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mark some target features as 'forbidden' so they cannot be (un)set with -Ctarget-feature #129884
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rustbot has assigned @petrochenkov. Use |
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Some changes occurred in compiler/rustc_codegen_gcc |
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Some changes occurred in tests/ui/check-cfg cc @Urgau |
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@@ -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 |
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ARM targets also have a soft-float
target feature. Aarch64 does not, though -- so we can't just add this for all targets, unfortunately.
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Aarch64 has abi: "softfloat"
:
abi: "softfloat".into(), |
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That seems like it cannot be changed with -Ctarget-feature
, which is good news.
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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 |
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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. |
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. |
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In fact rustc already checks that the content of the target spec file matches for custom target specs ever since #98225. |
AFAICT, on x86(-64), |
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The problematic case is 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. :) |
☔ The latest upstream changes (presumably #130473) made this pull request unmergeable. Please resolve the merge conflicts. |
…w a reason why it is forbidden
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MCP in rust-lang/compiler-team#780 has passed. The PR now does the conservative thing of emitting a warning rather than a hard error, pointing to #116344. @petrochenkov this is ready for review. |
@rustbot ready |
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Turns out fpregs is sometimes needed on ARM when -Ctarget-cpu enables too many things on a soft-float target, and we need to cut things back down to not have the FPU. This is sound as the target is +soft-float so toggling fpregs does not change the ABI -- but toggling fpregs on an eabihf target is not sound!
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Turns out ARM So we'll need to find a way to allow |
☔ The latest upstream changes (presumably #131511) made this pull request unmergeable. Please resolve the merge conflicts. |
Since we haven't heard from the reviewer in a while |
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
andx87
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