[llvm] [Bazel] Set HAVE_MALLINFO2=1 (PR #125681)

[Aaron Siddhartha Mondal via llvm-commits]

aaronmondal wrote:

Aargh what a rabbithole xDxD

So `__GLIBC_PREREQ` is defined by glibc's `features.h`. This means that if we substituted `HAVE_MALLINFO2` with it we'd make compilation dependent on this dynamic value after configuration stage, making the build irreproducible, given the same configuration. I'd argue that this is actually somewhat suboptimal for the sanitizers as well, but at least those are not "core" libraries.

In the HAVE_MALLINFO2 case however we'd be opening up libllvm to this nondeterminism which would harm portability of the build (the support lib built on a glibc2.33 system wouldn't work or even crash on a glibc2.31 system without an easy way to opt out, forcing users to either modify the source or to use crosscompilation sysroots).

Ignoring the deprecation warnings also seems suboptimal as it increases the "surprise factor" of breaking changes (IMO this is also an issue witht the current `-Wno-deprecated`).

Fortunately, the easy way out seems to be to just make this as an actual Bazel config.

Regarding the default I think it should be fine to make it `true` by default. According to https://reviews.llvm.org/D117916 mallinfo2 was released if Feb 2021. In terms of mainstream distros Ubuntu 22.04 runs on glibc2.35 so that should be fine. Users of Ubuntu 20.04 (glibc2.31) would have to flip the flag but that reaches EOL in May 2025 anyways.

... is what i *would've said*, but now it turns out that we're not actually using platforms for config settings. So I gess that needs fixing first lol.

https://github.com/llvm/llvm-project/pull/125681