Alexander Monakov <amona...@ispras.ru> writes: > On Fri, 1 Nov 2024, Jan Hubicka wrote: > >> > I have a vague memory that one of the tests in SPEC has a loop that >> > tries to malloc, doubling the size each time, until it fails. Would >> > the patch change the behavior of such a loop? >> >> If the resulting allocation is unused except for NULL check we will make >> it always "succeed" and thus the loop will likely loop forever. I >> wonder what this loop computes on systems, like linux, that overcommits >> memory. > > Even with overcommit, malloc is going to return NULL as soon as you pass > a half or more of the address space as the requested allocation size. > > Since the patch eliminates malloc with unknown size, I think this transform > is incorrect (wasn't correct for the same reason in LLVM either). > > I'm pretty sure it's going to break some Glibc tests that verify that > malloc(SIZE_MAX / 2 + 1) does not succeed. > > Please reconsider? Why to we need to match LLVM here?
Some references to feed discussion which I had in my logs from discussing it with someone the other week after that interaction we had with alanc: * https://github.com/llvm/llvm-project/issues/28790 (not very insightful, other than to show it has a history of confusing people, but one might argue that for other (definitely-valid) optimisations) * https://github.com/llvm/llvm-project/issues/49826 * https://github.com/llvm/llvm-project/issues/51476 Alexander may have more or better references. As I've complained about many times, I find GitHub Issues hard to search - sorry! sam
