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Implement support for become and explicit tail call codegen for the LLVM backend #144232

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@xacrimon xacrimon commented Jul 20, 2025 •
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This PR implements codegen of explicit tail calls via become in rustc_codegen_ssa and support within the LLVM backend. Completes a task on (#112788). This PR implements all the necessary bits to make explicit tail calls usable, other backends have received stubs for now and will ICE if you use become on them. I suspect there is some bikeshedding to be done on how we should go about implementing this for other backends, but it should be relatively straightforward for GCC after this is merged.

During development I also put together a POC bytecode VM based on tail call dispatch to test these changes out and analyze the codegen to make sure it generates expected assembly. That is available here.

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r? @oli-obk

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@rustbot rustbot added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. 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 20, 2025
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Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

@xacrimon xacrimon force-pushed the explicit-tail-call branch from 6a6d3c2 to f912c90 Compare July 20, 2025 22:49
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@xacrimon xacrimon force-pushed the explicit-tail-call branch 2 times, most recently from b36cf15 to ac87434 Compare July 20, 2025 23:10
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@xacrimon xacrimon force-pushed the explicit-tail-call branch from 138b2a7 to 3fac279 Compare July 21, 2025 00:14
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I suspect there is quite a bit of bikeshedding on how we should go about implementing this for WASM

I thought LLVM already supported the Wasm tail-call instructions — what's the missing piece that needs bikeshedding?

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oli-obk commented Jul 21, 2025

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it seems a tailcall test is still hitting a stack overflow, do you know why?

@rustbot rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 21, 2025
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rustbot commented Jul 21, 2025

Reminder, once the PR becomes ready for a review, use @rustbot ready.

@rustbot rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jul 21, 2025
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xacrimon commented Jul 21, 2025 •
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@rustbot author

it seems a tailcall test is still hitting a stack overflow, do you know why?

This was me mistakenly marking the rest @run-fail instead of @run-crash, that was intentional and it has now been fixed. That one is marked no-tce and doesn't use become. There is another one called tce instead of no-tce which uses become and is set to @run-pass.

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I suspect there is quite a bit of bikeshedding on how we should go about implementing this for WASM

I thought LLVM already supported the Wasm tail-call instructions — what's the missing piece that needs bikeshedding?

Ah, my knowledge is outdated then. I've edited my comment, though point still stands for backends that aren't llvm/gcc.

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@rustbot ready

@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 21, 2025
@xacrimon xacrimon requested review from bjorn3 and tmiasko July 21, 2025 18:37
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@WaffleLapkin I'm going to go ahead and guess this is something you want to have a look at.

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oli-obk commented Jul 22, 2025

@bors r+

Thanks! Glad we finally support it!

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bors commented Jul 22, 2025

📌 Commit ecf34e1 has been approved by oli-obk

It is now in the queue for this repository.

fmease added a commit to fmease/rust that referenced this pull request Jul 23, 2025
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Rollup merge of rust-lang#144232 - xacrimon:explicit-tail-call, r=oli…
cee3373 
…-obk

Implement support for `become` and explicit tail call codegen for the LLVM backend

This PR implements codegen of explicit tail calls via `become` in `rustc_codegen_ssa` and support within the LLVM backend. Completes a task on (rust-lang#112788). This PR implements all the necessary bits to make explicit tail calls usable, other backends have received stubs for now and will ICE if you use `become` on them. I suspect there is some bikeshedding to be done on how we should go about implementing this for other backends, but it should be relatively straightforward for GCC after this is merged.

During development I also put together a POC bytecode VM based on tail call dispatch to test these changes out and analyze the codegen to make sure it generates expected assembly. That is available [here](https://github.com/xacrimon/tcvm).
@fmease fmease mentioned this pull request Jul 23, 2025
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fmease commented Jul 23, 2025

https://github.com/rust-lang/rust/pull/144232/files#r2226977295 / #144382 (comment).
@bors r-

@bors bors 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-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jul 23, 2025
@fmease fmease closed this Jul 23, 2025
@rustbot rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Jul 24, 2025
@fmease fmease reopened this Jul 24, 2025
@rustbot rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 24, 2025
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@xacrimon xacrimon requested a review from fmease July 24, 2025 01:06
@xacrimon xacrimon force-pushed the explicit-tail-call branch from ecf34e1 to 4028c67 Compare July 24, 2025 01:07
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@rustbot ready

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xacrimon commented Jul 24, 2025 •
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@fmease @oli-obk This has now been rebased, please r+ when possible.

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fmease commented Jul 24, 2025

@bors r=oli-obk

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bors commented Jul 24, 2025

📌 Commit 4028c67 has been approved by oli-obk

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 24, 2025
fmease added a commit to fmease/rust that referenced this pull request Jul 24, 2025
@fmease
Rollup merge of rust-lang#144232 - xacrimon:explicit-tail-call, r=oli…
5b839dd 
…-obk

Implement support for `become` and explicit tail call codegen for the LLVM backend

This PR implements codegen of explicit tail calls via `become` in `rustc_codegen_ssa` and support within the LLVM backend. Completes a task on (rust-lang#112788). This PR implements all the necessary bits to make explicit tail calls usable, other backends have received stubs for now and will ICE if you use `become` on them. I suspect there is some bikeshedding to be done on how we should go about implementing this for other backends, but it should be relatively straightforward for GCC after this is merged.

During development I also put together a POC bytecode VM based on tail call dispatch to test these changes out and analyze the codegen to make sure it generates expected assembly. That is available [here](https://github.com/xacrimon/tcvm).
@fmease fmease mentioned this pull request Jul 24, 2025
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