Sv: Global register variables

[Marc Nieper-Wißkirchen]

Am Do., 2. Sept. 2021 um 19:06 Uhr schrieb Petter Tomner <tomner@kth.se>:

> > I don't like the idea of "magic" variable names, since it can change the
> meaning of existing client code.
> > I was thinking of an API, maybe something like:
>
> Sorry, I thought it looked bad enough. I should have been clearer that it
> was no serious patch and meant
> for Marc Nieper-Wißkirchen. I couldn't bother adding stuff all the call
> chain up to test it.
>

Yes, thanks, that was a fine proof-of-concept.  And looked bad enough for
me. ;)

The actual suggestion of "gcc_jit_global_set_register" looks good to me.
Marc


> > Is the name actually optional from the perspective of GCC's backend?
> > Maybe allow NULL for the regname param if it is?
>
> No it seems to be no support for no name. I guess any solution would have
> to poll for a register name first
> and then set the asm name.
>
> Regards
>
> -----Ursprungligt meddelande-----
> Från: David Malcolm <dmalcolm@redhat.com>
> Skickat: den 2 september 2021 18:45
> Till: Petter Tomner <tomner@kth.se>; Marc Nieper-Wißkirchen <
> marc.nieper+gnu@gmail.com>; jit@gcc.gnu.org
> Ämne: Re: Sv: Global register variables
>
> On Thu, 2021-09-02 at 16:05 +0000, Petter Tomner wrote:
> > Hi! Conceptually is seems quite straight forward if you want to try it
> > on some code you have.
> > See the attached code that hack any identifier starting with reg_* to
> > a "global register" named *.
> >
> > It seems to work with e.g "reg_r14" and "reg_r15" on my machine with
> > some silly getter and setter functions for those registers
>
> I don't like the idea of "magic" variable names, since it can change the
> meaning of existing client code.  I was thinking of an API, maybe something
> like:
>
> extern gcc_jit_lvalue *
> gcc_jit_global_set_register (gcc_jit_lvalue *global,
>                              const char *regname);
>
> (taking inspiration from gcc_jit_global_set_initializer) ?
>
> >  but there is an array "global_regs" in reginfo.c that is not reset
> > after each run so only run it once.
>
> Good catch.  Sounds like we'd need a reginfo_c_finalize, to be called from
> toplev::finalize in toplev.c
>
>
> > And as EXPORTED global type.
> >
> > I guess error handling and eg. having not to specify register
> > explicitly for portability would be the interesting and little bit
> > harder part.
>
> Is the name actually optional from the perspective of GCC's backend?
>
> Maybe allow NULL for the regname param if it is?
>
> Hope this is constructive
> Dave
>
> >
> >
> > Regards
> >
> > From f45d0e328b21b2a4b23021c099f1f3b0ad6e6002 Mon Sep 17 00:00:00
> > 2001
> > From: Petter Tomner <tomner@kth.se>
> > Date: Thu, 2 Sep 2021 17:23:24 +0200
> > Subject: [PATCH] Hack reg_* identifiers to global register
> >
> > ---
> >  gcc/jit/jit-playback.c | 15 ++++++++++++++-
> >  1 file changed, 14 insertions(+), 1 deletion(-)
> >
> > diff --git a/gcc/jit/jit-playback.c b/gcc/jit/jit-playback.c index
> > 79ac525e5df..fa011ed4c0c 100644
> > --- a/gcc/jit/jit-playback.c
> > +++ b/gcc/jit/jit-playback.c
> > @@ -40,7 +40,7 @@ along with GCC; see the file COPYING3.  If not see
> >  #include "gcc.h"
> >  #include "diagnostic.h"
> >  #include "stmt.h"
> > -
> > +#include "varasm.h"
> >  #include <pthread.h>
> >
> >  #include "jit-playback.h"
> > @@ -567,6 +567,19 @@ global_new_decl (location *loc,
> >        break;
> >      }
> >
> > +  { /* Filescope register variables on identifiers reg_REGNUM */
> > +    const char *key = "reg_";
> > +
> > +    for (int i = 0; i < 4; i++)
> > +      if (name[i] != key[i]) goto bail;
> > +    if (!name[4]) goto bail;
> > +
> > +    DECL_REGISTER (inner) = 1;
> > +    set_user_assembler_name (inner, name + 4);
> > +    DECL_HARD_REGISTER (inner) = 1;
> > +  }
> > +bail:
> > +
> >    if (loc)
> >      set_tree_location (inner, loc);
> >
> > --
> > 2.20.1
> >
> > -----Ursprungligt meddelande-----
> > Från: Jit <jit-bounces+tomner=kth.se@gcc.gnu.org> För David Malcolm
> > via Jit
> > Skickat: den 2 september 2021 16:34
> > Till: Marc Nieper-Wißkirchen <marc.nieper+gnu@gmail.com>;
> > jit@gcc.gnu.org
> > Ämne: Re: Global register variables
> >
> > On Tue, 2021-08-31 at 08:39 +0200, Marc Nieper-Wißkirchen via Jit
> > wrote:
> > > Does libgccjit support GCC's global register variables ([1])?
> >
> > Not currently.
> >
> > >
> > > If not, could we expect them to be included?
> >
> > > One major use case of
> > > libgccjit seems to be JIT compilation of some virtual machine
> > > bytecode and virtual machines usually need a number of global
> > > variables (like some virtual stack pointer or heap pointer, etc.).
> > > It makes a lot of sense to hold them in global (callee-saved)
> > > registers.
> >
> > Sounds like a useful feature; if someone wants to cook up a patch I
> > can review it.
> >
> > Dave
> >
>
>
>