Reduce float C99 CRT requirements

[seritools]

Continuing from #36

rust-lang#137578 lets core provide its own impls for various float math functions, removing the need for C99-level CRT (see this comment) for these.

Might want to just enable this when compiling for rust9x.

[seritools]

Oh, since Rust provides these as weakly-linked symbols already, it might just work™ after rebasing!

[seritools]

Oh, they are actually not available on windows :/

[stevefan1999-personal]

https://github.com/rust-lang/compiler-builtins/tree/main/libm/src/math
maybe useful

[stevefan1999-personal]

Another way to do this is like zig cc where the Zig author actually implemented a bunch of C99 math functions in Zig

[stevefan1999-personal]

Oh, they are actually not available on windows :/

In fact the intrinsic macro def is just right here: https://github.com/rust-lang/compiler-builtins/blob/4ce6a08da214ed1e0ea2da91baf46f9423785493/compiler-builtins/src/macros.rs#L52-L486

And from the looks of it, just try to find some way to expand what this is doing here: https://github.com/rust-lang/compiler-builtins/blob/82a32c6bd1b82b55de5aea0cddb707732b54855f/compiler-builtins/src/math/mod.rs#L46-L78

Then we can just export this as C function and implement it using libm, and let user use extern crate to force import the symbol as they wish...

[stevefan1999-personal]

https://crates.io/crates/externc-libm

Some one did that but the repo seems to be dead

[seritools]

yeah, excluding target_family = "rust9x" from rust-lang/compiler-builtins@82a32c6/compiler-builtins/src/math/mod.rs#L46-L78 cfg might work, and also enabling the i586 ones that are implemented here https://github.com/rust-lang/compiler-builtins/blob/main/libm/src/math/arch/i586.rs

[seritools]

A better way might be to provide the __use_sse2_mathfcns and __isa_available globals that usually get initialized by modern-ish msvcrt/libucrt versions, initialize them ourselves, then statically link against a modern libucrt.lib.

That way we wouldn't use x87 instructions on modern CPUs and still support old ones at the same time. One additional problem is that the OS also needs SSE store/restore support in the task scheduler/switcher (98SE+).

OTOH, an x87-only path might be fine for the i586 target. Lots of pros and cons for any solution, which is why this issue is still open :)

[repairman29]

Hi! I'm interested in contributing to this. Let me know if you'd like to discuss the implementation approach.

(Connected through Echeo)

[seritools]

Feel free to post any ideas, so far I don't see a good way that works well for all platforms.