[BranchFolding] Remove dubious assert from operator< #71639
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`MergePotentialElts::operator<` asserts that the two elements being compared are not equal. However, sorting functions are allowed to invoke the comparison function with equal arguments (though they usually don't for efficiency reasons). There is an existing special-case that disables the assert if _GLIBCXX_DEBUG is used, which may invoke the comparator with equal args to verify strict weak ordering. I believe libc++ also has strict weak ordering checks under some options nowadays. Most recently llvm#71312 was reported, where a change to glibc's qsort_r implementation can also result in comparison between equal elements. From what I understood this is an inefficiency that will be fixed on the glibc side as well, but I think at this point we should just remove this assertion. Fixes llvm#71312.
arsenm
approved these changes
Nov 8, 2023
Comment on lines
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| // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing | ||
| // an object with itself. | ||
| #ifndef _GLIBCXX_DEBUG | ||
| llvm_unreachable("Predecessor appears twice"); | ||
| #else | ||
| return false; | ||
| #endif |
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I don't really see the point of asserting this in the first place
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I think the point is to ensure that the array being sorted does not contain a pair of elements that compare equal, in which case it's more efficient to verify that by inspecting the sorted array:
for (MPIterator i = MergePotentials.begin(), n = std::next(i);
n != MergePotentials.end();
i = n, n++)
assert(*i < *n);
|
Do we need to backport this to release/17.x ? |
We don't know yet whether the glibc workaround is effective. In any case, it's something that is likely to impact other systems as well. |
|
Unfortunate that this was merged without acknowledging my response to @arsenm about the logic of the assert. |
tru
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Nov 14, 2023
`MergePotentialElts::operator<` asserts that the two elements being compared are not equal. However, sorting functions are allowed to invoke the comparison function with equal arguments (though they usually don't for efficiency reasons). There is an existing special-case that disables the assert if _GLIBCXX_DEBUG is used, which may invoke the comparator with equal args to verify strict weak ordering. I believe libc++ also has strict weak ordering checks under some options nowadays. Recently, #71312 was reported, where a change to glibc's qsort_r implementation can also result in comparison between equal elements. From what I understood, this is an inefficiency that will be fixed on the glibc side as well, but I think at this point we should just remove this assertion. Fixes #71312. (cherry picked from commit 74a76a2)
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MergePotentialElts::operator<asserts that the two elements being compared are not equal. However, sorting functions are allowed to invoke the comparison function with equal arguments (though they usually don't for efficiency reasons).There is an existing special-case that disables the assert if _GLIBCXX_DEBUG is used, which may invoke the comparator with equal args to verify strict weak ordering. I believe libc++ also has strict weak ordering checks under some options nowadays.
Recently, #71312 was reported, where a change to glibc's qsort_r implementation can also result in comparison between equal elements. From what I understoo,d this is an inefficiency that will be fixed on the glibc side as well, but I think at this point we should just remove this assertion.
Fixes #71312.