AIX: use align 8 for byval parameter

[mustartt]

On AIX, byval pointer arguments are aligned to 8 bytes based on the 64bit register size. For example, the C callee https://godbolt.org/z/5f4vnG6bh will expect the following argument.

ptr nocapture noundef readonly byval(%struct.TwoU64s) align 8 %0

This case is captured by run-make/extern-fn-explicit-align

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[gilamn5tr]

Looks good.

[workingjubilee]

...wait, so on AIX, loads from the stack aren't aligned to the size of the type? whee...

[mustartt]

...wait, so on AIX, loads from the stack aren't aligned to the size of the type? whee...

Not exactly the case. That's just how we are representing it in the IR. In reality, it almost always 16 byte aligned.

[workingjubilee]

??? what is the actuality then?

[hubert-reinterpretcast]

On AIX, byval pointer arguments have natural alignment of 4 bytes.

This sounds like a statement for 32-bit mode. TMK, in 64-bit mode, Clang produces 8-byte alignment.
Additionally, 16-byte alignment is used when the argument type involves a non-packed PPC vector.

[mustartt]

AIX, loads from the stack aren't aligned to the size of the type?

For input values of the parameter, yes, the loads from the stack can be not aligned to the type. But the function local have to have proper alignment.

That's just how we are representing it in the IR.

We do not (at the assembly level) pass pointers to the callee. The incoming parameter is represented as a pointer in the IR (associated to register-sized/aligned memory when not stored in the registers themselves). That is: The alignment is associated with the size of registers (except where a parameter type has an non-packed vector in it).

[hubert-reinterpretcast]

But the function local have to have proper alignment.

@mustartt, Clang creates the function local separately, does the Rust front-end do so as well?

[mustartt]

But the function local have to have proper alignment.

@mustartt, Clang creates the function local separately, does the Rust front-end do so as well?

Yes, the function locals will have the type's alignment. For example:

definei64@rust_func(ptrbyval([16 x i8]) align8%0) unnamed_addr #0personalityptr@rust_eh_personality {
start:
%val = alloca [16 x i8], align16callvoid@llvm.memcpy.p0.p0.i64(ptralign16%val, ptralign8%0, i6416, i1false)
...
}

[hubert-reinterpretcast]

Should there be a TODO regarding PowerPC SIMD vector types (https://doc.rust-lang.org/core/arch/powerpc64/index.html)?

[gilamn5tr]

Probably. Its 16 for vector types?

[hubert-reinterpretcast]

Probably. Its 16 for vector types?

It's 16 for vector types and aggregates with (non-packed) vector subobjects. There seems to be a bug in Clang for the packed cases.

[Nadrieril]

I know nothing about this :)
r? compiler

[wesleywiser]

This looks correct to me. Let's cc the AIX target maintainers though: @daltenty@gilamn5tr

[wesleywiser]

One more friendly reminder to the target maintainers to see if they spot an issue with this before merging: @daltenty@gilamn5tr

[gilamn5tr]

@wesleywiser We're happy with it for now. LGTM!

[wesleywiser]

Thanks for taking a look @gilamn5tr!

@bors r+

[bors]

📌 Commit 7bfcddf has been approved by wesleywiser

It is now in the queue for this repository.

[daltenty]

Confirming this looks good from the AIX target perspective