Implement support for `become` and explicit tail call codegen for the LLVM backend by xacrimon · Pull Request #144232 · rust-lang/rust

xacrimon · 2025-07-20T22:46:59Z

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.

rustbot · 2025-07-20T22:47:03Z

r? @oli-obk

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rustbot · 2025-07-20T22:47:05Z

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

jeffparsons · 2025-07-21T03:41:46Z

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?

oli-obk · 2025-07-21T06:56:48Z

@rustbot author

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

rustbot · 2025-07-21T06:56:52Z

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

xacrimon · 2025-07-21T15:55:42Z

@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.

xacrimon · 2025-07-21T16:36:03Z

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Ah, my knowledge is outdated then.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.

xacrimon · 2025-07-21T16:38:28Z

@rustbot ready

xacrimon · 2025-07-21T19:31:44Z

@WaffleLapkin I'm going to go ahead and guess this is something you want to have a look at.

oli-obk · 2025-07-22T07:42:20Z

@bors r+

Thanks! Glad we finally support it!

bors · 2025-07-22T07:42:23Z

📌 Commit ecf34e1 has been approved by oli-obk

It is now in the queue for this repository.

WaffleLapkin · 2025-07-30T14:48:53Z

Thanks!
@bors r+

bors · 2025-07-30T14:48:56Z

📌 Commit a448837 has been approved by WaffleLapkin

It is now in the queue for this repository.

…fleLapkin 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).

Rollup of 6 pull requests Successful merges: - #135975 (Implement `push_mut`) - #143672 (Fix Box allocator drop elaboration) - #144232 (Implement support for `become` and explicit tail call codegen for the LLVM backend) - #144663 (coverage: Re-land "Enlarge empty spans during MIR instrumentation") - #144683 (Simplify library dependencies on `compiler-builtins`) - #144685 (Only extract lang items once in codegen_fn_attrs) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 6 pull requests Successful merges: - #135975 (Implement `push_mut`) - #143672 (Fix Box allocator drop elaboration) - #144232 (Implement support for `become` and explicit tail call codegen for the LLVM backend) - #144663 (coverage: Re-land "Enlarge empty spans during MIR instrumentation") - #144685 (Only extract lang items once in codegen_fn_attrs) - #144717 (Move `rustc_middle::parameterized`) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #144232 - xacrimon:explicit-tail-call, r=WaffleLapkin 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 (#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).

…ffleLapkin Additional tce tests r? `@oli-obk` Adds known-bug tests for LLVM emissions regarding indirect operands for TCE. Also includes a test, `indexer.rs`, referring to function_table behavior described by the RFC. Depends on rust-lang#144232Closesrust-lang#144293

…ffleLapkin Additional tce tests r? ``@oli-obk`` Adds known-bug tests for LLVM emissions regarding indirect operands for TCE. Also includes a test, `indexer.rs`, referring to function_table behavior described by the RFC. Depends on rust-lang#144232Closesrust-lang#144293

…fleLapkin 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).

…ffleLapkin Additional tce tests r? ``@oli-obk`` Adds known-bug tests for LLVM emissions regarding indirect operands for TCE. Also includes a test, `indexer.rs`, referring to function_table behavior described by the RFC. Depends on rust-lang#144232Closesrust-lang#144293

L0uisc · 2025-08-06T19:20:36Z

 True,
 }

+/// Indicates to the call terminator codegen whether a cal


The final word on the line is misspelled - should be call if I'm not mistaken.
EDIT: I see the PR is merged last week. I'm not that familiar with GitHub's UI these days. Sorry for causing noise on a merged PR.

I've included a fix of this typo in #144865

…ffleLapkin,tgross35 Additional tce tests r? `@oli-obk` Adds known-bug tests for LLVM emissions regarding indirect operands for TCE. Also includes a test, `indexer.rs`, referring to function_table behavior described by the RFC. Depends on rust-lang#144232Closesrust-lang#144293

Rollup merge of #144650 - Borgerr:additional-tce-tests, r=WaffleLapkin,tgross35 Additional tce tests r? `@oli-obk` Adds known-bug tests for LLVM emissions regarding indirect operands for TCE. Also includes a test, `indexer.rs`, referring to function_table behavior described by the RFC. Depends on #144232Closes#144293

…n,tgross35 Additional tce tests r? `@oli-obk` Adds known-bug tests for LLVM emissions regarding indirect operands for TCE. Also includes a test, `indexer.rs`, referring to function_table behavior described by the RFC. Depends on rust-lang/rust#144232Closesrust-lang/rust#144293

Rollup of 6 pull requests Successful merges: - rust-lang/rust#135975 (Implement `push_mut`) - rust-lang/rust#143672 (Fix Box allocator drop elaboration) - rust-lang/rust#144232 (Implement support for `become` and explicit tail call codegen for the LLVM backend) - rust-lang/rust#144663 (coverage: Re-land "Enlarge empty spans during MIR instrumentation") - rust-lang/rust#144685 (Only extract lang items once in codegen_fn_attrs) - rust-lang/rust#144717 (Move `rustc_middle::parameterized`) r? `@ghost` `@rustbot` modify labels: rollup

…n,tgross35 Additional tce tests r? `@oli-obk` Adds known-bug tests for LLVM emissions regarding indirect operands for TCE. Also includes a test, `indexer.rs`, referring to function_table behavior described by the RFC. Depends on rust-lang/rust#144232Closesrust-lang/rust#144293

rustbot assigned oli-obk Jul 20, 2025

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

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

tmiasko reviewed Jul 21, 2025
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Comment threadcompiler/rustc_codegen_ssa/src/mir/block.rs Outdated

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 requested review from bjorn3 and tmiasko July 21, 2025 18:37

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 30, 2025

Zalathar mentioned this pull request Jul 31, 2025
Rollup of 6 pull requests #144709
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Zalathar mentioned this pull request Jul 31, 2025
Rollup of 6 pull requests #144718
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bors merged commit 8628b78 into rust-lang:masterJul 31, 2025
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rustbot added this to the 1.90.0 milestone Jul 31, 2025

WaffleLapkin added the F-explicit_tail_calls `#![feature(explicit_tail_calls)]` label Aug 1, 2025

tnuha mentioned this pull request Aug 3, 2025
tail calls: Passing Indirect Operands into LLVM hits a span_bug #144855
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L0uisc reviewed Aug 6, 2025
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InvalidPathException mentioned this pull request Sep 1, 2025
Support threaded dispatch InvalidPathException/wagmi#1
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jonathanpwang mentioned this pull request Sep 22, 2025
feat(nightly): execution becomes faster openvm-org/openvm#2013
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stevefan1999-personal mentioned this pull request Oct 2, 2025
Tracking issue for Boa's performance reports boa-dev/boa#2975
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zhuzhu81998 mentioned this pull request Mar 4, 2026
idea: opcode dispatch performance improvements boa-dev/boa#4858
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