fix for multiple #[repr(align(N))] on functions

[folkertdev]

tracking issue: #82232
fixes #132464

The behavior of align is specified at https://doc.rust-lang.org/reference/type-layout.html#r-layout.repr.alignment.align

For align, if the specified alignment is less than the alignment of the type without the align modifier, then the alignment is unaffected.

So in effect that means that the maximum of the specified alignments should be chosen. That is also the current behavior for align on ADTs:

#![feature(fn_align)]

#[repr(C,  align(32), align(64))]
struct Foo {
    x: u64,
}

const _: () = assert!(core::mem::align_of::<Foo>() == 64);

// See the godbolt LLVM output: the alignment of this function is 32
#[no_mangle]
#[repr(align(32))]
#[repr(align(64))]
fn foo() {}

// The current logic just picks the first alignment: the alignment of this function is 64
#[no_mangle]
#[repr(align(64))]
#[repr(align(32))]
fn bar() {}

https://godbolt.org/z/scco435jE

rust/compiler/rustc_middle/src/ty/mod.rs

Lines 1529 to 1532 in afa859f

	attr::ReprAlign(align) => {
	max_align = max_align.max(Some(align));
	ReprFlags::empty()
	}

The #132464 issue is really about parsing/representing the attribute, which has already been improved and now uses the "parse, don't validate" attribute approach. That means the behavior is already different from what the issue describes: on current main, the first value is chosen. This PR fixes a logic error, where we just did not check for the effect of two or more align modifiers. In combination, that fixes the issue.

cc @jdonszelmann if you do have further thoughs here

[rustbot]

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[rustbot]

Some changes occurred in compiler/rustc_codegen_ssa/src/codegen_attrs.rs

cc @jdonszelmann

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

[jdonszelmann]

I'll give this a review in a sec :)

[jdonszelmann]

This looks good! Simple change, and also closes the issue. Nice :) @bors r+ rollup

[bors]

📌 Commit a6dcd51 has been approved by jdonszelmann

It is now in the queue for this repository.