Compiling libcore without SSE leads to LLVM ERROR: SSE register return with SSE disabled

[gz]

May be related to: #26025
At least compilation seems to work for June 17th nightly and earlier.

I tried this (creating libcore library without the use of vector instructions; this is typically done in case one wants to use libcore in kernel level code):

rustc-nightly/src/libcore $ rustc --crate-type=lib --emit=link -O -C soft-float -C target-feature=-mmx,-sse,-sse2,-sse3,-ssse3,-sse4.1,-sse4.2,-3dnow,-3dnowa,-avx,-avx2 -g --emit=obj -C soft-float -C no-vectorize-slp -C no-vectorize-loops --crate-name core lib.rs 
LLVM ERROR: SSE register return with SSE disabled

I expected to see this happen: Successful compilation.

Instead, this happened: LLVM ERROR: SSE register return with SSE disabled

Meta

rustc --version --verbose:
rustc 1.2.0-nightly (2f56839 2015-06-18)
binary: rustc
commit-hash: 2f56839
commit-date: 2015-06-18
host: x86_64-unknown-linux-gnu
release: 1.2.0-nightly

[Roxxik]

I've investigated this a little further:
libcore cannot be compiled without mmx, sse and sse2
the following commands all fail

$ rustc -O -g --crate-type=lib --emit=link --emit=obj -C target-feature=-mmx --crate-name core lib.rs
LLVM ERROR: SSE register return with SSE disabled
$ rustc -O -g --crate-type=lib --emit=link --emit=obj -C target-feature=-sse --crate-name core lib.rs
LLVM ERROR: SSE register return with SSE disabled
$ rustc -O -g --crate-type=lib --emit=link --emit=obj -C target-feature=-sse2 --crate-name core lib.rs
LLVM ERROR: SSE2 register return with SSE2 disabled

the command above without those three disabled works for me

$ rustc --crate-type=lib --emit=link -O -C soft-float -C target-feature=-sse3,-ssse3,-sse4.1,-sse4.2,-3dnow,-3dnowa,-avx,-avx2 -g --emit=obj -C soft-float -C no-vectorize-slp -C no-vectorize-loops --crate-name core lib.rs

I haven't tested whether the resulting object file is functional.

$ rustc --version --verbose

rustc 1.3.0-dev (40db46c 2015-06-29)
binary: rustc
commit-hash: 40db46c
commit-date: 2015-06-29
host: x86_64-unknown-linux-gnu
release: 1.3.0-dev

[eternaleye]

To explain somewhat:

The x86_64 ABI states that it is mandatory for floating-point return values to be passed in SSE registers. On one level, this makes an enormous amount of sense: the 387 FPU being a weird 80-bit thing causes real issues, and every x86_64 CPU possesses SSE2.

On the other hand, LLVM doesn't offer any functionality along the lines of GCC's -mfpmath to override this behavior. At least at one point, adding +soft-float to the target fixed the issue, but I've heard that no longer does the trick.

[thepowersgang]

And to state why one would want to disable SSE etc. When writing kernel-mode code, it is very undesirable to touch the floating-point or vector units, as they both require explicit initialization, and have very large register states that would have to be saved on all context switches.

[eternaleye]

Personally, I'd actually like the idea of splitting libcore into float-free and float-using parts, as there are embedded CPUs with no FPU that are also of interest.

[gz]

Just adding a feature to disable floating point related functions/types etc. should be sufficient for architectures that do not support it or codebases that do not want to use it, right?

[thepowersgang]

@gz Adding a cfg() flag that disables compilation of the floating-point functions did allow libcore to be compiled with SSE etc disabled. See https://github.com/thepowersgang/rust_os/blob/master/libcore_nofp.patch for a patch on for the latest nightly.

I'm not sure what the best long-run solution would be, but probably having a #[cfg] flag is best (and making sure it gets tested correctly).

As a related note - Could that LLVM error message be changed to include the offending function? (I'm not sure where such a change would happen, in the rust branch, or in upstream LLVM)

[gz]

FYI I submitted a pull request for this: #26749

[gz]

FYI: https://llvm.org/bugs/show_bug.cgi?id=23203