From: korpela AT albert DOT ssl DOT berkeley DOT edu (Eric J. Korpela)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Optimization and Inline ASM
Date: 4 Oct 1996 23:22:45 GMT
Organization: Cal Berkeley-- Space Sciences Lab
Lines: 61
Message-ID: <534685$3ci@agate.berkeley.edu>
References: <199610040234 DOT TAA04831 AT bluesky DOT com>
NNTP-Posting-Host: albert.ssl.berkeley.edu
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

In article <199610040234 DOT TAA04831 AT bluesky DOT com>,
Kevin Baca <kbaca AT skygames DOT com> wrote:
>> c:/djgpp/tmp\cccaaaaa:66: Fatal error:Symbol loc1 already defined.
>> 
>> This error only occurs if I use the optimize switch:
>> 
>> gcc myfile.cpp -O3
>> 
>> Compiles flawlessly if I use just:
>> 
>> void myproc()
>> {
>>   __asm__ __volatile__("
>>  [code]
>> loc1:\n
    [code]
>> loc2:\n
    [code]
    );
>> }
>> 
>> What am I doing wrong? Help me, or I'll be bold soon! This error has driven
>> me NUTS!!!
>
>Try using different labels.

Not just different labels, local labels.  When the -O3 switch gets used
simple functions are compiled inline.  What this means is that every
place that myproc is called is replaced with the assembly code for
the above function.  If myproc() is called more than once in the code
you end up with multiple locations labeled loc1 and loc2.

Local labels are defined as <number>:  (for ex. 0:, 1:, 2:, etc.)

So if loc1: and loc2: were replaced by 1: and 2: things would be fine,
almost.

The reason I said almost is that when you reference a local label you
need to tell the assembler which direction to look for the label
(forward or backward).  To do this you append an "f" or a "b" to the
label name when it is referenced.  As a useless example...

.global _mypause
_mypause:
        pushl %ebp
        movl  %esp,%ebp
        movl  8(%ebp),%ecx
	testl %ecx,%ecx
	jz    0f
1:	decl  %ecx
        jnz   1b
0:      movl  %ebp,%esp
        popl  %ebp
        ret
       
Eric 
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