Undefined external symbols from compiler_builtins

[fenhl]

I have a project where I'm experimenting with introducing Rust code into an existing codebase by compiling it into a static library for a custom target and linking it using armips. This process no longer works on the latest nightly because armips complains about undefined external symbols such as _rnvntcse1eqnti5u6g_17compiler_builtins4math5atan2.

A minimal example is at https://github.com/fenhl/rust-builtins-min which can be tested by running python build.py after installing armips (scoop install extras/armips on Windows, see https://github.com/Kingcom/armips for binaries and build instructions for other platforms).

I ran cargo-bisect-rustc on the original project where I ran into this (not the minimal example, hence the different script in the command line) and it points to #123719 as the commit that introduced the regression.

cargo-bisect-rustc output

searched nightlies: from nightly-2022-06-21 to nightly-2024-04-16
regressed nightly: nightly-2024-04-15
searched commit range: 0bf471f...0d8b334
regressed commit: 29b1207

bisected with cargo-bisect-rustc v0.6.8

Host triple: x86_64-pc-windows-msvc
Reproduce with:

cargo bisect-rustc --start=2022-06-21 -c rust-src --script python -- .\cargo-bisect-rustc-test.py

[Noratrieb]

rust-lang/compiler-builtins@0.1.108...0.1.109

I suspect it's rust-lang/compiler-builtins#577

[Noratrieb]

@Amjad50 @Amanieu

[Amjad50]

Thanks for letting me know.

The changes in 0.1.109 do not work as expected, I'm working on a fix here rust-lang/compiler-builtins#594, I'll test it with this project.

[Amjad50]

So after looking around and trying to fix the issue, the main issue is from armips, it doesn't handle weak symbols correctly

Added a quick small fix here, Kingcom/armips#242, but not sure if the maintainer is around anymore

[Amjad50]

So, the PR in armips is fixed, and now it can handle weak symbols.

Though, there appear to be another issue.

When running armips with the library from latest nightly, it gives an error that memset is not found.

Looking at the llvm-ir for compiler_builtins, it seems the function compiler_builtins::math::libm::rem_pio2_large::rem_pio2_large calls

call void @llvm.memset.p0.i64(ptr noundef nonnull align 4 dereferenceable(80) %iq, i8 0, i64 80, i1 false)

which is converted into memset function call for some reason.

There is a way to fix this, which is to add -Zbuild-std-features=compiler-builtins-mem flag which would add the memory builtin functions.

I'm not sure if that is the correct behavior of llvm or not. since the target in the example has "os" = "none".

This is a static library, so maybe its assuming memset will be available when linking any executable?