From: Paul Shirley <Paul AT no DOT spam DOT please>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Disabling use of registers
Date: Mon, 10 Nov 1997 14:28:18 +0000
Organization: wot? me?
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References: <O6EQDcF78GA DOT 198 AT upnetnews03>
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In article <O6EQDcF78GA DOT 198 AT upnetnews03>, Carolyn Kelly-Pajot
<dehacked72 AT hotmail DOT com> writes
>Is there a way of making GCC not use a certain register? I'm trying to use some
>registers to speed up code, and want to make sure they are not used by the rest
>of my code.
>
>
Have a look at info /gcc/C extensions/Explicit reg vars/Global reg
vars/.

I believe GForth uses global registers for a large speedup, in most
normal code its hard to think of a case where it would be useful on
intel cpu's. I assume you know what you're doing but don't forget to
check wether the code actually runs faster: you could easily lose more
performance from register starvation than you gain.

Don't allocate eax, I'm not sure which other registers are preserved
across library calls. You must compile *all* modules with the same
global register settings *OR* take special care to explicitly
save/restore those registers at every external interface. Also you must
define global registers *before* any function prototypes or the compiler
chokes (because some machines may want to use those registers for
parameter passing I assume)


This is the code we use in DN3D (MIPS R3000 but it shows what you need
to do):
-------
register OPCODE *insptr     asm ("s0");
register OPCODE *script__   asm ("s1");
register void   *__s2       asm ("s2");
register void   *__s3       asm ("s3");
register void   *__s4       asm ("s4");

void execute(short i,short p,long x)
{
    //save all global registers
    OPCODE  *insptr_bak = insptr;   //##PS## must explicitly save insptr
    OPCODE  *script_bak = script__; //##PS## must explicitly save insptr
    void *old_s2 = __s2;
    void *old_s3 = __s3;
    void *old_s4 = __s4;

<lots of NDA'd code I can't show you ;) >

    //restore all touched global registers
    insptr = insptr_bak;    //##PS## must explicitly restore insptr
    script__ = script_bak;  //##PS## must explicitly save insptr
    __s2 = old_s2;
    __s3 = old_s3;
    __s4 = old_s4;
}




---
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