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 and ARM feature soft-float is on that list. We'll have to make some effort to collect more relevant features, and similar features from other targets, but that can happen after the basic infrastructure for this landed. (These features are being collected in #131799.)

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. :)

For now, this is just a warning, but should become a hard error in the future

RalfJung · 2024-11-04T21:57:19Z

@bors r=workingjubilee

bors · 2024-11-04T21:57:22Z

📌 Commit ffad9aa has been approved by workingjubilee

It is now in the queue for this repository.

…, r=workingjubilee mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature The context for this is rust-lang#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 and ARM feature `soft-float` is on that list. We'll have to make some effort to collect more relevant features, and similar features from other targets, but that can happen after the basic infrastructure for this landed. (These features are being collected in rust-lang#131799.) 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

…kingjubilee Rollup of 10 pull requests Successful merges: - rust-lang#129884 (mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature) - rust-lang#132153 (Stabilise `const_char_encode_utf16`.) - rust-lang#132473 ([core/fmt] Replace checked slice indexing by unchecked to support panic-free code) - rust-lang#132571 (add const_eval_select macro to reduce redundancy) - rust-lang#132587 (Revert "Avoid nested replacement ranges" from rust-lang#129346.) - rust-lang#132596 ([rustdoc] Fix `--show-coverage` when JSON output format is used) - rust-lang#132598 (Clippy: Move some attribute lints to be early pass (post expansion)) - rust-lang#132601 (Update books) - rust-lang#132606 (Improve example of `impl Pattern for &[char]`) - rust-lang#132609 (docs: fix grammar in doc comment at unix/process.rs) r? `@ghost` `@rustbot` modify labels: rollup

…r=workingjubilee mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature The context for this is rust-lang#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 and ARM feature `soft-float` is on that list. We'll have to make some effort to collect more relevant features, and similar features from other targets, but that can happen after the basic infrastructure for this landed. (These features are being collected in rust-lang#131799.) 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

bors · 2024-11-05T00:04:28Z

⌛ Testing commit ffad9aa with merge 4990a69...

rust-log-analyzer · 2024-11-05T02:04:08Z

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

bors · 2024-11-05T02:04:46Z

💔 Test failed - checks-actions

RalfJung · 2024-11-05T07:18:20Z

The musl job timed out?

@bors retry

workingjubilee · 2024-11-05T07:39:54Z

@bors rollup=iffy

bors · 2024-11-05T16:25:49Z

⌛ Testing commit ffad9aa with merge e8c698b...

bors · 2024-11-05T18:57:53Z

☀️ Test successful - checks-actions
Approved by: workingjubilee
Pushing e8c698b to master...

rust-timer · 2024-11-05T20:16:17Z

Finished benchmarking commit (e8c698b): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.3%	[1.3%, 1.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -0.9%, secondary -1.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.9%	[-0.9%, -0.9%]	1
Improvements ✅ (secondary)	-1.5%	[-2.0%, -1.1%]	6
All ❌✅ (primary)	-0.9%	[-0.9%, -0.9%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 780.065s -> 779.96s (-0.01%)
Artifact size: 335.31 MiB -> 335.30 MiB (-0.00%)

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

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RalfJung force-pushed the forbidden-target-features branch from b67a656 to 3e44287 Compare September 2, 2024 15:50

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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RalfJung force-pushed the forbidden-target-features branch from 3e44287 to 098469e Compare September 2, 2024 16:50

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

ffad9aa

For now, this is just a warning, but should become a hard error in the future

RalfJung force-pushed the forbidden-target-features branch from ebde449 to ffad9aa Compare November 4, 2024 21:56

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Rollup of 10 pull requests #132616

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Some -Ctarget-features must be restrained on RISCV #132618

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

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

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

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Overall result: ❌ regressions - no action needed

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