Auto merge of #104999 - saethlin:immediate-abort-inlining, r=thomcc

Adjust inlining attributes around panic_immediate_abort

The goal of `panic_immediate_abort` is to permit the panic runtime and formatting code paths to be optimized away. But while poking through some disassembly of a small program compiled with that option, I found that was not the case. Enabling LTO did address that specific issue, but enabling LTO is a steep price to pay for this feature doing its job.

This PR fixes that, by tweaking two things:
* All the slice indexing functions that we `const_eval_select` on get `#[inline]`. `objdump -dC` told me that originally some `_ct` functions could end up in an executable. I won't pretend to understand what's going on there.
* Normalize attributes across all `panic!` wrappers: use `inline(never) + cold` normally, and `inline` when `panic_immediate_abort` is enabled.

But also, with LTO and `panic_immediate_abort` enabled, this patch knocks ~709 kB out of the `.text` segment of `librustc_driver.so`. That is slightly surprising to me, my best theory is that this shifts some inlining earlier in compilation, enabling some subsequent optimizations. The size improvement of `librustc_driver.so` with `panic_immediate_abort` due to this patch is greater with LTO than without LTO, which I suppose backs up this theory.

I do not know how to test this. I would quite like to, because I think what this is solving was an accidental regression. This only works with `-Zbuild-std` which is a cargo flag, and thus can't be used in a rustc codegen test.

r? `@thomcc`

---

I do not seriously think anyone is going to use a compiler built with `panic_immediate_abort`, but I wanted a big complicated Rust program to try this out on, and the compiler is such.

3 files changed, 26 insertions, 24 deletions

diff --git a/library/core/src/panicking.rs b/library/core/src/panicking.rs
index 4fd1eb23413..a704a00faaa 100644
--- a/library/core/src/panicking.rs
+++ b/library/core/src/panicking.rs
@@ -38,10 +38,9 @@ use crate::panic::{Location, PanicInfo};
 /// site as much as possible (so that `panic!()` has as low an impact
 /// on (e.g.) the inlining of other functions as possible), by moving
 /// the actual formatting into this shared place.
-#[cold]
 // If panic_immediate_abort, inline the abort call,
 // otherwise avoid inlining because of it is cold path.
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 #[lang = "panic_fmt"] // needed for const-evaluated panics
@@ -67,8 +66,7 @@ pub const fn panic_fmt(fmt: fmt::Arguments<'_>) -> ! {
 
 /// Like panic_fmt, but without unwinding and track_caller to reduce the impact on codesize.
 /// Also just works on `str`, as a `fmt::Arguments` needs more space to be passed.
-#[cold]
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[rustc_nounwind]
 pub fn panic_str_nounwind(msg: &'static str) -> ! {
@@ -96,10 +94,9 @@ pub fn panic_str_nounwind(msg: &'static str) -> ! {
 // above.
 
 /// The underlying implementation of libcore's `panic!` macro when no formatting is used.
-#[cold]
 // never inline unless panic_immediate_abort to avoid code
 // bloat at the call sites as much as possible
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 #[rustc_const_unstable(feature = "core_panic", issue = "none")]
@@ -138,8 +135,8 @@ pub const fn panic_display<T: fmt::Display>(x: &T) -> ! {
     panic_fmt(format_args!("{}", *x));
 }
 
-#[cold]
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 #[lang = "panic_bounds_check"] // needed by codegen for panic on OOB array/slice access
 fn panic_bounds_check(index: usize, len: usize) -> ! {
@@ -154,8 +151,8 @@ fn panic_bounds_check(index: usize, len: usize) -> ! {
 ///
 /// This function is called directly by the codegen backend, and must not have
 /// any extra arguments (including those synthesized by track_caller).
-#[cold]
-#[inline(never)]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[lang = "panic_no_unwind"] // needed by codegen for panic in nounwind function
 #[rustc_nounwind]
 fn panic_no_unwind() -> ! {
@@ -185,7 +182,8 @@ pub enum AssertKind {
 }
 
 /// Internal function for `assert_eq!` and `assert_ne!` macros
-#[cold]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 #[doc(hidden)]
 pub fn assert_failed<T, U>(
@@ -202,7 +200,8 @@ where
 }
 
 /// Internal function for `assert_match!`
-#[cold]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 #[doc(hidden)]
 pub fn assert_matches_failed<T: fmt::Debug + ?Sized>(
@@ -221,6 +220,8 @@ pub fn assert_matches_failed<T: fmt::Debug + ?Sized>(
 }
 
 /// Non-generic version of the above functions, to avoid code bloat.
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 fn assert_failed_inner(
     kind: AssertKind,
diff --git a/library/core/src/slice/index.rs b/library/core/src/slice/index.rs
index 6d2f7330d5d..c295a0e0645 100644
--- a/library/core/src/slice/index.rs
+++ b/library/core/src/slice/index.rs
@@ -31,9 +31,8 @@ where
     }
 }
 
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
-#[cold]
 #[track_caller]
 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
@@ -48,19 +47,20 @@ const fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
 }
 
 // FIXME const-hack
+#[inline]
 #[track_caller]
 fn slice_start_index_len_fail_rt(index: usize, len: usize) -> ! {
     panic!("range start index {index} out of range for slice of length {len}");
 }
 
+#[inline]
 #[track_caller]
 const fn slice_start_index_len_fail_ct(_: usize, _: usize) -> ! {
     panic!("slice start index is out of range for slice");
 }
 
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
-#[cold]
 #[track_caller]
 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
@@ -71,19 +71,20 @@ const fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
 }
 
 // FIXME const-hack
+#[inline]
 #[track_caller]
 fn slice_end_index_len_fail_rt(index: usize, len: usize) -> ! {
     panic!("range end index {index} out of range for slice of length {len}");
 }
 
+#[inline]
 #[track_caller]
 const fn slice_end_index_len_fail_ct(_: usize, _: usize) -> ! {
     panic!("slice end index is out of range for slice");
 }
 
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
-#[cold]
 #[track_caller]
 #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 const fn slice_index_order_fail(index: usize, end: usize) -> ! {
@@ -92,27 +93,27 @@ const fn slice_index_order_fail(index: usize, end: usize) -> ! {
 }
 
 // FIXME const-hack
+#[inline]
 #[track_caller]
 fn slice_index_order_fail_rt(index: usize, end: usize) -> ! {
     panic!("slice index starts at {index} but ends at {end}");
 }
 
+#[inline]
 #[track_caller]
 const fn slice_index_order_fail_ct(_: usize, _: usize) -> ! {
     panic!("slice index start is larger than end");
 }
 
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
-#[cold]
 #[track_caller]
 const fn slice_start_index_overflow_fail() -> ! {
     panic!("attempted to index slice from after maximum usize");
 }
 
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
-#[cold]
 #[track_caller]
 const fn slice_end_index_overflow_fail() -> ! {
     panic!("attempted to index slice up to maximum usize");
diff --git a/library/std/src/panicking.rs b/library/std/src/panicking.rs
index d4976a469cc..1039835bbbd 100644
--- a/library/std/src/panicking.rs
+++ b/library/std/src/panicking.rs
@@ -594,8 +594,8 @@ pub fn begin_panic_handler(info: &PanicInfo<'_>) -> ! {
 // lang item for CTFE panic support
 // never inline unless panic_immediate_abort to avoid code
 // bloat at the call sites as much as possible
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
-#[cold]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 #[rustc_do_not_const_check] // hooked by const-eval
 pub const fn begin_panic<M: Any + Send>(msg: M) -> ! {