Enable -Zshare-generics for inline(never) functions

[Mark-Simulacrum]

This avoids inlining cross-crate generic items when possible that are
already marked inline(never), implying that the author is not intending
for the function to be inlined by callers. As such, having a local copy
may make it easier for LLVM to optimize but mostly just adds to binary
bloat and codegen time. In practice our benchmarks indicate this is
indeed a win for larger compilations, where the extra cost in dynamic
linking to these symbols is diminished compared to the advantages in
fewer copies that need optimizing in each binary.

It might also make sense it expand this with other heuristics (e.g.,
#[cold]) in the future, but this seems like a good starting point.

FWIW, I expect that doing cleanup in where we make the decision
what should/shouldn't be shared is also a good idea. Way too
much code needed to be tweaked to check this. But I'm hoping
to leave that for a follow-up PR rather than blocking this on it.

[Mark-Simulacrum]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 5702d83 with merge 1f2a5ec...

[bors]

☀️ Try build successful - checks-actions
Build commit: 1f2a5ec (1f2a5ecd17d6b0415946e70d192b88b566dc73f8)

[rust-timer]

Finished benchmarking commit (1f2a5ec): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.4%	[0.3%, 18.5%]	203
Regressions ❌ (secondary)	2.7%	[0.3%, 16.2%]	207
Improvements ✅ (primary)	-1.2%	[-4.1%, -0.3%]	28
Improvements ✅ (secondary)	-3.9%	[-6.8%, -2.7%]	4
All ❌✅ (primary)	1.1%	[-4.1%, 18.5%]	231

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.5%	[0.6%, 3.9%]	16
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.7%	[-4.5%, -1.0%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.7%	[-4.5%, 3.9%]	19

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.6%	[0.9%, 11.5%]	73
Regressions ❌ (secondary)	4.5%	[1.2%, 14.3%]	119
Improvements ✅ (primary)	-1.8%	[-3.8%, -0.9%]	12
Improvements ✅ (secondary)	-4.4%	[-7.8%, -2.7%]	4
All ❌✅ (primary)	2.0%	[-3.8%, 11.5%]	85

Binary size

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.3%]	15
Regressions ❌ (secondary)	0.2%	[0.0%, 0.3%]	38
Improvements ✅ (primary)	-0.7%	[-2.3%, -0.0%]	77
Improvements ✅ (secondary)	-8.7%	[-18.9%, -0.3%]	14
All ❌✅ (primary)	-0.6%	[-2.3%, 0.3%]	92

Bootstrap: 667.994s -> 656.525s (-1.72%)
Artifact size: 315.79 MiB -> 314.40 MiB (-0.44%)

[Mark-Simulacrum]

It looks like the majority of the additional cost comes from additional indirection in calls to standard library functions, e.g., a diff like this:

-  f61064:       e8 73 f4 49 03          call   44004dc <_RINvNtCsavh3npScQaX_5alloc7raw_vec11finish_growNtNtB4_5alloc6GlobalECs1UKmS5rlRwk_21rustc_trait_selection>
+  ffe57a:       ff 15 a0 24 03 03       call   *0x30324a0(%rip)        # 4030a20 <_ZN5alloc7raw_vec11finish_grow17h78aea5cebcfaa28aE@Base>

This means more work, particularly for short-lived compilations, since the symbol needs to get resolved at runtime now. That cost should be eliminated with #122362, which might take some time to land but is making progress now. Most downstream programs also don't pay it since they're not linking std dynamically.

A little of the extra cost from there seems to be due to these non-inlined functions now being codegen'd with frame pointers (due to #122646). That doesn't seem like something worth worrying about, we accepted some regression from those changes already.

My sense is that something like this is probably still a good idea despite the regressions. We do see good improvements in binary sizes (including >1MB of librustc_driver.so), and bootstrap times are significantly reduced. That suggests that this is a pretty good win for the larger crates while being a slight loss for smaller crates (instruction count timings are down for some of the larger primary crates as well, e.g., ripgrep and cranelift). That seems consistent with the loss due to additional indirection due to librustc_driver dynamically linking with the standard library.

Going to mark as ready for review as such.

r? compiler

[fmease]

r? compiler

[Mark-Simulacrum]

cc #14527

[Mark-Simulacrum]

Poking @wesleywiser as it's been ~3 weeks here.

[saethlin]

It's been another 3 weeks, and this looks really interesting.

r? saethlin

[saethlin]

Is this testing what we want? I was expecting to see a cross-crate call to a #[inline(never)] generic function in a test, because I think the point of this PR is to change the behavior for such calls, right?

[Mark-Simulacrum]

I think this is indeed checking that behavior, though it's rather obscure. As per the comment just above ("These should not contribute...") we are implicitly checking that these functions are absent in the mono-items of the downstream crate (tests/codegen-units/partitioning/extern-generic.rs) since they're not listed as MONO-ITEM declarations in that file. They are referenced though via the foo function above which has to get codegen'd in the downstream crate since it is generic and never called in this one.

I suppose we could try to write a more direct test case? But this also tests that the property holds transitively (i.e., we don't codegen the called function twice with inline(never) even if it's not directly being called).

IIRC, before my changes, this would fail.

[saethlin]

Oh I see! This is very interconnected, and I should really be used to that by now.

[saethlin]

Also I checked and yes the test does fail without the rest of this PR.

[bors]

☀️ Try build successful - checks-actions
Build commit: 38d8997 (38d8997653d314a96e1c58c74c13ca78b8c6ecf3)

[rust-timer]

Finished benchmarking commit (38d8997): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.5%	[0.2%, 16.8%]	154
Regressions ❌ (secondary)	2.7%	[0.1%, 15.0%]	210
Improvements ✅ (primary)	-0.9%	[-2.8%, -0.3%]	23
Improvements ✅ (secondary)	-3.8%	[-6.1%, -2.9%]	5
All ❌✅ (primary)	1.2%	[-2.8%, 16.8%]	177

Max RSS (memory usage)

Results (primary 1.5%, secondary 3.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.2%	[1.1%, 4.4%]	5
Regressions ❌ (secondary)	3.3%	[3.2%, 3.3%]	2
Improvements ✅ (primary)	-2.6%	[-2.7%, -2.4%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.5%	[-2.7%, 4.4%]	7

Cycles

Results (primary 2.3%, secondary 4.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.7%	[0.8%, 12.4%]	21
Regressions ❌ (secondary)	5.6%	[2.0%, 17.9%]	73
Improvements ✅ (primary)	-1.6%	[-2.7%, -0.8%]	8
Improvements ✅ (secondary)	-6.6%	[-9.5%, -3.2%]	10
All ❌✅ (primary)	2.3%	[-2.7%, 12.4%]	29

Binary size

Results (primary -0.3%, secondary -2.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.2%	[0.0%, 0.6%]	22
Regressions ❌ (secondary)	0.3%	[0.1%, 0.3%]	37
Improvements ✅ (primary)	-0.4%	[-1.6%, -0.0%]	69
Improvements ✅ (secondary)	-8.4%	[-18.6%, -0.2%]	14
All ❌✅ (primary)	-0.3%	[-1.6%, 0.6%]	91

Bootstrap: missing data
Artifact size: 319.71 MiB -> 318.41 MiB (-0.41%)

[Mark-Simulacrum]

@bors try @rust-timer queue

[bors]

⌛ Trying commit a14d42e with merge 29a8b2d...

[bors]

☀️ Try build successful - checks-actions
Build commit: 29a8b2d (29a8b2de729aa4db6332c99732870a877483da75)

[rust-timer]

Finished benchmarking commit (29a8b2d): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.5%	[0.2%, 17.5%]	150
Regressions ❌ (secondary)	2.8%	[0.1%, 15.5%]	205
Improvements ✅ (primary)	-1.0%	[-2.8%, -0.3%]	24
Improvements ✅ (secondary)	-3.6%	[-6.0%, -2.8%]	5
All ❌✅ (primary)	1.2%	[-2.8%, 17.5%]	174

Max RSS (memory usage)

Results (primary -1.6%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	3.5%	[2.2%, 4.7%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-4.1%	[-7.8%, -2.0%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.6%	[-7.8%, 4.7%]	6

Cycles

Results (primary 2.9%, secondary 5.2%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	4.1%	[1.1%, 13.6%]	23
Regressions ❌ (secondary)	5.7%	[2.0%, 15.0%]	74
Improvements ✅ (primary)	-1.8%	[-2.6%, -1.2%]	6
Improvements ✅ (secondary)	-4.2%	[-7.1%, -2.8%]	4
All ❌✅ (primary)	2.9%	[-2.6%, 13.6%]	29

Binary size

Results (primary -0.3%, secondary -2.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.3%]	21
Regressions ❌ (secondary)	0.3%	[0.1%, 0.3%]	37
Improvements ✅ (primary)	-0.4%	[-1.6%, -0.0%]	68
Improvements ✅ (secondary)	-8.4%	[-18.7%, -0.2%]	14
All ❌✅ (primary)	-0.3%	[-1.6%, 0.3%]	89

Bootstrap: missing data
Artifact size: 319.77 MiB -> 317.92 MiB (-0.58%)

[saethlin]

This list of affected benchmarks in terms of cycles is now way shorter, which is nice.

I'm still in favor of this PR because I think it makes the behavior of #[inline(never)] more intuitive; whether it's worth looking into other tuning (such as #126793) doesn't negate that.

The list of commits looks like this is still in a WIP state. I'm happy to approve this if you fix up the commits.

[saethlin]

Whoops I never changed the labels back to author. Did that now.

[Zoxc]

You could do another perf run now that #122362 has landed.

[Dylan-DPC]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

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

[bors]

🔒 Merge conflict

This pull request and the master branch diverged in a way that cannot be automatically merged. Please rebase on top of the latest master branch, and let the reviewer approve again.

How do I rebase?

Assuming self is your fork and upstream is this repository, you can resolve the conflict following these steps:

1. git checkout share-inline-never-generics (switch to your branch)
2. git fetch upstream master (retrieve the latest master)
3. git rebase upstream/master -p (rebase on top of it)
4. Follow the on-screen instruction to resolve conflicts (check git status if you got lost).
5. git push self share-inline-never-generics --force-with-lease (update this PR)

You may also read Git Rebasing to Resolve Conflicts by Drew Blessing for a short tutorial.

Please avoid the "Resolve conflicts" button on GitHub. It uses git merge instead of git rebase which makes the PR commit history more difficult to read.

Sometimes step 4 will complete without asking for resolution. This is usually due to difference between how Cargo.lock conflict is handled during merge and rebase. This is normal, and you should still perform step 5 to update this PR.

Error message

Auto-merging tests/codegen/avr/avr-func-addrspace.rs
Auto-merging library/std/src/panicking.rs
Auto-merging library/alloc/src/vec/mod.rs
Auto-merging library/alloc/src/raw_vec.rs
CONFLICT (content): Merge conflict in library/alloc/src/raw_vec.rs
Auto-merging compiler/rustc_monomorphize/src/partitioning.rs
Auto-merging compiler/rustc_middle/src/ty/instance.rs
Auto-merging compiler/rustc_middle/src/ty/context.rs
Auto-merging compiler/rustc_middle/src/mir/mono.rs
Auto-merging compiler/rustc_codegen_ssa/src/back/symbol_export.rs
Auto-merging compiler/rustc_codegen_llvm/src/callee.rs
Auto-merging Cargo.lock
warning: inexact rename detection was skipped due to too many files.
warning: you may want to set your merge.renamelimit variable to at least 1968 and retry the command.
Automatic merge failed; fix conflicts and then commit the result.

[alex-semenyuk]

@Mark-Simulacrum
From wg-triage. Could you please rebase this?