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[InstCombine] 'hoist xor-by-constant from xor-by-value': completely give up on constant exprs 

As Mikael Holmén is noting in the post-commit review for the first fix
https://reviews.llvm.org/rGd4ccef38d0bb#967466
not hoisting constantexprs is not enough,
because if the xor originally was a constantexpr (i.e. X is a constantexpr).
`SimplifyAssociativeOrCommutative()` in `visitXor()` will immediately
undo this transform, thus again causing an infinite combine loop.

This transform has resulted in a surprising number of constantexpr failures.
This commit is contained in:
Roman Lebedev 2020-12-29 16:27:55 +03:00
parent 7486de1b2e
commit 374ef57f13
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GPG Key ID: 083C3EBB4A1689E0
2 changed files with 25 additions and 3 deletions
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8
llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
View File

@ -3459,9 +3459,11 @@ Instruction *InstCombinerImpl::visitXor(BinaryOperator &I) {
// Otherwise, if all else failed, try to hoist the xor-by-constant:
// (X ^ C) ^ Y --> (X ^ Y) ^ C
// Just like we do in other places, we avoid the fold for constantexprs,
// at least to avoid endless combine loop.
if (match(&I, m_c_Xor(m_OneUse(m_Xor(m_Value(X), m_ImmConstant(C1))),
// Just like we do in other places, we completely avoid the fold
// for constantexprs, at least to avoid endless combine loop.
if (match(&I, m_c_Xor(m_OneUse(m_Xor(m_CombineAnd(m_Value(X),
m_Unless(m_ConstantExpr())),
m_ImmConstant(C1))),
m_Value(Y))))
return BinaryOperator::CreateXor(Builder.CreateXor(X, Y), C1);

20
llvm/test/Transforms/InstCombine/hoist-xor-by-constant-from-xor-by-value.ll
View File

@ -87,3 +87,23 @@ entry:
%r = xor i8 %or, xor (i8 xor (i8 ptrtoint (i32* @global_constant to i8), i8 -1), i8 xor (i8 ptrtoint (i32* @global_constant2 to i8), i8 -1))
ret i8 %r
}
@global_constant3 = external global [6 x [1 x i64]], align 1
@global_constant4 = external global i64, align 1
@global_constant5 = external global i16*, align 1
define i16 @constantexpr2() {
; CHECK-LABEL: @constantexpr2(
; CHECK-NEXT: [[I2:%.*]] = load i16*, i16** @global_constant5, align 1
; CHECK-NEXT: [[I3:%.*]] = load i16, i16* [[I2]], align 1
; CHECK-NEXT: [[I5:%.*]] = xor i16 [[I3]], xor (i16 zext (i1 icmp ne (i64* getelementptr inbounds ([6 x [1 x i64]], [6 x [1 x i64]]* @global_constant3, i64 0, i64 5, i64 0), i64* @global_constant4) to i16), i16 -1)
; CHECK-NEXT: ret i16 [[I5]]
;
%i0 = icmp ne i64* getelementptr inbounds ([6 x [1 x i64]], [6 x [1 x i64]]* @global_constant3, i16 0, i16 5, i16 0), @global_constant4
%i1 = zext i1 %i0 to i16
%i2 = load i16*, i16** @global_constant5, align 1
%i3 = load i16, i16* %i2, align 1
%i4 = xor i16 %i3, %i1
%i5 = xor i16 %i4, -1
ret i16 %i5
}