Re: [openrisc] Compiler register alloc doesn't match spec

[Marko Mlinar <markom@opencores.org>]

Hi Scott,

I just remembered that this 'bug' exists. Once I wrote an asm function called 
from C, and figured that out, because spec was not in sync ;)

This really has to be repaired. Since you have all the newest files 
downloaded, can you please change register numbering;
otherwise just say and I will update them.

thanks!
Marko

On Wednesday 05 March 2003 20:55, Scott Furman wrote:
> In the architecture spec, the following registers are listed as
> callee-saved, i.e. preserved across function calls:
>
>
>
> R1 (SP), R2 (FP), R9 (LR), R10, and all odd registers R13-R31
>
>
>
> The compiler, however, is defined such that the following registers are
> callee-saved:
>
>
>
>             R1 (SP), R2 (FP), R9 (LR),  and all even registers R10-R30
>
>
>
> In other words, the spec and the compiler don't match for registers
> R12-R31.
>
>
>
> (The relevant portion of gcc/gcc/config/or32.h is reproduced below)
>
>
>
> I suppose that the compiler could be changed to match the spec, but that
> would render all existing library binaries as incorrect and hence
> unlinkable against newer code that uses spec-conformant register
> allocation.  Probably the simpler thing to do is revamp the spec to
> match the implementation.
>
>
>
> On a related note, I noticed that R2, the frame pointer, is listed in
> the FIXED_REGISTERS array, preventing it from being used by the register
> allocator as a GPR.  For the case in which no frame pointer is needed
> for a function, there's no reason that R2 couldn't be used as a
> conventional callee-saved register, hence I would recommend removing it
> from the FIXED_REGISTERS array.  This change would allow
> backward-compatible linking with existing binaries.
>
>
>
> /* 1 for registers that have pervasive standard uses
>
>    and are not available for the register allocator.
>
>    On the or1k, these are r1 as stack pointer and
>
>    r2 as frame/arg pointer.  */
>
> #define FIXED_REGISTERS {1, 1, 1, 0, 0, 0, 0, 0,    0, 0, 0, 0, 0, 0, 0,
> 0 \
>
>                   , 0, 0, 0, 0, 0, 0, 0, 0,   0, 0, 0, 0, 0, 0, 0, 0}
>
> /* 1 for registers not available across function calls.
>
>    These must include the FIXED_REGISTERS and also any
>
>    registers that can be used without being saved.
>
>    The latter must include the registers where values are returned
>
>    and the register where structure-value addresses are passed.
>
>    Aside from that, you can include as many other registers as you like.
> */
>
> #define CALL_USED_REGISTERS {1, 1, 1, 1, 1, 1, 1, 1,   1, 1, 0, 1, 0, 1,
> 0, 1 \
>
>                       , 0, 1, 0, 1, 0, 1, 0, 1,  0, 1, 0, 1, 0, 1, 0, 1}

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