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

[xacrimon]

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 quite a bit of bikeshedding on how we should go about implementing this for WASM, 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]

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[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

[rust-log-analyzer]

The job aarch64-gnu-llvm-19-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

failures:

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

error: test did not exit with failure! code=None so test would pass with `run-crash`
status: signal: 6 (SIGABRT) (core dumped)
command: cd "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/explicit-tail-calls/recursion-no-tce" && RUSTC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" RUST_TEST_THREADS="4" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/explicit-tail-calls/recursion-no-tce/a"
stdout: none
--- stderr -------------------------------

thread '<unknown>' has overflowed its stack
fatal runtime error: stack overflow, aborting

[jeffparsons]

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]

@rustbot author

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

[rustbot]

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

[tmiasko]

lifetime_end after a musttail call will create ill-formed LLVM IR: "the call must immediately precede a ret instruction, or a pointer bitcast followed by a ret instruction".

It also hints at more fundamental challenge. Arguments using PassMode::Indirect are passed as pointers to stack allocations which will be deallocated by musttail call.

Perhaps it would make more sense to bug! in the situation, until PassMode::Indirect arguments are actually supported.