On Wed, Jul 7, 2021 at 4:40 AM Hongtao Liu via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Tue, Jul 6, 2021 at 9:37 AM Hongtao Liu <crazy...@gmail.com> wrote:
> >
> > On Tue, Jul 6, 2021 at 7:31 AM Segher Boessenkool
> > <seg...@kernel.crashing.org> wrote:
> > >
> > > Hi!
> > >
> > > I ran into this in shrink-wrap.c today.
> > >
> > > On Thu, Jun 03, 2021 at 02:54:07PM +0800, liuhongt via Gcc-patches wrote:
> > > > Use "used" flag for CALL_INSN to indicate it's a fake call. If it's a
> > > > fake call, it won't have its own function stack.
> > >
> > > Could you document somewhere what a "fake call" *is*? Including what
> > > that means to RTL, how this is expected to be used, etc.? In rtl.h is
> > fake call is used for TARGET_INSN_CALLEE_ABI, i'll add comments for
> > #define FAKE_CALL_P(RTX) in rtl.h
>
>
> Here's the patch I'm going to check in.
>
> Document FAKE_CALL_P in comments.
>
> gcc/ChangeLog:
>
> * rtl.h (FAKE_CALL_P): Add comments for FAKE_CALL_P.
>
> diff --git a/gcc/rtl.h b/gcc/rtl.h
> index 5ed0d6dd6fa..9afc60f08d8 100644
> --- a/gcc/rtl.h
> +++ b/gcc/rtl.h
> @@ -840,7 +840,13 @@ struct GTY(()) rtvec_def {
> #define CALL_P(X) (GET_CODE (X) == CALL_INSN)
>
> /* 1 if RTX is a call_insn for a fake call.
> - CALL_INSN use "used" flag to indicate it's a fake call. */
> + CALL_INSN use "used" flag to indicate it's a fake call.
> + Used by the x86 vzeroupper instruction,
> + in order to solve the problem of partial clobber registers,
> + vzeroupper is defined as a call_insn with a special callee_abi,
> + but it is not a real call and therefore has no function stack
> + of its own.
I think that's a big vague - you could then say a sibling or tail call
to a function
that doesn't set up a stack frame is fake as well? Maybe
"CALL_INSN use "used" flag to indicate the instruction
does not transfer control."
thus that this call is not affecting regular control flow? (it might
eventually still trap and thus cause non-call EH?)
Not sure if "no function stack of its own" is a good constraint,
vzeroupper does not perform any call or jump.
> + NB: FAKE_CALL_P is not handled thoroughly in the RTL. */
> #define FAKE_CALL_P(RTX) \
> (RTL_FLAG_CHECK1 ("FAKE_CALL_P", (RTX), CALL_INSN)->used)
>
>
>
>
> --
> BR,
> Hongtao
