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Optimizer does not take advantage of -value == ~(value - 1) #47903

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mahkoh opened this issue Dec 19, 2020 · 3 comments
Closed

Optimizer does not take advantage of -value == ~(value - 1) #47903

mahkoh opened this issue Dec 19, 2020 · 3 comments
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@mahkoh
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mahkoh commented Dec 19, 2020

Bugzilla Link 48559
Resolution FIXED
Resolved on Dec 21, 2020 09:52
Version 11.0
OS Linux
CC @rotateright
Fixed by commit(s) 38ca7fa

Extended Description

Test case:

long f(long align) {
return (align - 1) & -align;
}

long g(long align) {
return (align - 1) & ~(align - 1);
}

int h(long align) {
return -align == ~(align - 1);
}

Generated assembly:

f:
leaq -1(%rdi), %rax
negq %rdi
andq %rdi, %rax
retq
g:
xorl %eax, %eax
retq
h:
movl $1, %eax
retq
@mahkoh
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mahkoh commented Dec 19, 2020

assigned to @rotateright

@rotateright
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This is a special-case of a more general pattern that ends in a bitwise logic op and includes add/sub operands. For example:
https://alive2.llvm.org/ce/z/mxoxUz

I think these are the generalizations we should handle in instsimplify:
https://rise4fun.com/Alive/gR0

@rotateright
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https://reviews.llvm.org/rG38ca7face67e

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 11, 2021
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