Inconsistent truncation of -0.0

[tgross35]

#![feature(f128, f16)]

use core::hint::black_box;

extern "C" {
    fn __truncdfhf2(a: f64) -> f16;
}

fn main() {
    let a = -0_f64;
    let res_sys: f16 = unsafe { __truncdfhf2(a) };
    let res_llvm: f16 = black_box(a) as f16;
    let res_builtins: f16 = compiler_builtins::float::trunc::__truncdfhf2(a);
    dbg!(res_sys, res_llvm, res_builtins);
}

[src/lib.rs:108:1] res_sys = 0x0000
[src/lib.rs:108:1] res_llvm = 0x0000
[src/lib.rs:108:1] res_builtins = 0x8000

[tgross35]

Ok this is actually the builtin symbol doing the wrong thing. f64 -> f32 keeps the sign for -0, as does rustc_apfloat, so I think compiler-builtins is correct here.

[tgross35]

Even more float conversions seem wrong on some platforms when called directly. E.g. arm-unknown-linux-gnueabihf has the following:

add_f32(0.0, -inf): crate: -inf, sys: NaN
extend_f32_f64(-0.0,): crate: -0.0, sys: -1.0609978955e-314
mul_f32(1e-45, 1e-45): crate: 0.0, sys: NaN
trunc_f64_f32(-0.0,): crate: -0.0, sys: 0.0

I am getting these from the benchmark tests that call external symbols rather than assembly #617

[tgross35]

On x86_64-pc-windows-msvc, x86_64-pc-windows-gnu,

conv_f32_u128(0.0,): crate: 0, sys: 225082651231465353564815136481731410602
conv_f32_i128(0.0,): crate: 0, sys: -115199715689473109898559470950036800854

Interestingly, that test seems to pass on the i686 Windows targets

[Amanieu]

Perhaps this is due to an issue in libgcc's implementation of these functions?

[tgross35]

I suppose so. I need to dig a bit more into what is actually getting linked, I'll open issues in the relevant projects once I can reproduce them in C.

[tgross35]

Few more inconsistencies where it seems like the system symbol is wrong. I guess we are testing against the correct assembly versions, but the named symbols available are wrong.

On i586-unknown-linux-gnu:

add_f128(0x00000000000000000000000000000000, 0x00000000000000000000000000000000): crate: 0x00000000000000000000000000000000, sys: 0x00000001000000040008000000000000
sub_f128(0x00000000000000000000000000000000, 0x00000000000000000000000000000000): crate: 0x00000000000000000000000000000000, sys: 0x80000001000000040008000000000000
powi_f32(1.0000001, -32768): crate: 0.99610233, sys: 0.9961014

On powerpc64le-unknown-linux-gnu:

extend_f32_f128(0.0,): crate: 0x00000000000000000000000000000000, sys: 0x80000000000000000000000000000000

Possibly related to the other ABI problems like llvm/llvm-project#92246.

Also on powerpc64le-unknown-linux-gnu, __fixunssfsi has a shift right overflow panic. I am pretty suspicious of this one, maybe it's just an old qemu version since I get a different error testing locally.

thread 'main' panicked at /rustc/791adf759cc065316f054961875052d5bc03e16c/library/core/src/ops/bit.rs:614:1:
attempt to shift right with overflow
stack backtrace:
qemu: Unsupported syscall: 383
qemu: Unsupported syscall: 383
...
  14:       0x4000523670 - rust_begin_unwind
                               at /rustc/791adf759cc065316f054961875052d5bc03e16c/library/std/src/panicking.rs:652:5
  15:       0x400002cd94 - core::panicking::panic_fmt::hb7734d8d5340d0ff
                               at /rustc/791adf759cc065316f054961875052d5bc03e16c/library/core/src/panicking.rs:72:14
  16:       0x400002d970 - core::panicking::panic_const::panic_const_shr_overflow::h18e0443d138c13b2
                               at /rustc/791adf759cc065316f054961875052d5bc03e16c/library/core/src/panicking.rs:179:21
  17:       0x40004f8adc - <u32 as core::ops::bit::Shr>::shr::h3d32457f86482582
                               at /rustc/791adf759cc065316f054961875052d5bc03e16c/library/core/src/ops/bit.rs:588:17
  18:       0x40004f57e8 - compiler_builtins::float::conv::float_to_int_inner::h20f2990f410a885c
                               at /checkout/src/float/conv.rs:223:24
  19:       0x40004f4318 - compiler_builtins::float::conv::float_to_unsigned_int::h2c46ea36e0a8c0b6
                               at /checkout/src/float/conv.rs:161:5
  20:       0x40004f258c - compiler_builtins::float::conv::__fixunssfsi::h499e282469decc4b
                               at /checkout/src/float/conv.rs:237:9
  21:       0x40000da6f0 - float_conv::conv_f32_u32::crate_fn::h3d40db525d4f4959
                               at /checkout/testcrate/src/bench.rs:123:17
  22:       0x40000da268 - float_conv::conv_f32_u32::h8400d8bbfdc9131d
                               at /checkout/testcrate/src/bench.rs:158:33
  23:       0x40000df4d8 - float_conv::float_conv::h7a3178d7d3386116
                               at /cargo/registry/src/index.crates.io-6f17d22bba15001f/criterion-0.5.1/src/macros.rs:71:17