mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature #129884

RalfJung · 2024-09-02T10:09:57Z

The context for this is #116344: some target features change the way floats are passed between functions. Changing those target features is unsound as code compiled for the same target may now use different ABIs.

So this introduces a new concept of "forbidden" target features (on top of the existing "stable " and "unstable" categories), and makes it a hard error to (un)set such a target feature. For now, the x86 features soft-float and x87 are on that list. We'll have to make some effort to collect similar features from other targets, but that can happen after the basic infrastructure for this landed.

I've made this a warning for now to give people some time to speak up if this would break something.

MCP: rust-lang/compiler-team#780

rustbot · 2024-09-02T10:10:05Z

r? @petrochenkov

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rustbot · 2024-09-02T11:54:48Z

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

rustbot · 2024-09-02T13:04:26Z

Some changes occurred in tests/ui/check-cfg

cc @Urgau

compiler/rustc_middle/src/query/mod.rs

compiler/rustc_target/src/target_features.rs

compiler/rustc_codegen_llvm/src/llvm_util.rs

RalfJung · 2024-09-02T14:22:14Z

compiler/rustc_target/src/target_features.rs

@@ -328,6 +355,7 @@ const X86_ALLOWED_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
 ("sha512", Unstable(sym::sha512_sm_x86), &["avx2"]),
 ("sm3", Unstable(sym::sha512_sm_x86), &["avx"]),
 ("sm4", Unstable(sym::sha512_sm_x86), &["avx2"]),
+ ("soft-float", Forbidden, &[]), // changes float ABI


ARM targets also have a soft-float target feature. Aarch64 does not, though -- so we can't just add this for all targets, unfortunately.

Aarch64 has abi: "softfloat":

rust/compiler/rustc_target/src/spec/targets/aarch64_unknown_none_softfloat.rs

Line 15 in 9b82580

abi: "softfloat".into(),

That seems like it cannot be changed with -Ctarget-feature, which is good news.

VorpalBlade · 2024-09-02T15:02:07Z

How does this affect kernel / bare metal builds on x86 where you don't enable any floating point support whatsoever? I know the Linux kernel uses that. I'm not sure what the status for Redox, etc is.

RalfJung · 2024-09-02T15:28:28Z

How does this affect kernel / bare metal builds on x86 where you don't enable any floating point support whatsoever? I know the Linux kernel uses that. I'm not sure what the status for Redox, etc is.

We have a x86_64-unknown-none soft-float target for them. I sure hope they don't use some other target and -Ctarget-features=+soft-float.

bjorn3 · 2024-09-02T16:11:05Z

We have a x86_64-unknown-none soft-float target for them. I sure hope they don't use some other target and -Ctarget-features=+soft-float.

The kernel currently has a script generating a target spec for x86: https://github.com/Rust-for-Linux/linux/blob/rust-next/scripts/generate_rust_target.rs Arm64, riscv and loongarch use builtin bare-metal targets though.

RalfJung · 2024-09-02T16:13:24Z

Yeah anyone using custom targets can still set these target features in the target spec. I assume it is generally understood that code using different custom target specs cannot be linked together, so the same concerns do not apply there.

bjorn3 · 2024-09-02T16:18:38Z

I assume it is generally understood that code using different custom target specs cannot be linked together

In fact rustc already checks that the content of the target spec file matches for custom target specs ever since #98225.

beetrees · 2024-09-02T16:35:58Z

The one exception is that it may be the case that on a target with +soft-float, it is okay to enable x87 as floats will still be returned in regular registers... I haven't checked this, so I am not sure. I would say this seems sufficiently niche that it's okay to reject this for now, and if the need really comes up we'll have to add a special case.

AFAICT, on x86(-64), soft-float completely disables use of floating-point registers and always uses software floating point operations regardless of which other target features are enabled, meaning that it's safe to enable the x87 feature (and other floating-point features) but completely useless (e.g. if you enable the sse feature and try to use an xmm_reg input/output in inline ASM, LLVM will emit rustc-LLVM ERROR: Do not know how to soften this operator's operand!).

RalfJung · 2024-09-02T16:51:35Z

AFAICT, on x86(-64), soft-float completely disables use of floating-point registers and always uses software floating point operations regardless of which other target features are enabled, meaning that it's safe to enable the x87 feature (and other floating-point features) but completely useless (e.g. if you enable the sse feature and try to use an xmm_reg input/output in inline ASM, LLVM will emit rustc-LLVM ERROR: Do not know how to soften this operator's operand!).

The problematic case is -x87 on what is usually a hardfloat target -- that will change ABI. So we have to make x87 "forbidden".

We could make it "forbidden to disable but not forbidden to enable", but if you're saying that enabling it is useless anyway then I suggest we go with "forbidden" now. :)

Urgau · 2024-09-02T18:06:32Z

compiler/rustc_target/src/target_features.rs

+ /// This feature can not be set via `-Ctarget-feature` or `#[target_feature]`, it can only be set in the basic
+ /// target definition. Used in particular for features that change the floating-point ABI.
+ Forbidden,


It's written that those target features cannot be set (-Ctarget-feature or #[target_feature]), but can they be conditional used (in #[cfg]) ?

In llvm_util.rs code has been added to not print them with --print target-features, yet the check-cfg tests output now show that soft-float, fpregs and x87 as possible values for #[cfg(target_feature = "...")], which seems inconsistent.

If they are not supposed to be "used", the same filtering should be applied here.

rust/compiler/rustc_session/src/config/cfg.rs

Lines 373 to 376 in a460185

rustc_target::target_features::all_known_features()

.map(|(f, _sb)| f)

.chain(rustc_target::target_features::RUSTC_SPECIFIC_FEATURES.iter().cloned())

.map(Symbol::intern),

It's written that those target features cannot be set (-Ctarget-feature or #[target_feature]), but can they be conditional used (in #[cfg]) ?

That's a good question. The soft-float feature actually does differ between soft-float and hard-float targets, so I guess it makes sense to have it available in cfg?

I guess the question is whether --print target-features should print what can be set with -Ctarget-feature, or print what can be queried with cfg.

Being able to cfg on various features seems essential, to be able to conditionally use different algorithms (e.g. fixed point vs float) or even to determine if certain compiler intrinsics should be provided to implement soft float or if they are unneeded (especially on embedded).

(I have no opinion on the output of print, if it is made for human consumption rather than machine readable you could perhaps add a (read only, determined by target tuple) after the entry?)

I'm not sure that soft-float should be exposed as #[cfg(target_feature = "soft-float")]: it's the inverse of a normal target feature in that enabling it disables functionality (maybe Rust should have a hard-float target feature that's the inverse of the soft-float LLVM feature?).

I'm also fine with not exposing them in cfg for now. That's the more conservative choice as it preserves the status quo.

rustbot · 2024-09-03T06:58:44Z

Some changes occurred in cfg and check-cfg configuration

cc @Urgau

petrochenkov · 2024-09-04T16:25:53Z

Blocked on rust-lang/compiler-team#780.
@rustbot blocked

bors · 2024-09-12T07:48:58Z

☔ The latest upstream changes (presumably #117465) made this pull request unmergeable. Please resolve the merge conflicts.

…again and again

RalfJung · 2024-09-12T18:45:18Z

compiler/rustc_codegen_ssa/src/target_features.rs

+ .emit();
+ }
+ Stability::Forbidden { reason } => {
+ tcx.dcx().emit_err(errors::ForbiddenTargetFeature {


This code here is only used for #[target_features] (the attribute, not the -C flag), where unknown features are a hard error. Thus forbidden features should also be immediately a hard error here, this is not a breaking change.

…w a reason why it is forbidden

bors · 2024-09-17T20:25:15Z

☔ The latest upstream changes (presumably #130473) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot assigned petrochenkov Sep 2, 2024

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The ABI of float types on can be changed by -Ctarget-feature #116344

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RalfJung changed the title ~~mark some target features as 'forbidden' so they cannot be (un)set~~ mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature Sep 2, 2024

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mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature rust-lang/compiler-team#780

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RalfJung added 7 commits September 12, 2024 19:57

mark some target features as 'forbidden' so they cannot be (un)set

b190bd9

make target_feature cfg computation not iterate over the same vector …

0ec1af6

…again and again

fix GCC backend

92b849c

rename known_target_features → rust_target_features

1ea42d3

also ban soft-float target features on ARM

2ec471c

do not add forbidden features to cfg

b1db7ea

extend comments for RUSTC_SPECIFIC_FEATURES and RUSTC_SPECIAL_FEATURES

a69de2c

RalfJung force-pushed the forbidden-target-features branch 2 times, most recently from 62e2638 to 0a1bba2 Compare September 12, 2024 18:33

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Retpoline support in rustc (target features or dedicated flags) #116852

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make this just a warning (for use with the -C flag) for now; also sho…

691bcaa

…w a reason why it is forbidden

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mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature #129884

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	rustc_target::target_features::all_known_features()
	.map(\|(f, _sb)\| f)
	.chain(rustc_target::target_features::RUSTC_SPECIFIC_FEATURES.iter().cloned())
	.map(Symbol::intern),

mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature #129884

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mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature #129884

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