.min and .floor operations for f64 types are non-functional on MIPS

[johnnyman727]

I'm building Rust code for Tessel 2 using some instructions created by @badboy here.

Simple scripts compiled on the Rust playground and my personal computer work fine but fail for different reasons on Tessel (which has a MIPS architecture).

let testVal: f64 = 546.54;
assert!(testVal.floor(), 546); // Returns 546.54 instead
assert!(testVal.min(0)); // Returns 546.54 instead

Let me know if there are any other details I can provide to help debug!

[nagisa]

Output of --emit=llvm-ir,asm (should output two files with .ll and .s extensions) would be useful.

testVal.float()

Contradiction with the issue title. Moreover assert is a macro (you’re missing a !) and assert!(testVal.floor(), 546); wouldn’t compile due various reasons.

[kevinmehall]

I suspect this is due to a mismatch of float ABI (Tessel's OpenWrt uses soft float, while Rust's mipsel-unknown-linux-gnu default target uses hard float).

The following program succeeded on Tessel 2 with a custom target.json. I rebuilt libstd using that target, using a compiler-rt built with the GCC from the OpenWrt build system.

fn main() {
    let test_val: f64 = 546.54;
    assert_eq!(test_val.floor(), 546.0);
    assert_eq!(test_val.min(0.0), 0.0);
    println!("ok");
}

So this isn't really a rust-lang/rust bug, beyond a 👍 for something like RFC 1133 that makes it easier to use customized target options.

[japaric]

@kevinmehall Note that you are mixing different ABIs here: The OpenWRT toolchain is using uclibc and the Rust target (mips*-*-*-gnu) expects glibc. These two libcs are not ABI compatible (actually, uclibc never became ABI stable). This combination is not guaranteed to always work.

(Thought most of the problems you observed are likely caused by the float ABI mismatch.)

[alexcrichton]

Thanks for the investigation @kevinmehall! Sounds like this is expected from ABI differences, so I'm going to close.

[johnnyman727]

Thanks @nagisa for pointing out the syntax/functional issues with my example. I've updated for posterity's sake.

Thanks @kevinmehall for investigating the ABI.