Re: Sv: Global register variables

[David Malcolm <dmalcolm@redhat.com>]

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
>