Reenable the use of the force-soft-floats feature

[antoyo]

This fixes an infinite recursion in sqrt for codegen backends that implement intrinsics like simd_fsqrt by calling sqrt on every element of the vector.

Fix #649

This requires rust-lang/libm#532 to fix the issue, so I'll need to update the submodule when this PR is merged.

[tgross35]

Sorry about the regression here. Could this be temporally fixed by setting the compiler-builtins-no-asm feature when building with cg_gcc https://github.com/rust-lang/rust/blob/d4f880f8ce832cd7560bb2f1ebc34f967055ffd7/library/std/Cargo.toml#L99? I think bootstrap has a way to configure this.

The change in libm was made to move away from subtractive features like force-soft-float because that doesn't work well with Cargo and config was getting complicated (now gone in libm but still exists in compiler-builtins). If something like this is needed, I'd prefer splitting out separate features to enable assembly and to enable the rest of core::arch, rather than adding back force-soft-float. I can do this work, just let me know if no-asm is an option.

[antoyo]

just let me know if no-asm is an option.

Just to make sure, this will disable some optimizations, right?
If so, I'd prefer to not use no-asm since rustc_codegen_gcc supports inline assembly.

[tgross35]

Yeah you're right, that would affect compiler-builtins. It's not a problem in libm since there are no uses of asm! though.

So what might be best is to comment out println!("cargo:rustc-cfg=arch_enabled"); with a FIXME to add this back in the future once libm drops all core::arch calls. This is the plan after the functions are in core (I'm hoping rust-lang/rust#138087 can go to stable after only a couple of cycles).

The existing mention of force-soft-floats can be deleted too, it isn't used in either crate anymore so I guess I forgot to clean it up.

[antoyo]

So what might be best is to comment out println!("cargo:rustc-cfg=arch_enabled"); with a FIXME to add this back in the future once libm drops all core::arch calls.

Ok, do you want me to make a PR for this or will you do it?
I might need more details if you want me to make the PR.

Thanks!

[tgross35]

You can probably just update this PR, something like:

    // The arch module may contain assembly.
    if !cfg!(feature = "no-asm") {
        // FIXME: when part of builtins, `libm` can use assembly but cannot use vector intrinsics
        // from `core::arch` in order to avoid recursion problems. Once basic `libm` functions
        // are available in `core`, we will be able to remove these problematic calls and add back
        // `arch_enabled`.
        // Cc: <https://github.com/rust-lang/compiler-builtins/issues/649>
        // println!("cargo:rustc-cfg=arch_enabled");
    }