diff --git a/compiler/rustc_feature/src/builtin_attrs.rs b/compiler/rustc_feature/src/builtin_attrs.rs
index 99875ec540545..003baf071b8c8 100644
--- a/compiler/rustc_feature/src/builtin_attrs.rs
+++ b/compiler/rustc_feature/src/builtin_attrs.rs
@@ -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:
diff --git a/compiler/rustc_mir_transform/src/inline.rs b/compiler/rustc_mir_transform/src/inline.rs
index 2009539d4d084..36546a03cdfc5 100644
--- a/compiler/rustc_mir_transform/src/inline.rs
+++ b/compiler/rustc_mir_transform/src/inline.rs
@@ -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");
}
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 46472a131ff4b..609ab054da21e 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -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,
diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index e42b2315ea5d7..f9d89795a9988 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -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 {
diff --git a/tests/mir-opt/inline/rustc_no_mir_inline.caller.Inline.panic-abort.diff b/tests/mir-opt/inline/rustc_no_mir_inline.caller.Inline.panic-abort.diff
new file mode 100644
index 0000000000000..dd79cff1dcf80
--- /dev/null
+++ b/tests/mir-opt/inline/rustc_no_mir_inline.caller.Inline.panic-abort.diff
@@ -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;
+ }
+ }
+
diff --git a/tests/mir-opt/inline/rustc_no_mir_inline.caller.Inline.panic-unwind.diff b/tests/mir-opt/inline/rustc_no_mir_inline.caller.Inline.panic-unwind.diff
new file mode 100644
index 0000000000000..4506a338edd7a
--- /dev/null
+++ b/tests/mir-opt/inline/rustc_no_mir_inline.caller.Inline.panic-unwind.diff
@@ -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;
+ }
+ }
+
diff --git a/tests/mir-opt/inline/rustc_no_mir_inline.caller.PreCodegen.after.panic-abort.mir b/tests/mir-opt/inline/rustc_no_mir_inline.caller.PreCodegen.after.panic-abort.mir
new file mode 100644
index 0000000000000..d0772e51a07fe
--- /dev/null
+++ b/tests/mir-opt/inline/rustc_no_mir_inline.caller.PreCodegen.after.panic-abort.mir
@@ -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;
+ }
+}
diff --git a/tests/mir-opt/inline/rustc_no_mir_inline.caller.PreCodegen.after.panic-unwind.mir b/tests/mir-opt/inline/rustc_no_mir_inline.caller.PreCodegen.after.panic-unwind.mir
new file mode 100644
index 0000000000000..39ad4f1010b79
--- /dev/null
+++ b/tests/mir-opt/inline/rustc_no_mir_inline.caller.PreCodegen.after.panic-unwind.mir
@@ -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;
+ }
+}
diff --git a/tests/mir-opt/inline/rustc_no_mir_inline.rs b/tests/mir-opt/inline/rustc_no_mir_inline.rs
new file mode 100644
index 0000000000000..b008df32726b5
--- /dev/null
+++ b/tests/mir-opt/inline/rustc_no_mir_inline.rs
@@ -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();
+}