On Mon, 7 Jun 2021, Qing Zhao wrote:
> Hi,
>
> > On Jun 7, 2021, at 2:53 AM, Richard Biener <rguent...@suse.de> wrote:
> >
> >>
> >> To address the above suggestion:
> >>
> >> My study shows: the call to __builtin_clear_padding is expanded during
> >> gimplification phase.
> >> And there is no __bultin_clear_padding expanding during rtx expanding
> >> phase.
> >> However, for -ftrivial-auto-var-init, padding initialization should be
> >> done both in gimplification phase and rtx expanding phase.
> >> since the __builtin_clear_padding might not be good for rtx expanding,
> >> reusing __builtin_clear_padding might not work.
> >>
> >> Let me know if you have any more comments on this.
> >
> > Yes, I didn't suggest to literally emit calls to __builtin_clear_padding
> > but instead to leverage the lowering code, more specifically share the
> > code that figures _what_ is to be initialized (where the padding is)
> > and eventually the actual code generation pieces. That might need some
> > refactoring but the code where padding resides should be present only
> > a single time (since it's quite complex).
>
> Okay, I see your point here.
>
> >
> > Which is also why I suggested to split out the padding initialization
> > bits to a separate patch (and option).
>
> Personally, I am okay with splitting padding initialization from this current
> patch,
> Kees, what’s your opinion on this? i.e, the current -ftrivial-auto-var-init
> will NOT initialize padding, we will add another option to
> Explicitly initialize padding.
It would also be possible to have -fauto-var-init, -fauto-var-init-padding
and have -ftrivial-auto-var-init for clang compatibility enabling
both. Or -fauto-var-init={zero,pattern,padding} and allow
-fauto-var-init=pattern,padding to be specified. Note there's also
padding between auto variables on the stack - that "trailing"
padding isn't initialized either? (yes, GCC sorts variables to minimize
that padding) For example for
void foo()
{
char a[3];
bar (a);
}
there's 12 bytes padding after 'a', shouldn't we initialize that? If not,
why's other padding important to be initialized?
Richard.
