On Fri, Sep 18, 2020 at 1:41 PM Luc Van Oostenryck
<luc.vanoostenryck@xxxxxxxxx> wrote:
>
> I also have 2 questions here under.
>
> > struct bad {
> > unsigned long long a;
> > char b[];
> > };
> ...
> > // The layout is odd
> > // The code does "info->align_size = 0" for unsized arrays, but it
> > still works?
> > int odd(struct bad *a)
> > {
> > return __alignof__(*a);
> > }
>
> This returns 8. What's odd here?
The fact that it works correctly.
> The 0 align_size is only for the member 'b' and shouldn't have any
> effect on the alignment of the whole struct. What am I missing?
I wrote that code by looking at the sparse source, and _expected_ it
to return the wrong value.
Because the sparse code does
/*
* Unsized arrays cause us to not align the resulting
* structure size
*/
if (base_size < 0) {
info->align_size = 0;
base_size = 0;
}
so I expected that when base_size < 0, we'd drop the _previous_
alignment we saved.
But what I suspect goes on is that base_size is actually 0, not < 0.
But I didn't verify.
> > // Arrays of flexible-array structures are pretty nonsensical
> > // Plus we don't even optimize the constant return. Sad.
> > int not_nice(struct bad p[2])
> > {
> > return (void *)(p+1) - (void *)p;
> > }
>
> I don't understand what you mean by 'optimize the constant return'.
> test-linearize returns the only possible sensical answer (if the size
> of the structure is accepted to be 8):
> not_nice:
> .L2:
> <entry-point>
> ret.32 $8
That's not what I see. I see
not_nice:
.L2:
<entry-point>
add.64 %r3 <- %arg1, $8
sub.64 %r5 <- %r3, %arg1
trunc.32 %r6 <- (64) %r5
ret.32 %r6
which is rather different and not exactly optimal.
That wasn't what I _intended_ to look for, obviously. I expected the
code you quote.
I wonder why it works for you but not me.
Linus
