Can't use `f32` transcendental functions (`sin`, `cos`) because lto `fat` is broken in rust 1.66

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The Xous microkernel

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Can't use `f32` transcendental functions (`sin`, `cos`) because lto `fat` is broken in rust 1.66 #285

Open bunnie opened 1 year ago

bunnie commented 1 year ago

Rust 1.66 breaks lto fat due to a panic inside sin()/cos() routines.

This issue is to investigate this further. Two options:

Create pure Rust versions of sin/cos that don't break in LTO
Pull out the entire FFT analysis routine and do it on the tester host side

2 is probably the better outcome but definitely a lot more work and risk on production quality.

bunnie commented 1 year ago

Note: Losing fat and going to thin adds 600kiB to the Xous binary size

bunnie commented 1 year ago

Did some investigations.

[x] see if mold can work with this - yes, but: can't handle custom kernel link.x script (not a feature of mold), requires PIC, which isn't enabled in current std, thus would require a patch to std that could cause it to not be upstreamable. aborting approach. Note build time of services goes from 28s to 20s, so about 30% improvement with mold.
[x] see if gold can work with this - NO, gold is bit-rotted and does not support risc-v
[x] see if ld can work with this xobs found that the bug still exists (see image below)

Seems like we have to wait for llvm to fix this.

Next step: try to excise out the one broken function, or maybe it can be bodged around in some way...

bunnie commented 1 year ago

lto fat restored in 79be209790518d13541e5d0e3c8ec0eedbb00ff7 because I have gone through and eliminated calls to f32::cos() through the entire system and replaced it with a less-accurate, but more compact and more importantly not crashing quadrant table-lookup based cosine implementation.

The llvm issue is still real. Renaming the bug and leaving it open to remind ourselves to revisit the issue someday if the llvm issue (https://github.com/rust-lang/rust/issues/105734) ever gets fixed, especially if users eventually want to use cos(), sin(), etc.