Re: Two entry points for a function

[Paul Cercueil]

Le lun., août 8 2022 at 23:04:02 +0200, Marc Nieper-Wißkirchen 
<marc.nieper+gnu@gmail.com> a écrit :
> Hi Paul,
>
> thanks for chiming in.
>
> I didn't know about jit_align.  Maybe it should be documented (along 
> with some instruction that embeds given bytes in the code stream?).
>
> So what I have in mind could probably be done with GNU lightning as 
> follows:

I guess that works, yes. Be careful that your "short piece of code" 
cannot do much (Lightning won't save/restore the callee-saved registers 
for you, stack won't work, jit_ret() won't work, etc).

-Paul

> jit_prolog ();
> jit_tramp (256);
> jit_align (16);
> entry1 = label_indirect ();
> /* short piece of code */
> jit_align (16);
> entry2 = label_indirect ();
> ...
> jit_epilog ();
> jit_emit ();
> char *addr1 = jit_address (entry1);
> char *addr2 = jit_address (entry2);
> assert (addr2 - addr1 == 16);
> ...
>
> Am Mo., 8. Aug. 2022 um 22:53 Uhr schrieb Paul Cercueil 
> <paul@crapouillou.net>:
> > Hi Marc,
> >
> >  I don't think you can have multiple entry points of a function (or
> >  maybe with jit_indirect()?) but you can cheese things a bit with a
> >  trampoline.
> >
> >  Here's what I do, more or less:
> >
> >  ----
> >
> >  jit_prolog();
> >  jit_tramp(256);
> >
> >  for (i = 0; i < nb_entry_points; i++) {
> >      jit_addi(JIT_R0, 1);
> >      jit_align(8);
> >  }
> >
> >  jit_epilog();
> >  jit_prolog();
> >
> >  /* Read JIT_R0, its value give us what entry point was taken */
> >
> >  jit_epilog();
> >
> >  ----
> >
> >  In that example my JIT_R0 is always 0 on entry. The jit_align() 
> > makes
> >  sure that each one of your entry points is aligned to 8 bytes. 
> > There is
> >  still some decoding needed (I read a C function pointer from a 
> > table at
> >  the index pointed by JIT_R0), but the decoding is done in that 
> > function
> >  and not in the callers.
> >
> >  Cheers,
> >  -Paul
> >
> >  Le lun., août 8 2022 at 22:16:28 +0200, Marc Nieper-Wißkirchen
> >  <marc.nieper+gnu@gmail.com> a écrit :
> >  > I am looking for a way to implement a function (pointer) with GNU
> >  > lightning that has two entry points.
> >  >
> >  > In native assembly, I would code something like this:
> >  >
> >  >         .align 16
> >  > entry1: jmp L0
> >  >         .align 16
> >  > entry2: ...
> >  >         ...
> >  > L0:     ...
> >  >
> >  > The function would be represented by a pointer to entry1.  If I
> >  > wanted to jump to the second entry point, I would just add 16 to 
> > the
> >  > pointer.
> >  >
> >  > How can a similar thing be achieved with GNU lightning?  So far, 
> > the
> >  > best that has come to my mind is to use one more indirection.  The
> >  > "function pointer" points to a structure having the addresses of
> >  > entry1 and entry2 as fields.  (Let us assume we are in a 
> > trampoline
> >  > so that the stack is already set up).  But this is less efficient 
> > due
> >  > to the extra indirection.
> >  >
> >  > Is there a way to emulate the above native code with GNU 
> > lightning?
> >  > And, if not, what would have to be added?
> >  >
> >  > Marc