Make the allocator shim participate in LTO again

[bjorn3]

This is likely the cause of the perf regression in #145955. It also caused some functional regressions.

Fixes #146235
Fixes #146239

[bjorn3]

@bors try @rust-timer queue

[bjorn3]

Forgot to revert the changes in exported_symbols_for_lto. This shouldn't affect the perf run other than possibly showing less of a performance improvement than it should give in the end.

[rust-bors]

☀️ Try build successful (CI)
Build commit: 5ab6398 (5ab63980021f7c1ae280eba3261d66240d594007, parent: ad85bc524b1ad696e42061ad8338d382dffbdbe5)

[rustbot]

r? @fee1-dead

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[rustbot]

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

[lqd]

This test reproduces some from of these other two issues (rust-lld: error: undefined hidden symbol: __rustc::__rg_oom without this PR). Can you add it to the PR?

//@ compile-flags: --crate-type cdylib -C lto 

use std::alloc::{GlobalAlloc, Layout};

struct MyAllocator;

unsafe impl GlobalAlloc for MyAllocator {
    unsafe fn alloc(&self, _layout: Layout) -> *mut u8 {
        todo!()
    }

    unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {
    }
}

#[global_allocator]
static GLOBAL: MyAllocator = MyAllocator;

Forgot to revert the changes in exported_symbols_for_lto

You've since done this, IIUC.

[bjorn3]

Thanks! Had to modify it slightly to work with compiletest.

Forgot to revert the changes in exported_symbols_for_lto

You've since done this, IIUC.

Correct

[lqd]

Otherwise this looks good to me and fixes the regressions, so that's great, thanks!

I'm not sure we care about the perf results, but they should be available in 3-4hours. You can r=me at your preference.

[rust-timer]

Finished benchmarking commit (5ab6398): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.1%, 0.3%]	2
Improvements ✅ (primary)	-1.3%	[-27.1%, -0.3%]	229
Improvements ✅ (secondary)	-1.1%	[-46.7%, -0.0%]	264
All ❌✅ (primary)	-1.3%	[-27.1%, -0.3%]	229

Max RSS (memory usage)

Results (primary 1.8%, secondary -2.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	7.4%	[7.4%, 7.4%]	1
Regressions ❌ (secondary)	2.1%	[1.4%, 2.7%]	5
Improvements ✅ (primary)	-0.9%	[-1.4%, -0.5%]	2
Improvements ✅ (secondary)	-4.5%	[-6.5%, -2.5%]	9
All ❌✅ (primary)	1.8%	[-1.4%, 7.4%]	3

Cycles

Results (primary -13.2%, secondary -8.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.9%	[2.9%, 2.9%]	1
Improvements ✅ (primary)	-13.2%	[-24.6%, -2.4%]	6
Improvements ✅ (secondary)	-10.8%	[-44.4%, -1.6%]	6
All ❌✅ (primary)	-13.2%	[-24.6%, -2.4%]	6

Binary size

Results (primary 53.0%, secondary 59.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	70.9%	[70.9%, 71.0%]	3
Regressions ❌ (secondary)	118.8%	[118.8%, 118.8%]	1
Improvements ✅ (primary)	-0.9%	[-0.9%, -0.9%]	1
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.4%]	1
All ❌✅ (primary)	53.0%	[-0.9%, 71.0%]	4

Bootstrap: 467.829s -> 466.151s (-0.36%)
Artifact size: 390.58 MiB -> 387.87 MiB (-0.69%)

[bjorn3]

It improves things even more than it previously regressed.

$ echo 'fn main() {}' | RUSTC_BOOTSTRAP=1 rustc +nightly-2025-09-01  - -Zhuman-readable-cgu-names -O -Csave-temps -Clto=true && ls -l
-rwxrwxr-x 1 bjorn bjorn 1772392  5 sep 20:53 rust_out
-rw-rw-r-- 1 bjorn bjorn 2609776  5 sep 20:53 rust_out.rust_out.49aad25d9732bf04-cgu.0.rcgu.bc
-rw-rw-r-- 1 bjorn bjorn 6979364  5 sep 20:53 rust_out.rust_out.49aad25d9732bf04-cgu.0.rcgu.lto.after-restriction.bc
-rw-rw-r-- 1 bjorn bjorn 6979324  5 sep 20:53 rust_out.rust_out.49aad25d9732bf04-cgu.0.rcgu.lto.input.bc
-rw-rw-r-- 1 bjorn bjorn    4512  5 sep 20:53 rust_out.rust_out.49aad25d9732bf04-cgu.0.rcgu.no-opt.bc
-rw-rw-r-- 1 bjorn bjorn 3211728  5 sep 20:53 rust_out.rust_out.49aad25d9732bf04-cgu.0.rcgu.o
$ echo 'fn main() {}' | RUSTC_BOOTSTRAP=1 rustc +nightly  - -Zhuman-readable-cgu-names -O -Csave-temps -Clto=true && ls -l
-rwxrwxr-x 1 bjorn bjorn 1766032  5 sep 20:54 rust_out
-rw-rw-r-- 1 bjorn bjorn 2612612  5 sep 20:53 rust_out.rust_out.f8d6093b640b0034-cgu.0.rcgu.bc
-rw-rw-r-- 1 bjorn bjorn 6982788  5 sep 20:53 rust_out.rust_out.f8d6093b640b0034-cgu.0.rcgu.lto.after-restriction.bc
-rw-rw-r-- 1 bjorn bjorn 6982752  5 sep 20:53 rust_out.rust_out.f8d6093b640b0034-cgu.0.rcgu.lto.input.bc
-rw-rw-r-- 1 bjorn bjorn    4512  5 sep 20:53 rust_out.rust_out.f8d6093b640b0034-cgu.0.rcgu.no-opt.bc
-rw-rw-r-- 1 bjorn bjorn 3188680  5 sep 20:54 rust_out.rust_out.f8d6093b640b0034-cgu.0.rcgu.o
-rw-rw-r-- 1 bjorn bjorn    3484  5 sep 20:53 rust_out.rust_out.f8d6093b640b0034-crate.allocator.rcgu.bc
-rw-rw-r-- 1 bjorn bjorn    3224  5 sep 20:53 rust_out.rust_out.f8d6093b640b0034-crate.allocator.rcgu.o
$ echo 'fn main() {}' | RUSTC_BOOTSTRAP=1 rustc +5ab63980021f7c1ae280eba3261d66240d594007 - -Zhuman-readable-cgu-names -O -Csave-temps -Clto=true && ls -l
-rwxrwxr-x 1 bjorn bjorn 2264800  5 sep 20:55 rust_out
-rw-rw-r-- 1 bjorn bjorn    4512  5 sep 20:55 rust_out.rust_out.dbca3ea46f37a61b-cgu.0.rcgu.no-opt.bc
-rw-rw-r-- 1 bjorn bjorn 2619640  5 sep 20:55 rust_out.rust_out.dbca3ea46f37a61b-crate.allocator.rcgu.bc
-rw-rw-r-- 1 bjorn bjorn 6982380  5 sep 20:55 rust_out.rust_out.dbca3ea46f37a61b-crate.allocator.rcgu.lto.after-restriction.bc
-rw-rw-r-- 1 bjorn bjorn 6982344  5 sep 20:55 rust_out.rust_out.dbca3ea46f37a61b-crate.allocator.rcgu.lto.input.bc
-rw-rw-r-- 1 bjorn bjorn 3701008  5 sep 20:55 rust_out.rust_out.dbca3ea46f37a61b-crate.allocator.rcgu.o

I suspect what happened is that I didn't restore the check in fat LTO that ensures the allocator module is not used as base to merge all other modules into. The allocator module is not configured to be optimized, so we probably skipped all optimizations after doing the module merging pass of fat LTO. I've added a new commit to fix this.

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[fee1-dead]

r? @lqd or codegen

[rust-bors]

☀️ Try build successful (CI)
Build commit: 499d4b9 (499d4b9d218a37f0dd293ed772cc9ee39f778836, parent: 9cd272dc85320e85a8c83a1a338870de52c005f3)

[rust-timer]

Queued 499d4b9 with parent 9cd272d, future comparison URL.
There is currently 1 preceding artifact in the queue.
It will probably take at least ~1.6 hours until the benchmark run finishes.