Message-Id: <m0xlZKT-000S2XC@inti.gov.ar>
Comments: Authenticated sender is <salvador AT natacha DOT inti DOT gov DOT ar>
From: "Salvador Eduardo Tropea (SET)" <salvador AT inti DOT gov DOT ar>
Organization: INTI
To: "Richard" <RiM AT msn DOT com>, djgpp AT delorie DOT com
Date: Fri, 26 Dec 1997 16:03:21 +0000
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Subject: Re: Extended Assembly
In-reply-to: <e$JXWMAE9GA.170@upnetnews02.moswest.msn.net>
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> From:          "Richard" <RiM AT msn DOT com>
> Subject:       Extended Assembly
> Date:          Wed, 24 Dec 1997 01:25:30 +0100
> To:            djgpp AT delorie DOT com

>  I ran into a problem with extended assembly. It seems to ignore my
> clobber list and reuse the
> registers. Usually it works save in some situation such as the
> following :
> 
> (I tried to narrow it down to a example not too artificial)
> 
> **********************************
> long overflow;
> #define addll(x, y)                     \
> ({                                      \
>  long __result;                         \
>  asm(                                   \
>      "movl    %1,%%eax \n\t"            \
>      "xorl    %%edx,%%edx \n\t"         \
>      "addl    %2,%%eax \n\t"            \
>      "adcl    %%edx,%%edx \n\t"         \
>      "movl    %%edx,_overflow \n\t"     \
>      :"=a" (__result)                   \
>      :"g" (x), "g" (y)                  \
>      :"%eax", "%edx", "memory"                    \
>     );                                  \
>   __result;                             \
> })
> long * f(long *x, long *y, long *z,int lx){
>   int i, j;
>   for(i=1;i<lx;i++)
>    z[i] = addll( x[i], y[i]);
>   return z;
> }
> 
> **********************************
> Which gives such code : (basic prolog/epilog trimmed)
> movl 12(%ebp),%edi
>  movl 16(%ebp),%esi
>  movl 20(%ebp),%ebx
>  movl $1,%ecx
>  cmpl %ebx,%ecx
>  jge L3
>  .align 2,0x90
> L5:
>  movl 8(%ebp),%eax
> /APP
>  movl    (%eax,%ecx,4),%eax           <---- Here is the problem !
>  xorl    %edx,%edx
>  addl    (%edi,%ecx,4),%eax
>  adcl    %edx,%edx
>  movl    %edx,_overflow
> 
> /NO_APP
>  movl %eax,(%esi,%ecx,4)
>  incl %ecx
>  cmpl %ebx,%ecx
>  jl L5
> L3:
>  movl %esi,%eax
>  leal -12(%ebp),%esp
> ***************************************
>  The trouble is the use of the "eax" register as base pointer in the
> instruction :
> movl    (%eax,%ecx,4),%eax
>  If it was three local arrays, such case would not arise as in every
> array the "ebp"
> register would hold the base pointer.
> 
>  In this very case, I can tell the gcc to use "esi" and "edi" to hold
> the x and y
> parameters. But it does not cure the problem in other situations.
> 
>  Is it simply that gcc run out of register ?  So, why it does not take
> some variable
> out of the registers, such as "ebx" (my "lx") ?
> 
> Or is there a bug in my clobber list ? I tried many variations, but it
> changed nothing.
> Why gcc does not take into account my clobber list ?
> 
>  Please shed some light on my troubles !
> 
> --Richard.
> 
> PS :
> * "gcc --version " give me "2.7.2". It should be the one bundled in
> the 2.00 djgpp.
> * The optimization level doesn't change anything on that problem.
> * I know that here I could ask gcc to load "eax" for me. But in a
> slightly more
> complicated assembly function, I had to load "eax" after a few other
> instructions.
> *If I ask gcc to use :"r" (x), "r" (y), this give me the following
> code which is hardly
> better !
> L5:
>  movl 8(%ebp),%eax
>  movl (%eax,%ecx,4),%ebx
>  movl 12(%ebp),%eax
>  movl (%eax,%ecx,4),%eax
> /APP
>  movl    %ebx,%eax                 <-- Overwrite eax
>  xorl    %edx,%edx
>  addl    %eax,%eax                  <-- and add it to myself !
>  adcl    %edx,%edx
>  movl    %edx,_overflow
> 
> /NO_APP
> --.
> 
> 
> 
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