From: korpela AT albert DOT ssl DOT berkeley DOT edu (Eric J. Korpela)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: FPU instructions in inline assembler. Error: operands given don't match any known 386 instruction
Date: 21 Jan 1997 00:48:25 GMT
Organization: Cal Berkeley-- Space Sciences Lab
Lines: 79
Message-ID: <5c13oq$6kt@agate.berkeley.edu>
References: <01bc02c4$15049a40$a6cde182 AT abcblk4 DOT rci>
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To: djgpp AT delorie DOT com
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In article <01bc02c4$15049a40$a6cde182 AT abcblk4 DOT rci>,
Steffen Waldersdorff Løffler <waldersdorff AT forensic DOT ku DOT dk> wrote:
>#define add_asm(t1, t2, result)                     \
>        asm volatile                                \
>        (   "flds   %1;"                            \
>            "fadds  %2;"                            \
>            "fstps  %0;"                            \
>            : "=g"  (result)                        \
>            : "g" (t1), "g" (t2))                   \

The "g" code means any register, memory or immediate operand is allowed.
This is certainly not the case for the above code.  For example, what
if GCC decides to use %eax for %1.  It's not possible to load directly from
in integer register to a floating point register.  Your options are...

1) tell gcc that the operands are memory locations.

  asm volatile
  ( "flds %1"
    "fadds %2"
    "fstps %0"
    :: "m" (result), "m" (t1), "m" (t2));

2) let gcc do the loading and storing for you (t= top of stack, u=next level)

  asm volatile
  (
    "faddp"  /* leaves the result at the top of the stack */
  : "=t" (result) : "u" (t1), "0" (t2));

3) The best answer is probably a combination of both, as that will compile
   to the fewest instructions....  (Or so it would seem)

  asm volatile
  (
    "fadds %2"
    : "=t" (result) : "0" (t1), "m" (t2));

Good luck,

Eric
 

>
>void main()
>{
>        float t1=2.0;
>        float t2=2.0;
>        float res;
>
>        add_asm(t1, t2, res);
>
>        printf("%f\n", res);
>}
>
>
>If it's compiled with f.ex. the -O1 or -O2 switch it will generate the
>following error:
>
>....15: Error: operands given don't match any known 386 instruction
>
>What am I doing wrong??? 
>
>
>Thanks! 
>
>Steffen
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-- 
Eric Korpela                        |  An object at rest can never be
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