[PATCH] D100879: [Clang] Propagate guaranteed alignment for malloc and others

[David Goldblatt via Phabricator via cfe-commits]

davidtgoldblatt added a comment.

(For background: I'm a jemalloc maintainer and wrote N2293).

In D100879#3145361 <https://reviews.llvm.org/D100879#3145361>, @rjmccall wrote:

> Platforms are permitted to make stronger guarantees than the C standard requires, and it is totally reasonable for compilers to assume that `malloc` meets the target platform's documented guarantees.  Arguably, libraries should not be replacing `malloc` if they fail to meet the platform's documented guarantees.

I agree generally that assuming platform guarantees is reasonable for the compiler in targeting that platform, but I don't think I agree that the dlmalloc alignment is one of them. Supporting malloc replacement is a first-class feature for glibc malloc, and related projects avoid relying on alignment guarantees (e.g. libstdc++ at one point considered assuming that 8-byte allocs were 16-byte aligned, and decided not to). At least one Linux distribution using clang (Alpine) uses musl, which is a weak-alignment implementation (contrary to what I claimed in N2293; I screwed up my background research).

> With that said, it's probably not unreasonable for Clang to provide an option that controls its assumptions about `malloc`.  We should think somewhat carefully about whether there are other behaviors we'd want to include in that, though.

This makes sense to me; this is broadly how clang handles `operator new` alignment as well.


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https://reviews.llvm.org/D100879