On Wed, May 8, 2024 at 1:33 AM Kewen.Lin <li...@linux.ibm.com> wrote:
>
> Hi Richi,
>
> >> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> >> index c584664e168..58e48f7dc55 100644
> >> --- a/gcc/doc/invoke.texi
> >> +++ b/gcc/doc/invoke.texi
> >> @@ -18363,11 +18363,11 @@ If @code{N=0}, no pad location is recorded.
> >> The NOP instructions are inserted at---and maybe before, depending on
> >> @var{M}---the function entry address, even before the prologue. On
> >> PowerPC with the ELFv2 ABI, for a function with dual entry points,
> >> -the local entry point is this function entry address.
> >> +@var{M} NOP instructions are inserted before the global entry point and
> >> +@var{N} - @var{M} NOP instructions are inserted after the local entry
> >> +point, which means the NOP instructions may not be consecutive.
> >
> > Isn't it @var{M-1} NOP instructions before the global entry? I suppose
>
> No, the existing documentation is a bit confusing, sigh ...
>
> > the existing
> >
> > "... with the function entry point before the @var{M}th NOP.
> > If @var{M} is omitted, it defaults to @code{0} so the
> > function entry points to the address just at the first NOP."
> >
> > wording is self-contradicting in a way since before the 0th NOP (default)
> > to me is the same as before the 1st NOP (M == 1). So maybe that should
> > be _after_ the @var{M}th NOP instead which would be consistent with your
> > ELFv2 docs? Maybe the sentence should be re-worded similar to your
> > ELVv2 one, specifying the number of NOPs before and after the entry point.
> >
>
> ... the current "with the function entry point before the Mth NOP."
> has the 0th NOP assumption, so the default (0th) NOP and 1st NOP (M == 1)
> are actually different, such as:
>
> -fpatchable-function-entry=3,0
>
> foo:
> nop
> nop
> nop
>
> -fpatchable-function-entry=3,1
>
> nop
> foo:
> nop
> nop
>
> Alan also had the similar concern on this wording before:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99888#c8
>
> " Alan Modra 2022-08-12 03:00:29 UTC
> "
> "(In reply to Segher Boessenkool from comment #7)
> "> '-fpatchable-function-entry=N[,M]'
> "> Generate N NOPs right at the beginning of each function, with the
> "> function entry point before the Mth NOP.
> "
> " Bad doco. Should be "after the Mth NOP" I think.
> " Or better written to avoid the concept of a 0th nop.
> " Default for M is zero, placing all nops after the function entry and
> " before normal function prologue code.
>
> BR,
> Kewen
>
> >> -The maximum value of @var{N} and @var{M} is 65535. On PowerPC with the
> >> -ELFv2 ABI, for a function with dual entry points, the supported values
> >> -for @var{M} are 0, 2, 6 and 14.
> >> +The maximum value of @var{N} and @var{M} is 65535.
> >> @end table
> >>
> So this patch is to change the current implementation by
> emitting the "before" NOPs before global entry point and
> the "after" NOPs after local entry point. The new behavior
Thanks. This looks good to me :)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99888#c5
--
宋方睿
