i686 MinGW32 legacy longdouble build issue

[Barough]

Build attempt is made using media-autobuild_suite

┌ gifski git ............................................ [Updates found]
├ Running rust.update...
├ Running rust.build...
Likely error (tail of the failed operation logfile):
Compiling wasm-bindgen-rayon v1.3.0
error[E0080]: attempt to compute 8_usize - 12_usize, which would overflow
--> D:\m_ab_s\build\gifski-git\target\i686-pc-windows-gnu\release\build\ffmpeg-sys-next-86c818efbefaccc2\out/bindings.rs:3:123503
|
3 | ..."Size of _LONGDOUBLE"] [:: std :: mem :: size_of :: < _LONGDOUBLE > () - 12usize] ; ["Alignment of _LONGDOUBLE"] [:: std :: mem :: ali...
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of _ failed here

For more information about this error, try rustc --explain E0080.
error: could not compile ffmpeg-sys-next (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
rust.build failed. Check D:/m_ab_s/build/gifski-git/ab-suite.rust.build.log
This is required for other packages, so this script will exit.

logs.zip

[kornelski]

The error is from bindgen, which detected a potential ABI mismatch between Rust and C libraries.

My first suggestion would be to get rid of the i686 target and the 32-bit-emulating MinGW, and install everything in 64-bit versions. You're probably using the i686 target by mistake, since the actual computers needing this architecture are in museums, and wouldn't have enough RAM to load a program containing modern ffmpeg. If you don't have particular requirements for using the -gnu targets, I'd also suggest switching to -msvc one that is more native on Windows.

But if you really want take the route of building for retrocomputing CPUs running a half-hearted Windows-Unix hybrid, then you need to set CFLAGS to -mlong-double-64.

The mismatch is caused by i686 target still allowing use of legacy 80-bit floating point types of the x87 coprocessor, which has been obsolete since late 1990s. It was a bad design that made it harder to control fp precision, and emulating it on CPUs produced in the last 20 years makes programs slower. Rust never bothered to support a type that makes everything worse, so you need to pass -mlong-double-64 to C compilers to disable it there too.

I'm not sure if the flag alone will be enough, because the old 32-bit-only environment may have precompiled libraries that use legacy ABIs. Compiling ffmpeg is a challenge even with good compilers on contemporary platforms, and I don't know if 32-bit MinGW is even capable of doing it.