parent d3b7a7cfa0
commit 9697c664a6
3 changed files with 31 additions and 28 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

39
lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
View File

@ -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),

8
test/Transforms/InstCombine/assume2.ll
View File

@ -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
;

12
test/Transforms/InstCombine/demorgan.ll
View File

@ -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