Allow using `-C force-unwind-tables=no` when `panic=unwind` #83482

ghost · 2021-03-25T18:46:42Z

It seems LLVM still generates proper unwind tables even there is no uwtable attribute, unless I looked at the wrong place 🤔:
https://github.com/llvm/llvm-project/blob/c21016715f0ee4a36affdf7150ac135ca98b0eae/llvm/include/llvm/IR/Function.h#L666

Therefore, I assume it's safe to omit uwtable even when panic=unwind, and this PR removes the restriction that disallows using -C force-unwind-tables=no when panic=unwind.

rust-highfive · 2021-03-25T18:46:45Z

r? @davidtwco

(rust-highfive has picked a reviewer for you, use r? to override)

nagisa · 2021-03-25T21:40:25Z

So, what the linked LLVM code says is that unwinding tables will be generated if a function has a personality function.

A function will only have such a function associated if it has any in scope cleanups, or other unwind-interrupting mechanisms. A plain function that does not have any cleanups would still break unwinding here, i believe.

Basically this needs a test that verifies a stack along the lines of can work:

#0 panics
#1 no_cleanups_1
#2 has_cleanups_or_catches_panics
#3 no_cleanups_2

and similar. The calls mustn't be inlined nor tail calls.

nikic · 2021-03-25T22:28:20Z

@nagisa The code also checks for !doesNotThrow(), which is true for all functions which aren't nounwind, regardless of whether they have a personality function.

nagisa · 2021-03-25T23:27:30Z

Ah, seems good to me then. Definitely can avoid generation of a fair amount of unnecessary data by default, then.

ghost · 2021-03-26T06:10:32Z

I think some more tests could still be helpful... @rustbot label -S-waiting-on-review S-waiting-on-author

davidtwco · 2021-03-26T10:18:17Z

r? @nagisa

ghost · 2021-03-26T15:45:26Z

@rustbot label S-waiting-on-review -S-waiting-on-author

src/test/assembly/panic-unwind-no-uwtable.rs

src/test/codegen/force-no-unwind-tables.rs

nagisa · 2021-03-28T00:25:28Z

r=me once the comments about tests are addressed and the commits squashed.

compiler/rustc_session/src/session.rs

nagisa · 2021-04-04T13:11:24Z

@bors r+ Thanks!

bors · 2021-04-04T13:11:26Z

📌 Commit d1c591b has been approved by nagisa

Allow using `-C force-unwind-tables=no` when `panic=unwind` It seems LLVM still generates proper unwind tables even there is no `uwtable` attribute, unless I looked at the wrong place 🤔: https://github.com/llvm/llvm-project/blob/c21016715f0ee4a36affdf7150ac135ca98b0eae/llvm/include/llvm/IR/Function.h#L666 Therefore, I *assume* it's safe to omit `uwtable` even when `panic=unwind`, and this PR removes the restriction that disallows using `-C force-unwind-tables=no` when `panic=unwind`.

bors · 2021-04-10T18:48:37Z

⌛ Testing commit 40f51bb5ecae520228b79bdd69e33dbf64468c4a with merge 4f5e9c3324a8ffce36bb8d13139b1c74e9f51c14...

bors · 2021-04-10T19:20:08Z

💔 Test failed - checks-actions

ghost · 2021-04-11T14:49:00Z

The failure was caused by dso_local between define and void (#83592). I rebased and changed // CHECK-LABEL: define void @foo to // CHECK-LABEL: define{{.*}}void @foo. Now codegen tests pass for me locally with --target wasm32-unknown-emscripten.

But the UI test failed...

   = note: error: undefined symbol: __gxx_personality_v0 (referenced by top-level compiled C/C++ code)
           warning: Link with `-s LLD_REPORT_UNDEFINED` to get more information on undefined symbols
           warning: To disable errors for undefined symbols use `-s ERROR_ON_UNDEFINED_SYMBOLS=0`
           warning: ___gxx_personality_v0 may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
           Error: Aborting compilation due to previous errors
           emcc: error: '/usr/bin/node /usr/share/emscripten/src/compiler.js /tmp/tmpsj7faogo.txt' failed (1)
           

error: aborting due to previous error

EDIT: Never mind. src/test/ui/builtin-clone-unwind.rs failed for me too. Maybe it's my local Emscripten issue...

nagisa · 2021-04-11T18:17:23Z

@bors r+

bors · 2021-04-11T18:17:24Z

📌 Commit 2fd4dd2 has been approved by nagisa

bors · 2021-04-11T19:50:28Z

⌛ Testing commit 2fd4dd2 with merge a866124...

bors · 2021-04-11T22:11:25Z

☀️ Test successful - checks-actions
Approved by: nagisa
Pushing a866124 to master...

ghost · 2021-06-03T04:08:19Z

The documentation seems still indicating that panic=unwind might require force-unwind-tables to not be set to "no":

n, no, or off: Unwind tables are not forced to be generated. If unwind
tables are required by the target or -C panic=unwind, an error will be
emitted.

Created #85951 to update it.

…eklabnik Update the documentation of `-C force-unwind-tables` for rust-lang#83482 `panic=unwind` does not require `force-unwind-tables` to be "yes" anymore. I forgot to update this in rust-lang#83482.

Rollup of 11 pull requests Successful merges: - rust-lang#85906 (Use `Iterator::find` instead of open-coding it) - rust-lang#85951 (Update the documentation of `-C force-unwind-tables` for rust-lang#83482) - rust-lang#85985 (Clarify documentation of slice sorting methods) - rust-lang#85989 (Remove rustfmt tests from top-level .gitattributes) - rust-lang#86074 (Default panic message should print Box<dyn Any>) - rust-lang#86078 (Type page font weight) - rust-lang#86090 (:arrow_up: rust-analyzer) - rust-lang#86095 (Search description codeblock) - rust-lang#86096 (Comment out unused error codes and add description for E0316) - rust-lang#86101 (Correct type signature in doc for Bound::as_mut) - rust-lang#86103 (Remove lifetime hack) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

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Rollup of 8 pull requests #83847

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Rollup of 8 pull requests #83851

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Allow using -C force-unwind-tables=no when panic=unwind

2fd4dd2

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ghost mentioned this pull request Jun 3, 2021

Update the documentation of -C force-unwind-tables for #83482 #85951

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Rollup of 11 pull requests #86126

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JohnTitor mentioned this pull request Jun 8, 2021

Rollup of 11 pull requests #86127

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Tracking issue for const generics (RFC 2000) #44580

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ice: Mismatched open/close delims #116781

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Allow using -C force-unwind-tables=no when panic=unwind #83482

Allow using -C force-unwind-tables=no when panic=unwind #83482

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Allow using `-C force-unwind-tables=no` when `panic=unwind` #83482

Allow using `-C force-unwind-tables=no` when `panic=unwind` #83482

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