Refactor the way cg_ssa handles indirect returns (returns via sret) — Take 2

[antoyo]

This continues the work that @FractalFir done in #144976.

I managed to make this work, so this confirms the original approach works and is ready to merge.

r? workingjubilee

(assigning the same reviewer as the original PR)

The problem

The current way cg_ssa handles PassMode::Indirect is fundamentally incompatible with the requirements of the GCC backend. This is currently worked around in a very brittle way, that breaks on ARM.

In order to function correctly, cg_gcc requires the sret(indirect return) pointer to be treated in a special way.

The solution

This PR separates the sret pointer from all the other arguments, allowing each backend to decide how it wants to handle that pointer. This will allow GCC to do it's own thing, while changing nothing on the LLVM side. Currently, the PR just makes both backends preappend the sret arg, mimicking the previous behaviour.

The PR is based on this zulip suggestion.

Assumptions I made.

The PR makes certain assumptions about the ABI handling.

1. No LLVM intrinsic returns via sret. According to my sources, this is always true.
2. Tail calls don't currently return via sret. From asking about, it seems like the tail call handling does not currently support indirect returns. So, I did not add support for that.
3. TLS shims don't perform indirect returns, and asserts and drops also don't perform indirect returns(since they return nothing) - those assumptions seemed reasonable.
4. Inline assembly calls don't use indirect returns. I assumed functions like inline_asm_call will not need the sret handling code. I am not 100% sure about this, tough.

[rustbot]

Some changes occurred in compiler/rustc_codegen_llvm/src/builder/autodiff.rs

cc @ZuseZ4

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @GuillaumeGomez

[rustbot]

workingjubilee is currently at their maximum review capacity.
They may take a while to respond.

[antoyo]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 3ae8918 (3ae8918f6a7874e42bc8432d0339e0d172a2daa6)
Base parent: 4fefe36 (4fefe3656700e5528edddc2336667818917dfc70)

[rust-bors]

☔ The latest upstream changes (presumably #159844) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.