Re: [PATCH 1/2] CALL_INSN may not be a real function call.

[Hongtao Liu <crazylht@gmail.com>]

On Tue, Jul 6, 2021 at 9:37 AM Hongtao Liu <crazylht@gmail.com> wrote:
>
> On Tue, Jul 6, 2021 at 7:31 AM Segher Boessenkool
> <segher@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.
+   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