explicit tail calls: support indirect arguments #151143

folkertdev · 2026-01-14T21:26:52Z

tracking issue: #112788

After discussion in #144855, I was wrong and it is actually possible to support tail calls with PassMode::Indirect { on_stack: false, .. } arguments.

Normally an indirect argument with on_stack: false would be passed as a pointer into the caller's stack frame. For tail calls, that would be unsound, because the caller's stack frame is overwritten by the callee's stack frame.

Therefore we store the argument for the callee in the corresponding caller's slot. Because guaranteed tail calls demand that the caller's signature matches the callee's, the corresponding slot has the correct type.

To handle cases like the one below, the tail call arguments must first be copied to a temporary, and can only then be copied to the caller's argument slots.

// A struct big enough that it is not passed via registers.
pub struct Big([u64; 4]);

fn swapper(a: Big, b: Big) -> (Big, Big) {
    become swapper_helper(b, a);
}

I'm not really familiar with MIR and what tricks/helpers we have, so I'm just cobbling this together. Hopefully we can arrive at something more elegant.

compiler/rustc_codegen_ssa/src/mir/block.rs

folkertdev · 2026-01-14T21:36:17Z

compiler/rustc_codegen_ssa/src/mir/block.rs

+                        PassMode::Indirect { on_stack: true, .. } => {
+                            // FIXME: some LLVM backends (notably x86) do not correctly pass byval
+                            // arguments to tail calls (as of LLVM 21). See also:
+                            //
+                            // - https://github.com/rust-lang/rust/pull/144232#discussion_r2218543841
+                            // - https://github.com/rust-lang/rust/issues/144855
+                            span_bug!(
+                                fn_span,
+                                "arguments using PassMode::Indirect {{ on_stack: true, .. }} are currently not supported for tail calls"
+                            )


I've kept the span_bug! for now.

It looks like a fix for x86 might get cherry-picked into LLVM 22. If so, I think that is enough support to allow this variant too. At that point riscv would be the next most commonly used target that would miscompile.

folkertdev · 2026-01-15T17:32:28Z

@bors try=x86_64-msvc-1,x86_64-msvc-2

rust-bors · 2026-01-15T17:32:31Z

Unknown command "try". Run @bors help to see available commands.

folkertdev · 2026-01-15T17:32:48Z

@bors try job=x86_64-msvc-1,x86_64-msvc-2

explicit tail calls: support indirect arguments try-job: x86_64-msvc-1 try-job: x86_64-msvc-2

rust-bors · 2026-01-15T19:20:43Z

💔 Test for e8cb9bb failed: CI. Failed job:

try - x86_64-msvc-1 (web logs, enhanced plaintext logs)

folkertdev · 2026-01-15T22:22:55Z

@bors try job=x86_64-msvc-1,x86_64-msvc-2

rust-bors · 2026-01-15T22:22:59Z

⌛ Trying commit 33942c6 with merge 282e2c8…

To cancel the try build, run the command @bors try cancel.

Workflow: https://github.com/rust-lang/rust/actions/runs/21048427234

explicit tail calls: support indirect arguments try-job: x86_64-msvc-1 try-job: x86_64-msvc-2

folkertdev added the F-explicit_tail_calls `#![feature(explicit_tail_calls)]` label Jan 14, 2026

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jan 14, 2026

folkertdev force-pushed the tail-call-indirect branch 2 times, most recently from 8a1cd78 to 6f7eede Compare January 14, 2026 21:41

folkertdev commented Jan 14, 2026

View reviewed changes