Pass `rustc` the equivalent of `-Wframe-larger-than=1000`

[ojeda]

[mpe]

Is there such a thing? I had a look and couldn't find it, but I'm not a rustc expert.

[ojeda]

Not sure -- we may need to implement it ourselves! :)

[bjorn3]

No, there isn't a direct option, but there are things that can help. There is rust-lang/rust#83519 for moves of large values. This includes assignments. For stack frame sizes, only LLVM knows about it. With -Z emit-stack-sizes it is possible to tell LLVM to add a section containing info about stack frame sizes. This option was added to rustc for https://github.com/japaric/cargo-call-stack.

[mpe]

OK. I used -Z emit-stack-sizes to debug issue #155, which worked well enough to find the large functions. I guess that's not useful for an upstream stack size warning option though, because it would require nightly rustc (AIUI).

[TheSven73]

@mpe would you mind sharing how you obtained a list of function stack sizes using -Z emit-stack-sizes? I'd like to do the same for arm 32-bit. It's quite possible that stack overflows are happening there too.

[mpe]

Something like this:

$ make KRUSTCFLAGS="-Z emit-stack-sizes"
$ cargo install stack-sizes
$ stack-sizes vmlinux | sort -nk 2

[ojeda]

I guess that's not useful for an upstream stack size warning option though, because it would require nightly rustc (AIUI).

Yeah -- we should use it nevertheless. That way the rustc folks know it is useful and being used (see the #2 list), and that we will want it stabilized at some point. If really needed, we can always gate it or take it out later.

[bjorn3]

I don't know what stability guarantee LLVM has for this feature. If LLVM doesn't guarantee that it will remain the same forever, I think it is unlikely that it will be stabilized in rustc.

[ojeda]

My thinking was that given Clang supports -Wframe-larger-than=, LLVM will need to support the feature one way or another, which means it should be fairly easy for rustc to keep providing it too.

[bjorn3]

Clang doesn't use the stack size section. Instead it instructs LLVM to emit a diagnostic whenever the stack size is above a certain threshold. Rustc doesn't have any support for handling LLVM diagnostics except for inline asm.

https://github.com/llvm/llvm-project/blob/a9968c0a339ab47f5d6da1c513b2754f1e98a596/llvm/lib/CodeGen/PrologEpilogInserter.cpp#L278-L284

llvm/llvm-project@728c554