diff options
Diffstat (limited to 'compiler/rustc_target/src/target_features.rs')
| -rw-r--r-- | compiler/rustc_target/src/target_features.rs | 24 |
1 files changed, 11 insertions, 13 deletions
diff --git a/compiler/rustc_target/src/target_features.rs b/compiler/rustc_target/src/target_features.rs index 9fd07c8634a..3bc4b92987f 100644 --- a/compiler/rustc_target/src/target_features.rs +++ b/compiler/rustc_target/src/target_features.rs @@ -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. |