To: djgpp AT delorie DOT com
Subject: Re: Inline Asm?
Message-ID: <19970106.200214.4975.4.chambersb@juno.com>
References: <19961231 DOT 204032 DOT 5015 DOT 0 DOT chambersb AT juno DOT com>
	<5as4du$jd6 AT mack DOT rt66 DOT com>
From: chambersb AT juno DOT com (Benjamin D Chambers)
Date: Mon, 06 Jan 1997 22:56:41 EST


On 6 Jan 1997 17:12:46 -0700 brennan AT mack DOT rt66 DOT com (Brennan "The Rev.
Bas" Underwood) writes:
>In article <19961231 DOT 204032 DOT 5015 DOT 0 DOT chambersb AT juno DOT com>,
>Benjamin D Chambers <chambersb AT juno DOT com> wrote:
>>Well, I spent some time stepping through my program with GDB, and 
>found
>>that the following lines caused problems:
>>
>>cmpl	$0,	%%edx  (This is, of course, in the c file)
>>jne	0
>>
>>Now, to me, this means that if edx is not zero, jump to the label 0
>>(which was defined earlier) and, if edx is zero, keep executing as
>>normal.  However, when I stepped through this with GDB, I found that 
>no
>>matter what value edx held, the loop fell through (that is, no jump 
>was
>>performed).  Am I doing something wrong here?  Again, thanx in 
>advance...
>
>Tell you what, just use jnz as below... (JNZ = jump if not zero)
>
>>  asm("0:                                       \n
>>        movb    $1,             (,%%eax,1)      \n
>>        incl    %%eax                           \n
>>        decl    %%edx                           \n
>         jnz     0b                              \n
>>       "
>>  :     // No Output
>>  : "d" (size),
>>    "a" (data)
>>  : "memory", "%cc"
>>  );
>
>Also, the decl and jnz will pair into 1 cycle on the Pentium.
>And, I put a 'b' on the '0', to make 0 into a local label in case of
>loop unrolling.
>
I think I figured this out...  The 0 was treated as an offset, rather
than a label.  (I had tried jnz originally, then changed it to the more
readable jne).  When jne 0 was used, it always jumped 0 bytes ahead -
with the no effect I was observing.  When I used labels like LoopTop or
LOOP or something, it jumped to the label correctly.

Perhaps this tidbit should be placed in the FAQ - it certainly can be
confusing :)

...Chambers