wrong C ABI for `@alignOf(@Vector(4, f32))` on arm

[xxxbxxx]

Zig Version

0.10.0-dev.3008

Steps to Reproduce

There seem to be a special case in the ABI for arm (and armeb, thumb, thumbeb) with a reduced alignment for 16-bytes vectors.

empirically:
https://godbolt.org/z/ca6G58G19

but I don't know the exact rules.

Expected Behavior

@alignOf(@Vector(4, f32))==8
@alignOf(@Vector(16, i8))==8

Actual Behavior

@alignOf(@Vector(4, f32))==16
@alignOf(@Vector(16, i8))==16

[andrewrk]

What's wrong with the bigger alignment?

[xxxbxxx]

mmm I don't remember if I had a practical problem with this or not.

But more likely I stumbled upon this when investigating #12026
and trying to write a test: https://github.com/ziglang/zig/pull/12028/files

however, I reckon there will be ABI issues if sharing an extern struct with C code.

[xxxbxxx]

yeah, here is a test case:
(but to be precise, it's a madeup exemple, not something I encountered doing usefull code - I seldom use arm anyway)

repro.c:

#include <arm_neon.h>

typedef struct {
    int a, b;
    int32x4_t vec0;
    int c;
} my_struct;

int read_value(my_struct* S) { return S->c; }

repro.zig:

const std = @import("std");

const MyStruct = extern struct {
    a: c_int,
    b: c_int,
    vec0: @Vector(4, i32),
    c: c_int,
};

extern fn read_value(s: *MyStruct) c_int;

test {
    var s: MyStruct = .{ .a = 0, .b = 0, .vec0 = .{ 0, 0, 0, 0 }, .c = 123 };
    const v = read_value(&s);
    try std.testing.expectEqual(@as(c_int, 123), v);
}

zig test -target arm-linux-musl -mcpu cortex_a9 repro.c repro.zig
expected 123, found 0
1/1 repro.test_0...FAIL (TestExpectedEqual)

whereas if I move a,b after vec0, the test passes.

[andrewrk]

Ah, so it's a C ABI issue.

[lerno]

If I recall what the Clang sources say about this, is that ARM_AAPCS and ARM_AAPCS_LINUX are limited to 8 byte max alignment. For Android, AAPCS16 and APCS_GNU, there is no limit.