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Add #[rustc_no_mir_inline] for standard library UB checks
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Co-authored-by: Ben Kimock <kimockb@gmail.com>
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Noratrieb and saethlin committed Feb 24, 2024
1 parent 933a05b commit 81d7069
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Showing 9 changed files with 107 additions and 3 deletions.
4 changes: 4 additions & 0 deletions compiler/rustc_feature/src/builtin_attrs.rs
Original file line number Diff line number Diff line change
Expand Up @@ -792,6 +792,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
rustc_intrinsic, Normal, template!(Word), ErrorFollowing,
"the `#[rustc_intrinsic]` attribute is used to declare intrinsics with function bodies",
),
rustc_attr!(
rustc_no_mir_inline, Normal, template!(Word), WarnFollowing,
"#[rustc_no_mir_inline] prevents the MIR inliner from inlining a function while not affecting codegen"
),

// ==========================================================================
// Internal attributes, Testing:
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4 changes: 4 additions & 0 deletions compiler/rustc_mir_transform/src/inline.rs
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Expand Up @@ -421,6 +421,10 @@ impl<'tcx> Inliner<'tcx> {
callee_attrs: &CodegenFnAttrs,
cross_crate_inlinable: bool,
) -> Result<(), &'static str> {
if self.tcx.has_attr(callsite.callee.def_id(), sym::rustc_no_mir_inline) {
return Err("#[rustc_no_mir_inline]");
}

if let InlineAttr::Never = callee_attrs.inline {
return Err("never inline hint");
}
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1 change: 1 addition & 0 deletions compiler/rustc_span/src/symbol.rs
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Expand Up @@ -1441,6 +1441,7 @@ symbols! {
rustc_mir,
rustc_must_implement_one_of,
rustc_never_returns_null_ptr,
rustc_no_mir_inline,
rustc_nonnull_optimization_guaranteed,
rustc_nounwind,
rustc_object_lifetime_default,
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18 changes: 15 additions & 3 deletions library/core/src/intrinsics.rs
Original file line number Diff line number Diff line change
Expand Up @@ -2706,13 +2706,25 @@ pub const unsafe fn const_deallocate(_ptr: *mut u8, _size: usize, _align: usize)
macro_rules! assert_unsafe_precondition {
($message:expr, ($($name:ident:$ty:ty = $arg:expr),*$(,)?) => $e:expr $(,)?) => {
{
// #[cfg(bootstrap)] (this comment)
// When the standard library is compiled with debug assertions, we want the check to inline for better performance.
// This is important when working on the compiler, which is compiled with debug assertions locally.
// When not compiled with debug assertions (so the precompiled std) we outline the check to minimize the compile
// time impact when debug assertions are disabled.
// It is not clear whether that is the best solution, see #120848.
#[cfg_attr(debug_assertions, inline(always))]
#[cfg_attr(not(debug_assertions), inline(never))]
// The proper solution to this is the `#[rustc_no_mir_inline]` below, but we still want decent performance for cfg(bootstrap).
#[cfg_attr(all(debug_assertions, bootstrap), inline(always))]
#[cfg_attr(all(not(debug_assertions), bootstrap), inline(never))]

// This check is inlineable, but not by the MIR inliner.
// The reason for this is that the MIR inliner is in an exceptionally bad position
// to think about whether or not to inline this. In MIR, this call is gated behind `debug_assertions`,
// which will codegen to `false` in release builds. Inlining the check would be wasted work in that case and
// would be bad for compile times.
//
// LLVM on the other hand sees the constant branch, so if it's `false`, it can immediately delete it without
// inlining the check. If it's `true`, it can inline it and get significantly better performance.
#[cfg_attr(not(bootstrap), rustc_no_mir_inline)]
#[cfg_attr(not(bootstrap), inline)]
#[rustc_nounwind]
fn precondition_check($($name:$ty),*) {
if !$e {
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Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
- // MIR for `caller` before Inline
+ // MIR for `caller` after Inline

fn caller() -> () {
let mut _0: ();
let _1: ();

bb0: {
StorageLive(_1);
_1 = callee() -> [return: bb1, unwind unreachable];
}

bb1: {
StorageDead(_1);
_0 = const ();
return;
}
}

Original file line number Diff line number Diff line change
@@ -0,0 +1,19 @@
- // MIR for `caller` before Inline
+ // MIR for `caller` after Inline

fn caller() -> () {
let mut _0: ();
let _1: ();

bb0: {
StorageLive(_1);
_1 = callee() -> [return: bb1, unwind continue];
}

bb1: {
StorageDead(_1);
_0 = const ();
return;
}
}

Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
// MIR for `caller` after PreCodegen

fn caller() -> () {
let mut _0: ();
let _1: ();

bb0: {
_1 = callee() -> [return: bb1, unwind unreachable];
}

bb1: {
return;
}
}
Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
// MIR for `caller` after PreCodegen

fn caller() -> () {
let mut _0: ();
let _1: ();

bb0: {
_1 = callee() -> [return: bb1, unwind continue];
}

bb1: {
return;
}
}
17 changes: 17 additions & 0 deletions tests/mir-opt/inline/rustc_no_mir_inline.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,17 @@
// EMIT_MIR_FOR_EACH_PANIC_STRATEGY
#![crate_type = "lib"]
#![feature(rustc_attrs)]

//@ compile-flags: -Zmir-opt-level=2 -Zinline-mir

#[inline]
#[rustc_no_mir_inline]
pub fn callee() {}

// EMIT_MIR rustc_no_mir_inline.caller.Inline.diff
// EMIT_MIR rustc_no_mir_inline.caller.PreCodegen.after.mir
pub fn caller() {
// CHECK-LABEL: fn caller(
// CHECK: callee()
callee();
}

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