Add -Zuse-sync-unwind #117744

quininer · 2023-11-09T08:59:04Z

Currently Rust uses async unwind by default, but async unwind will bring non-negligible size overhead. it would be nice to allow users to choose this.

In addition, async unwind currently prevents LLVM from generate compact unwind for MachO, if one wishes to generate compact unwind for MachO, then also needs this flag.

rustbot · 2023-11-09T08:59:12Z

r? @wesleywiser

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bjorn3 · 2023-11-09T10:53:06Z

In addition, async unwind currently prevents LLVM from generate compact unwind for MachO, if one wishes to generate compact unwind for MachO, then also needs this flag.

For compact unwind info there is also the issue that compact unwinding only supports 3 different personality functions, all of which are used by apple itself (__gxx_personality_v0, __gcc_personality_v0 and __objc_personality_v0): #102754 The fix for that may be to only use compact unwind info when using one of these personality functions, which would mean that rust doesn't get to use compact unwinding at all.

quininer · 2023-11-09T11:01:05Z

The fix for that may be to only use compact unwind info when using one of these personality functions, which would mean that rust doesn't get to use compact unwinding at all.

Yes, if want to compact unwind you also must use -femit-compact-unwind-non-canonical=true flag. and __gcc_personality_v0 is rarely used, in production we only have __gxx_personality_v0, __objc_personality_v0 and _rust_eh_personality.

quininer · 2023-11-14T07:29:49Z

ping @wesleywiser

quininer · 2023-11-16T09:34:31Z

r? @bjorn3 Can you help take a look?

bjorn3 · 2023-11-16T13:09:05Z

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -95,11 +95,12 @@ pub fn sanitize_attrs<'ll>(

 /// Tell LLVM to emit or not emit the information necessary to unwind the stack for the function.
 #[inline]
-pub fn uwtable_attr(llcx: &llvm::Context) -> &Attribute {
+pub fn uwtable_attr(llcx: &llvm::Context, use_sync_unwind: Option<bool>) -> &Attribute {
    // NOTE: We should determine if we even need async unwind tables, as they
    // take have more overhead and if we can use sync unwind tables we
    // probably should.


@durin42 do you think unconditionally (or by default) using sync only unwind tables would work?

quininer · 2023-11-23T07:42:41Z

ping? @bjorn3 @durin42

Noratrieb · 2023-11-30T10:13:03Z

Can you link some more material on the advantages and disadvantages of sync unwind vs async unwind? What are the advantages of async unwind?

quininer · 2023-11-30T10:49:18Z

@Nilstrieb I'm not an expert on async unwind but I found some links.

https://maskray.me/blog/2020-11-08-stack-unwinding
https://groups.google.com/g/llvm-dev/c/KH4QAKPBQ5A
https://gcc.gnu.org/legacy-ml/gcc-help/2009-10/msg00195.html

I understand that async unwind is mainly used to support unwinding in function prologue/epilogue. It may be used by tools such as profiling tools (but usually frame pointer should be used). this is achieved by add more cfi to eh_frame, so there is an impact on binary size.

apiraino · 2023-12-28T11:38:07Z

Do the proposed changes here need an MCP?

(mostly asking to someone from compiler team)

edit: PR was discussed by T-compiler meeting (on Zulip) and it was decided it doesnt need one

rustbot · 2023-12-31T07:04:30Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

These commits modify compiler targets.
(See the Target Tier Policy.)

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

This flag specifies whether LLVM generates async unwind or sync unwind.

quininer · 2024-01-08T11:46:57Z

what do I need to do to move this PR forward?

bjorn3 · 2024-01-08T13:02:19Z

@bors r+

bors · 2024-01-08T13:02:22Z

📌 Commit 12784c3 has been approved by bjorn3

It is now in the queue for this repository.

Add -Zuse-sync-unwind Currently Rust uses async unwind by default, but async unwind will bring non-negligible size overhead. it would be nice to allow users to choose this. In addition, async unwind currently prevents LLVM from generate compact unwind for MachO, if one wishes to generate compact unwind for MachO, then also needs this flag.

…iaskrgr Rollup of 8 pull requests Successful merges: - rust-lang#117744 (Add -Zuse-sync-unwind) - rust-lang#118649 (Make inductive cycles in coherence ambiguous always) - rust-lang#118979 (Use `assert_unsafe_precondition` for `char::from_u32_unchecked`) - rust-lang#119562 (Rename `pointer` field on `Pin`) - rust-lang#119619 (mir-opt and custom target fixes) - rust-lang#119632 (Fix broken build for ESP IDF due to rust-lang#119026) - rust-lang#119712 (Adding alignment to the cases to test for specific error messages.) - rust-lang#119734 (Miri subtree update) r? `@ghost` `@rustbot` modify labels: rollup

…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#117744 (Add -Zuse-sync-unwind) - rust-lang#118649 (Make inductive cycles in coherence ambiguous always) - rust-lang#118979 (Use `assert_unsafe_precondition` for `char::from_u32_unchecked`) - rust-lang#119619 (mir-opt and custom target fixes) - rust-lang#119632 (Fix broken build for ESP IDF due to rust-lang#119026) - rust-lang#119712 (Adding alignment to the cases to test for specific error messages.) - rust-lang#119734 (Miri subtree update) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of rust-lang#117744 - quininer:add-z-sync-uw, r=bjorn3 Add -Zuse-sync-unwind Currently Rust uses async unwind by default, but async unwind will bring non-negligible size overhead. it would be nice to allow users to choose this. In addition, async unwind currently prevents LLVM from generate compact unwind for MachO, if one wishes to generate compact unwind for MachO, then also needs this flag.

rustbot assigned wesleywiser Nov 9, 2023

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Nov 9, 2023

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rustbot assigned bjorn3 and unassigned wesleywiser Nov 16, 2023

bjorn3 reviewed Nov 16, 2023

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quininer force-pushed the add-z-sync-uw branch 2 times, most recently from 24d5856 to ff2d246 Compare December 31, 2023 07:04

rustbot added O-unix Operating system: Unix-like T-bootstrap Relevant to the bootstrap subteam: Rust's build system (x.py and src/bootstrap) labels Dec 31, 2023

Add -Zuse-sync-unwind

12784c3

This flag specifies whether LLVM generates async unwind or sync unwind.

quininer force-pushed the add-z-sync-uw branch from ff2d246 to 12784c3 Compare December 31, 2023 07:27

quininer requested a review from bjorn3 January 2, 2024 04:06

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 8, 2024

matthiaskrgr mentioned this pull request Jan 8, 2024

Rollup of 8 pull requests #119755

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matthiaskrgr mentioned this pull request Jan 9, 2024

Rollup of 7 pull requests #119760

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bors merged commit 9e4843e into rust-lang:master Jan 9, 2024
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rustbot added this to the 1.77.0 milestone Jan 9, 2024

quininer deleted the add-z-sync-uw branch January 9, 2024 08:22

ojeda mentioned this pull request Mar 5, 2024

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Add -Zuse-sync-unwind #117744

Add -Zuse-sync-unwind #117744

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Add -Zuse-sync-unwind #117744

Add -Zuse-sync-unwind #117744

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