d1fc8f7211
In certain cases, lowering negations can cause an infinite loop in SelectionDAG on X86. The following snippet shows that behaviour: https://godbolt.org/z/5hP45T4hY What happens is that ADD(XOR(..., -1), 1) is detected as the two's complement and transformed into SUB(0, ...) However, immediates can not be encoded as the LHS of a SUB on X86. Therefore it is transformed back into an ADD/XOR pair, which is then again transformed into a SUB and so on. In that specific case, I still think it is valid to display this as a SUB(0,...) , because it should eventually be lowered as a NEG. Which seems better than an ADD/XOR pair. Adding an exception to the X86 specific handling for SUBs with 0 LHS operand fixes this infinite loop. Differential Revision: https://reviews.llvm.org/D154575
18 lines
522 B
LLVM
18 lines
522 B
LLVM
; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
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; RUN: llc < %s -mtriple=i386-unknown-unknown | FileCheck %s
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@value1 = external hidden constant i32
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define i32 @function(i32 %arg1) {
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; CHECK-LABEL: function:
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; CHECK: # %bb.0: # %entry
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; CHECK-NEXT: movl $value1, %eax
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; CHECK-NEXT: xorl {{[0-9]+}}(%esp), %eax
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; CHECK-NEXT: negl %eax
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; CHECK-NEXT: retl
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entry:
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%0 = xor i32 %arg1, xor (i32 ptrtoint (i32* @value1 to i32), i32 -1)
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%.neg = add i32 %0, 1
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ret i32 %.neg
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}
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