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MIPS delay slot codegen issue #91442
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@rustbot label: +O-mips |
@rustbot label: +A-codegen |
Simplified example: #![no_std]
#![no_main]
#![feature(bench_black_box)]
psp::module!("crash-test",1,0);
fn psp_main() {
let dx = core::hint::black_box(0.0);
let tdx = do_calc(dx);
psp::dprintln!("{}", tdx);
}
#[no_mangle]
#[inline(never)]
fn do_calc(dx: f32) -> f32 {
if dx == 0.0 {
core::f32::MAX
} else {
1.0 / dx
}
}
|
Nevermind, this appears to be a possible problem with the FPU of the PSP doing the comparison to 0, combined with my dodgy understanding of MIPS assembly. |
Followup: The real problem is that MIPS revs <=3 have delay slots for floating point comparison instructions, (such as c.eq.s), and LLVM does not appear to have accounted for this. Sources: |
I tried this code:
and compiled with the official tier 3 target, mipsel-sony-psp, the psp crate, and the cargo-psp crate. This should compile for any other mips target by renaming
psp_main
tomain
, removing#[no_main]
, removingpsp::enable_home_button
, removingpsp::module!
and replacingpsp::dprintln!
withprintln!
. I suspect the problem would be the same on other MIPS targets, PSP is MIPS2 ISA.I expected to see this happen: The if statement should prevent the case of divide by zero
Instead, this happened: The division instruction is scheduled in the delay slot of the jump (2b0), rendering the if statement useless. This leads to a floating point exception for dividing by zero.
Meta
rustc --version --verbose
:I used gdb for backtrace since I can't run RUST_BACKTRACE=1 on PSP.
Backtrace
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