On Fri, Sep 18, 2020 at 01:49:46PM -0700, Linus Torvalds wrote:
> 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.
OK, I see. I'll check this.
> > > // 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.
My fault, sorry. By error, I didn't run these tests on the head but in
one of my branches that coincidentally had some patches doing some
reassociation which then triggers the optimization.
-- Luc
