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Implement a first version of RFC 3525: struct target features

This PR is an attempt at implementing rust-lang/rfcs#3525, behind a feature gate `struct_target_features`.

There's obviously a few tasks that ought to be done before this is merged; in no particular order:
- add proper error messages
- add tests
- create a tracking issue for the RFC
- properly serialize/deserialize the new target_features field in `rmeta` (assuming I even understood that correctly :-))

That said, as I am definitely not a `rustc` expert, I'd like to get some early feedback on the overall approach before fixing those things (and perhaps some pointers for `rmeta`...), hence this early PR :-)

Here's an example piece of code that I have been using for testing - with the new code, the calls to intrinsics get correctly inlined:
```rust
#![feature(struct_target_features)]

use std::arch::x86_64::*;

/*
// fails to compile
#[target_feature(enable = "avx")]
struct Invalid(u32);
*/

#[target_feature(enable = "avx")]
struct Avx {}

#[target_feature(enable = "sse")]
struct Sse();

/*
// fails to compile
extern "C" fn bad_fun(_: Avx) {}
*/

/*
// fails to compile
#[inline(always)]
fn inline_fun(_: Avx) {}
*/

trait Simd {
    fn do_something(&self);
}

impl Simd for Avx {
    fn do_something(&self) {
        unsafe {
            println!("{:?}", _mm256_setzero_ps());
        }
    }
}

impl Simd for Sse {
    fn do_something(&self) {
        unsafe {
            println!("{:?}", _mm_setzero_ps());
        }
    }
}

struct WithAvx {
    #[allow(dead_code)]
    avx: Avx,
}

impl Simd for WithAvx {
    fn do_something(&self) {
        unsafe {
            println!("{:?}", _mm256_setzero_ps());
        }
    }
}

#[inline(never)]
fn dosomething<S: Simd>(simd: &S) {
    simd.do_something();
}

fn main() {
    /*
    // fails to compile
    Avx {};
    */

    if is_x86_feature_detected!("avx") {
        let avx = unsafe { Avx {} };
        dosomething(&avx);
        dosomething(&WithAvx { avx });
    }
    if is_x86_feature_detected!("sse") {
        dosomething(&unsafe { Sse {} })
    }
}
```

Tracking:

- rust-lang#129107
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bors committed Aug 28, 2024
2 parents 100fde5 + 7eb4cfe commit acb4e8b
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Showing 25 changed files with 511 additions and 27 deletions.
125 changes: 115 additions & 10 deletions compiler/rustc_codegen_ssa/src/codegen_attrs.rs
Original file line number Diff line number Diff line change
@@ -1,5 +1,6 @@
use rustc_ast::{ast, attr, MetaItemKind, NestedMetaItem};
use rustc_attr::{list_contains_name, InlineAttr, InstructionSetAttr, OptimizeAttr};
use rustc_data_structures::fx::FxHashSet;
use rustc_errors::codes::*;
use rustc_errors::{struct_span_code_err, DiagMessage, SubdiagMessage};
use rustc_hir as hir;
Expand All @@ -8,7 +9,7 @@ use rustc_hir::def_id::{DefId, LocalDefId, LOCAL_CRATE};
use rustc_hir::weak_lang_items::WEAK_LANG_ITEMS;
use rustc_hir::{lang_items, LangItem};
use rustc_middle::middle::codegen_fn_attrs::{
CodegenFnAttrFlags, CodegenFnAttrs, PatchableFunctionEntry,
CodegenFnAttrFlags, CodegenFnAttrs, PatchableFunctionEntry, TargetFeature,
};
use rustc_middle::mir::mono::Linkage;
use rustc_middle::query::Providers;
Expand All @@ -17,6 +18,7 @@ use rustc_session::lint;
use rustc_session::parse::feature_err;
use rustc_span::symbol::Ident;
use rustc_span::{sym, Span};
use rustc_target::abi::VariantIdx;
use rustc_target::spec::{abi, SanitizerSet};

use crate::errors;
Expand Down Expand Up @@ -78,23 +80,26 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
let mut link_ordinal_span = None;
let mut no_sanitize_span = None;

let fn_sig_outer = || {
use DefKind::*;

let def_kind = tcx.def_kind(did);
if let Fn | AssocFn | Variant | Ctor(..) = def_kind { Some(tcx.fn_sig(did)) } else { None }
};

for attr in attrs.iter() {
// In some cases, attribute are only valid on functions, but it's the `check_attr`
// pass that check that they aren't used anywhere else, rather this module.
// In these cases, we bail from performing further checks that are only meaningful for
// functions (such as calling `fn_sig`, which ICEs if given a non-function). We also
// report a delayed bug, just in case `check_attr` isn't doing its job.
let fn_sig = || {
use DefKind::*;

let def_kind = tcx.def_kind(did);
if let Fn | AssocFn | Variant | Ctor(..) = def_kind {
Some(tcx.fn_sig(did))
} else {
let sig = fn_sig_outer();
if sig.is_none() {
tcx.dcx()
.span_delayed_bug(attr.span, "this attribute can only be applied to functions");
None
}
sig
};

let Some(Ident { name, .. }) = attr.ident() else {
Expand Down Expand Up @@ -613,7 +618,93 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
}
}

// If a function uses #[target_feature] it can't be inlined into general
if let Some(sig) = fn_sig_outer() {
// Collect target features from types reachable from arguments.
// We define a type as "reachable" if:
// - it is a function argument
// - it is a field of a reachable struct
// - there is a reachable reference to it
// FIXME(struct_target_features): we may want to cache the result of this computation.
let mut visited_types = FxHashSet::default();
let mut reachable_types: Vec<_> = sig.skip_binder().inputs().skip_binder().to_owned();
let mut additional_tf = vec![];

while let Some(ty) = reachable_types.pop() {
if visited_types.contains(&ty) {
continue;
}
visited_types.insert(ty);
match ty.kind() {
ty::Alias(..) => {
if let Ok(t) =
tcx.try_normalize_erasing_regions(tcx.param_env(did.to_def_id()), ty)
{
reachable_types.push(t)
}
}

ty::Ref(_, inner, _) => reachable_types.push(*inner),
ty::Tuple(tys) => reachable_types.extend(tys.iter()),
ty::Adt(adt_def, args) => {
additional_tf.extend_from_slice(tcx.struct_target_features(adt_def.did()));
// This only recurses into structs as i.e. an Option<TargetFeature> is an ADT
// that doesn't actually always contain a TargetFeature.
if adt_def.is_struct() {
reachable_types.extend(
adt_def
.variant(VariantIdx::from_usize(0))
.fields
.iter()
.map(|field| field.ty(tcx, args)),
);
}
}
ty::Bool
| ty::Char
| ty::Int(..)
| ty::Uint(..)
| ty::Float(..)
| ty::Foreign(..)
| ty::Str
| ty::Array(..)
| ty::Pat(..)
| ty::Slice(..)
| ty::RawPtr(..)
| ty::FnDef(..)
| ty::FnPtr(..)
| ty::Dynamic(..)
| ty::Closure(..)
| ty::CoroutineClosure(..)
| ty::Coroutine(..)
| ty::CoroutineWitness(..)
| ty::Never
| ty::Param(..)
| ty::Bound(..)
| ty::Placeholder(..)
| ty::Infer(..)
| ty::Error(..) => (),
}
}

// FIXME(struct_target_features): is this really necessary?
if !additional_tf.is_empty() && sig.skip_binder().abi() != abi::Abi::Rust {
tcx.dcx().span_err(
tcx.hir().span(tcx.local_def_id_to_hir_id(did)),
"cannot use a struct with target features in a function with non-Rust ABI",
);
}
if !additional_tf.is_empty() && codegen_fn_attrs.inline == InlineAttr::Always {
tcx.dcx().span_err(
tcx.hir().span(tcx.local_def_id_to_hir_id(did)),
"cannot use a struct with target features in a #[inline(always)] function",
);
}
codegen_fn_attrs
.target_features
.extend(additional_tf.iter().map(|tf| TargetFeature { implied: true, ..*tf }));
}

// If a function uses non-default target_features it can't be inlined into general
// purpose functions as they wouldn't have the right target features
// enabled. For that reason we also forbid #[inline(always)] as it can't be
// respected.
Expand Down Expand Up @@ -758,6 +849,20 @@ fn check_link_name_xor_ordinal(
}
}

fn struct_target_features(tcx: TyCtxt<'_>, def_id: LocalDefId) -> &[TargetFeature] {
let mut features = vec![];
let supported_features = tcx.supported_target_features(LOCAL_CRATE);
for attr in tcx.get_attrs(def_id, sym::target_feature) {
from_target_feature(tcx, attr, supported_features, &mut features);
}
tcx.arena.alloc_slice(&features)
}

pub fn provide(providers: &mut Providers) {
*providers = Providers { codegen_fn_attrs, should_inherit_track_caller, ..*providers };
*providers = Providers {
codegen_fn_attrs,
should_inherit_track_caller,
struct_target_features,
..*providers
};
}
2 changes: 2 additions & 0 deletions compiler/rustc_feature/src/unstable.rs
Original file line number Diff line number Diff line change
Expand Up @@ -594,6 +594,8 @@ declare_features! (
(unstable, strict_provenance, "1.61.0", Some(95228)),
/// Allows string patterns to dereference values to match them.
(unstable, string_deref_patterns, "1.67.0", Some(87121)),
/// Allows structs to carry target_feature information.
(incomplete, struct_target_features, "CURRENT_RUSTC_VERSION", Some(129107)),
/// Allows the use of `#[target_feature]` on safe functions.
(unstable, target_feature_11, "1.45.0", Some(69098)),
/// Allows using `#[thread_local]` on `static` items.
Expand Down
35 changes: 35 additions & 0 deletions compiler/rustc_hir/src/def.rs
Original file line number Diff line number Diff line change
Expand Up @@ -326,6 +326,41 @@ impl DefKind {
| DefKind::ExternCrate => false,
}
}

/// Whether `query struct_target_features` should be used with this definition.
pub fn has_struct_target_features(self) -> bool {
match self {
DefKind::Struct | DefKind::Union | DefKind::Enum => true,
DefKind::Fn
| DefKind::AssocFn
| DefKind::Ctor(..)
| DefKind::Closure
| DefKind::Static { .. }
| DefKind::Mod
| DefKind::Variant
| DefKind::Trait
| DefKind::TyAlias
| DefKind::ForeignTy
| DefKind::TraitAlias
| DefKind::AssocTy
| DefKind::Const
| DefKind::AssocConst
| DefKind::Macro(..)
| DefKind::Use
| DefKind::ForeignMod
| DefKind::OpaqueTy
| DefKind::Impl { .. }
| DefKind::Field
| DefKind::TyParam
| DefKind::ConstParam
| DefKind::LifetimeParam
| DefKind::AnonConst
| DefKind::InlineConst
| DefKind::SyntheticCoroutineBody
| DefKind::GlobalAsm
| DefKind::ExternCrate => false,
}
}
}

/// The resolution of a path or export.
Expand Down
2 changes: 2 additions & 0 deletions compiler/rustc_hir_typeck/src/coercion.rs
Original file line number Diff line number Diff line change
Expand Up @@ -850,6 +850,8 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
}

// Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).
// FIXME(struct_target_features): should this be true also for functions that inherit
// target features from structs?

if b_hdr.safety == hir::Safety::Safe
&& !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
Original file line number Diff line number Diff line change
Expand Up @@ -224,6 +224,7 @@ provide! { tcx, def_id, other, cdata,
variances_of => { table }
fn_sig => { table }
codegen_fn_attrs => { table }
struct_target_features => { table }
impl_trait_header => { table }
const_param_default => { table }
object_lifetime_default => { table }
Expand Down
3 changes: 3 additions & 0 deletions compiler/rustc_metadata/src/rmeta/encoder.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1392,6 +1392,9 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
if def_kind.has_codegen_attrs() {
record!(self.tables.codegen_fn_attrs[def_id] <- self.tcx.codegen_fn_attrs(def_id));
}
if def_kind.has_struct_target_features() {
record_array!(self.tables.struct_target_features[def_id] <- self.tcx.struct_target_features(def_id));
}
if should_encode_visibility(def_kind) {
let vis =
self.tcx.local_visibility(local_id).map_id(|def_id| def_id.local_def_index);
Expand Down
3 changes: 2 additions & 1 deletion compiler/rustc_metadata/src/rmeta/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -19,7 +19,7 @@ use rustc_macros::{
Decodable, Encodable, MetadataDecodable, MetadataEncodable, TyDecodable, TyEncodable,
};
use rustc_middle::metadata::ModChild;
use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrs;
use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrs, TargetFeature};
use rustc_middle::middle::debugger_visualizer::DebuggerVisualizerFile;
use rustc_middle::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo};
use rustc_middle::middle::lib_features::FeatureStability;
Expand Down Expand Up @@ -427,6 +427,7 @@ define_tables! {
variances_of: Table<DefIndex, LazyArray<ty::Variance>>,
fn_sig: Table<DefIndex, LazyValue<ty::EarlyBinder<'static, ty::PolyFnSig<'static>>>>,
codegen_fn_attrs: Table<DefIndex, LazyValue<CodegenFnAttrs>>,
struct_target_features: Table<DefIndex, LazyArray<TargetFeature>>,
impl_trait_header: Table<DefIndex, LazyValue<ty::ImplTraitHeader<'static>>>,
const_param_default: Table<DefIndex, LazyValue<ty::EarlyBinder<'static, rustc_middle::ty::Const<'static>>>>,
object_lifetime_default: Table<DefIndex, LazyValue<ObjectLifetimeDefault>>,
Expand Down
8 changes: 4 additions & 4 deletions compiler/rustc_middle/src/middle/codegen_fn_attrs.rs
Original file line number Diff line number Diff line change
Expand Up @@ -26,8 +26,8 @@ pub struct CodegenFnAttrs {
/// be set when `link_name` is set. This is for foreign items with the
/// "raw-dylib" kind.
pub link_ordinal: Option<u16>,
/// The `#[target_feature(enable = "...")]` attribute and the enabled
/// features (only enabled features are supported right now).
/// All the target features that are enabled for this function. Some features might be enabled
/// implicitly.
pub target_features: Vec<TargetFeature>,
/// The `#[linkage = "..."]` attribute on Rust-defined items and the value we found.
pub linkage: Option<Linkage>,
Expand Down Expand Up @@ -55,8 +55,8 @@ pub struct CodegenFnAttrs {
pub struct TargetFeature {
/// The name of the target feature (e.g. "avx")
pub name: Symbol,
/// The feature is implied by another feature, rather than explicitly added by the
/// `#[target_feature]` attribute
/// The feature is implied by another feature or by an argument, rather than explicitly
/// added by the `#[target_feature]` attribute
pub implied: bool,
}

Expand Down
7 changes: 6 additions & 1 deletion compiler/rustc_middle/src/query/mod.rs
Original file line number Diff line number Diff line change
Expand Up @@ -47,7 +47,7 @@ use {rustc_ast as ast, rustc_attr as attr, rustc_hir as hir};
use crate::infer::canonical::{self, Canonical};
use crate::lint::LintExpectation;
use crate::metadata::ModChild;
use crate::middle::codegen_fn_attrs::CodegenFnAttrs;
use crate::middle::codegen_fn_attrs::{CodegenFnAttrs, TargetFeature};
use crate::middle::debugger_visualizer::DebuggerVisualizerFile;
use crate::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo};
use crate::middle::lib_features::LibFeatures;
Expand Down Expand Up @@ -1245,6 +1245,11 @@ rustc_queries! {
feedable
}

query struct_target_features(def_id: DefId) -> &'tcx [TargetFeature] {
separate_provide_extern
desc { |tcx| "computing target features for struct `{}`", tcx.def_path_str(def_id) }
}

query asm_target_features(def_id: DefId) -> &'tcx FxIndexSet<Symbol> {
desc { |tcx| "computing target features for inline asm of `{}`", tcx.def_path_str(def_id) }
}
Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_middle/src/ty/parameterized.rs
Original file line number Diff line number Diff line change
Expand Up @@ -59,6 +59,7 @@ trivially_parameterized_over_tcx! {
std::string::String,
crate::metadata::ModChild,
crate::middle::codegen_fn_attrs::CodegenFnAttrs,
crate::middle::codegen_fn_attrs::TargetFeature,
crate::middle::debugger_visualizer::DebuggerVisualizerFile,
crate::middle::exported_symbols::SymbolExportInfo,
crate::middle::lib_features::FeatureStability,
Expand Down
16 changes: 16 additions & 0 deletions compiler/rustc_mir_build/messages.ftl
Original file line number Diff line number Diff line change
Expand Up @@ -125,6 +125,17 @@ mir_build_initializing_type_with_requires_unsafe_unsafe_op_in_unsafe_fn_allowed
.note = initializing a layout restricted type's field with a value outside the valid range is undefined behavior
.label = initializing type with `rustc_layout_scalar_valid_range` attr
mir_build_initializing_type_with_target_feature_requires_unsafe =
initializing type with `target_feature` attr is unsafe and requires unsafe block
.note = this struct can only be constructed if the corresponding `target_feature`s are available
.label = initializing type with `target_feature` attr
mir_build_initializing_type_with_target_feature_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
initializing type with `target_feature` attr is unsafe and requires unsafe function or block
.note = this struct can only be constructed if the corresponding `target_feature`s are available
.label = initializing type with `target_feature` attr
mir_build_inline_assembly_requires_unsafe =
use of inline assembly is unsafe and requires unsafe block
.note = inline assembly is entirely unchecked and can cause undefined behavior
Expand Down Expand Up @@ -387,6 +398,11 @@ mir_build_unsafe_op_in_unsafe_fn_initializing_type_with_requires_unsafe =
.note = initializing a layout restricted type's field with a value outside the valid range is undefined behavior
.label = initializing type with `rustc_layout_scalar_valid_range` attr
mir_build_unsafe_op_in_unsafe_fn_initializing_type_with_target_feature_requires_unsafe =
initializing type with `target_feature` attr is unsafe and requires unsafe block
.note = this struct can only be constructed if the corresponding `target_feature`s are available
.label = initializing type with `target_feature` attr
mir_build_unsafe_op_in_unsafe_fn_inline_assembly_requires_unsafe =
use of inline assembly is unsafe and requires unsafe block
.note = inline assembly is entirely unchecked and can cause undefined behavior
Expand Down
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