Re: [PATCH] rust: enable fentry support

On Tue, Jul 28, 2026 at 5:58 PM Vasily Gorbik <[email protected]> wrote:

> So I do not think -mrecord-mcount Rust support is only needed for
> s390. The Rust build path needs an equivalent of -mrecord-mcount when
> FTRACE_MCOUNT_USE_CC is selected. And additionally an equivalent of
> -mnop-mcount for s390 (this is debatable, we could check if this brings
> us much or we could drop it).

Right, as background, the llvm x86 maintainer were uninterested in
supporting these options [1] when I created the PR. I think the
argument is roughly that these options are bespoke features for
building this project. mrecordmcount already does that, independent of
the compiler. I think some llvm maintainers hold the opinion that
patchable-function-entries is already a sufficient replacement for
fentry.

Digging through commit history, the answer is unsurprisingly that
using recordmcount is slower. It's certainly more convenient to have
the compiler record them.

[1] https://github.com/llvm/llvm-project/pull/184641