translate-c: emit alignCast when casting pointers

[raulgrell]

This small patch makes all pointer casts include an align cast as well. This isn't great, but I haven't quite worked out how to work out when they're needed. Tests still need to be changed.

Is there a simple way of running only the translate-c tests?

[LemonBoy]

This small patch makes all pointer casts include an align cast as well. This isn't great, but I haven't quite worked out how to work out when they're needed.

I think you can use getTypeAlignIfKnown and only emit a @alignCast if the pointee (is that even a word) types alignment differ.

unsigned ZigClangASTContext_getTypeAlignIfKnown(const ZigClangASTContext* self, ZigClangQualType T) {
    return reinterpret_cast<const clang::ASTContext *>(self)->getTypeAlignIfKnown(bitcast(T));
}

Is there a simple way of running only the translate-c tests?

./zig build --build-file ../build.zig test-translate-c

[raulgrell]

@LemonBoy Thanks, that really helped! I still need to trigger a test case, but in theory this should be enough. The other places where ptrCast is emitted don't seem to be an issue.

I'll let you know once I've got some tests

EDIT: Nope, I'm not really doing the right thing. I'll try again tonight.

[raulgrell]

Turns out I was checking the alignment of the pointer and not the pointee... At this point, this handles the following test code if you ignore the c alignment attributes:

typedef unsigned char U8;
typedef unsigned short __attribute__((aligned(1))) U16;
typedef unsigned int __attribute__((aligned(1))) U32;

void main() {
    U8 a;
    U8 b;
    U16 c;

    void * pa = (U8 *)(&a);
    void * px = (U16 *)(&a);
    U32 * pxx = (U16 *)(&a);
    void * ppx = (U32 *)(&a);
}

generating

pub const U8 = u8;
pub const U16 = c_ushort;
pub const U32 = c_uint;
pub export fn main() void {
    var a: U8 = undefined;
    var b: U8 = undefined;
    var c: U16 = undefined;
    var pa: ?*c_void = @ptrCast(?*c_void, &a);
    var px: ?*c_void = @ptrCast(?*c_void, @ptrCast([*c]U16, @alignCast(@alignOf([*c]U16), &a)));
    var pxx: [*c]U32 = @ptrCast([*c]U32, @alignCast(@alignOf([*c]U32), @ptrCast([*c]U16, @alignCast(@alignOf([*c]U16), &a))));
    var ppx: ?*c_void = @ptrCast(?*c_void, @ptrCast([*c]U32, @alignCast(@alignOf([*c]U32), &a)));
}

Once translate-c can handle alignment attributes, it should skip the alignCast

[raulgrell]

Questions:

• If the bit size is known, should it be included directly instead of calling @Alignof on the target type?
• Should I extract the alignment comparison into a function? Or is the duplication ok?
• If the alignment is not known, should the alignCasts be skipped?

[raulgrell]

@LemonBoy's implementation in #2318 is more robust than this one. The only practical difference is hardcoding the alignment vs calling alignOf, which was already mentioned in the comments of his PR.