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

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.

compiler/rustc_codegen_ssa/src/mir/block.rs

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Ah, my knowledge is outdated then.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.

xacrimon · 2025-07-24T01:07:06Z

@rustbot ready

xacrimon · 2025-07-24T01:07:35Z

@fmease @oli-obk This has now been rebased, please r+ when possible.

fmease · 2025-07-24T01:38:05Z

@bors r=oli-obk

bors · 2025-07-24T01:38:07Z

📌 Commit 4028c67 has been approved by oli-obk

It is now in the queue for this repository.

…-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).

Rollup of 16 pull requests Successful merges: - #143374 (Unquerify extern_mod_stmt_cnum.) - #143838 (std: net: uefi: Add support to query connection data) - #144014 (don't link to the nightly version of the Edition Guide in stable lints) - #144094 (Ensure we codegen the main fn) - #144218 (Use serde for target spec json deserialize) - #144221 (generate elf symbol version in raw-dylib) - #144232 (Implement support for `become` and explicit tail call codegen for the LLVM backend) - #144240 (Add more test case to check if the false note related to sealed trait suppressed) - #144247 (coretests/num: use ldexp instead of hard-coding a power of 2) - #144276 (Use less HIR in check_private_in_public.) - #144278 (add Rev::into_inner) - #144317 (pass build.npm from bootstrap to tidy and use it for npm install) - #144320 (rustdoc: avoid allocating a temp String for aliases in search index) - #144334 (rustc_resolve: get rid of unused rustdoc::span_of_fragments_with_expansion) - #144335 (Don't suggest assoc ty bound on non-angle-bracketed problematic assoc ty binding) - #144358 (Stop using the old `validate_attr` logic for stability attributes) r? `@ghost` `@rustbot` modify labels: rollup

jieyouxu · 2025-07-24T10:01:34Z

Failed in rollup: #144387 (comment)

---- [ui] tests/ui/explicit-tail-calls/recursion-no-tce.rs stdout ----

error: test did not crash! code=Some(0) so test would pass with `run-pass`
status: exit status: 0
command: cd "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/explicit-tail-calls/recursion-no-tce" && RUSTC="/checkout/obj/build/unpacked-dist/rustc-nightly-x86_64-unknown-linux-gnu/rustc/bin/rustc" RUST_TEST_THREADS="36" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/explicit-tail-calls/recursion-no-tce/a"
--- stdout -------------------------------
1000000
------------------------------------------
stderr: none

@bors r-

oli-obk · 2025-07-24T10:11:14Z

Yea... I think you can just remove that test, it depends too much on optimizations and stack size and other things outside of our control.

…nd the LLVM codegen bckend.

…eed tail calls.

…ust-lang#144232 (comment)

xacrimon · 2025-07-24T16:05:07Z

@rustbot ready

Alright, that makes sense. It's been removed.

rustbot assigned oli-obk Jul 20, 2025

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xacrimon requested a review from fmease July 24, 2025 01:06

xacrimon force-pushed the explicit-tail-call branch from ecf34e1 to 4028c67 Compare July 24, 2025 01:07

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 mentioned this pull request Jul 24, 2025

Rollup of 16 pull requests #144387

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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 24, 2025

xacrimon added 10 commits July 24, 2025 18:03

Implement support for explicit tail calls in the MIR block builders a…

7552e6b

…nd the LLVM codegen bckend.

chore: clang-format

9762aef

move tests in codegen to codegen-llvm

6c1f0b9

chore: fix typo

ad4a5c0

don't end lifetimes after call

6f98ddc

recursion-no-tce.rs should be run-crash

08d4616

fix typo in codegen test

ec64d6f

add an error message if you attempt to pass indirect args via guarant…

b1e4463

…eed tail calls.

use span_bug instead of bug

e0448a3

remove the test that attempts to indice a stack overflow crash. see r…

58c6633

…ust-lang#144232 (comment)

xacrimon force-pushed the explicit-tail-call branch from 4028c67 to 58c6633 Compare July 24, 2025 16:05

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 24, 2025

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

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

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

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