do not add prolog for variadic naked functions

[goolmoos]

fixes #99858

[rustbot]

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[Nadrieril]

I am not knowledgeable in this part of the compiler, reassigning

r? compiler

[bors]

☔ The latest upstream changes (presumably #123838) made this pull request unmergeable. Please resolve the merge conflicts.

[pnkfelix]

This change looks like a strict improvement, so I'm going to approve the PR.

---

However, reading it raised a question for me.

According to my understanding of this PR, its effectively ensuring that args will be empty here:

rust/compiler/rustc_codegen_ssa/src/mir/mod.rs

Lines 223 to 224 in 9139d72

	let local_values = {
	let args = arg_local_refs(&mut start_bx, &mut fx, &memory_locals);

which then implies that this loop will not have any args to iterate over:

rust/compiler/rustc_codegen_ssa/src/mir/mod.rs

Lines 250 to 256 in 9139d72

	let retptr = allocate_local(mir::RETURN_PLACE);
	iter::once(retptr)
	.chain(args.into_iter())
	.chain(mir.vars_and_temps_iter().map(allocate_local))
	.collect()
	};
	fx.initialize_locals(local_values);

What about the other components in that chained iterator, namely the retptr and the vars_and_temps_iter ? In particular, does processing the retptr ever imply that a function epilogue will be emitted (which should not be the case for #[naked]) ?

[pnkfelix]

@bors r+

[bors]

📌 Commit 9139d72 has been approved by pnkfelix

It is now in the queue for this repository.