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Make UB transmutes really UB in LLVM #143718

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@scottmcm scottmcm commented Jul 10, 2025 •
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Ralf suggested in #143410 (comment) that UB transmutes shouldn't be trapping, which happened for the one path that PR was changing, but there's another path as well, so this PR changes that other path to match.

r? codegen

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Make UB transmutes really UB in LLVM
58d7c2d 
Ralf suggested in <rust-lang#143410 (comment)> that UB transmutes shouldn't be trapping, which happened for the one path that PR was changing, but there's another path as well, so this PR changes that other path to match.
@rustbot 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 Jul 10, 2025
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rustbot commented Jul 10, 2025

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

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@nikic is this the best way to indicate "unreachable" to LLVM in the middle of a block? Does LLVM quickly normalize this by making the entire block unreachable or can such an assume cause problems?

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dianqk commented Jul 10, 2025

@nikic is this the best way to indicate "unreachable" to LLVM in the middle of a block? Does LLVM quickly normalize this by making the entire block unreachable or can such an assume cause problems?

According to https://llvm.org/docs/Frontend/PerformanceTips.html#id11, this should be store i1 true, ptr poison, align 1.

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Ah! We should have a helper method for this then.

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scottmcm commented Jul 10, 2025 •
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Oh, interesting! I'll switch it (both here and in the other spot).

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@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 10, 2025
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dianqk commented Jul 10, 2025

Ah! We should have a helper method for this then.

Yes. This makes sence to me.
BTW, this reminds me of #127740.

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nikic commented Jul 10, 2025

FWIW I don't think it particularly matters how Rust emits this.

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Well, if nothing else it's reasonable for there to be a method for "I wanted unreachable but not a terminator", so I can do that and if it ends up not mattering which code pattern it is, then fine.

I do think here is different from #127740 because there it was a "runtime" value being passed to the assert, not a constant being created to be passed to it, so there I think it's better to let LLVM normalize, but here where I never specifically wanted an assume in the first place I don't mind just emitting using a different pattern.

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Add BuilderMethods::unreachable_nonterminator
f5fc872 
So places that need `unreachable` but in the middle of a basic block can call that instead of figuring out the best way to do it.
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@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 10, 2025
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@bors r+

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bors commented Jul 10, 2025

📌 Commit f5fc872 has been approved by WaffleLapkin

It is now in the queue for this repository.

@bors 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 Jul 10, 2025
tgross35 added a commit to tgross35/rust that referenced this pull request Jul 10, 2025
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Rollup merge of rust-lang#143718 - scottmcm:ub-transmute-is-ub, r=Waf…
c6d2963 
…fleLapkin

Make UB transmutes really UB in LLVM

Ralf suggested in <rust-lang#143410 (comment)> that UB transmutes shouldn't be trapping, which happened for the one path *that* PR was changing, but there's another path as well, so *this* PR changes that other path to match.

r? codegen
@tgross35 tgross35 mentioned this pull request Jul 10, 2025
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bors added a commit that referenced this pull request Jul 11, 2025
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Auto merge of #143760 - tgross35:rollup-hyioynq, r=tgross35
5752c21 
Rollup of 7 pull requests

Successful merges:

 - #142391 (rust: library: Add `setsid` method to `CommandExt` trait)
 - #143301 (`tests/ui`: A New Order [26/N])
 - #143302 (`tests/ui`: A New Order [27/N])
 - #143303 (`tests/ui`: A New Order [28/28] FINAL PART)
 - #143568 (std: sys: net: uefi: tcp4: Add timeout support)
 - #143708 (fix: Include frontmatter in -Zunpretty output )
 - #143718 (Make UB transmutes really UB in LLVM)

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