From:	"Richard" <RiM AT msn DOT com>
Subject:	Extended Assembly
Date:	Wed, 24 Dec 1997 01:25:30 +0100
Lines:	107
MIME-Version:	1.0
Message-ID:	<e$JXWMAE9GA.170@upnetnews02.moswest.msn.net>
Newsgroups:	comp.os.msdos.djgpp
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

 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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