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Ralf Jung 2025-01-27 20:17:39 +01:00
parent 93ee180cfa
commit 3f6ffa1462
1 changed files with 11 additions and 13 deletions
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24
compiler/rustc_target/src/target_features.rs
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@ -108,21 +108,19 @@ impl Stability {
// per-function level, since we would then allow safe calls from functions with `+soft-float` to
// functions without that feature!
//
// It is important for soundness that features allowed here do *not* change the function call ABI.
// For example, disabling the `x87` feature on x86 changes how scalar floats are passed as
// arguments, so enabling toggling that feature would be unsound. In fact, since `-Ctarget-feature`
// will just allow unknown features (with a warning), we have to explicitly list features that change
// the ABI as `Forbidden` to ensure using them causes an error. Note that this is only effective if
// such features can never be toggled via `-Ctarget-cpu`! If that is ever a possibility, we will need
// extra checks ensuring that the LLVM-computed target features for a CPU did not (un)set a
// `Forbidden` feature. See https://github.com/rust-lang/rust/issues/116344 for some more context.
// FIXME: add such "forbidden" features for non-x86 targets.
// It is important for soundness to consider the interaction of targets features and the function
// call ABI. For example, disabling the `x87` feature on x86 changes how scalar floats are passed as
// arguments, so letting people toggle that feature would be unsound. To this end, the
// `abi_required_features` function computes which target features must and must not be enabled for
// any given target, and individual features can also be marked as `Forbidden`.
// See https://github.com/rust-lang/rust/issues/116344 for some more context.
//
// The one exception to features that change the ABI is features that enable larger vector
// registers. Those are permitted to be listed here. This is currently unsound (see
// https://github.com/rust-lang/rust/issues/116558); in the future we will have to ensure that
// functions can only use such vectors as arguments/return types if the corresponding target feature
// is enabled.
// registers. Those are permitted to be listed here. The `*_FOR_CORRECT_VECTOR_ABI` arrays store
// information about which target feature is ABI-required for which vector size; this is used to
// ensure that vectors can only be passed via `extern "C"` when the right feature is enabled. (For
// the "Rust" ABI we generally pass vectors by-ref exactly to avoid these issues.)
// Also see https://github.com/rust-lang/rust/issues/116558.
//
// Stabilizing a target feature requires t-lang approval.