Re: inline asm: How to push PIC reg before seven input operands get loaded?

[lynx dot abraxas at freenet dot de]

On 24/03/07 18:16:51, Andrew Haley wrote:
>  > If I do eg:
>  > 
>  > Pixel16 *tileData[4]; uint16 *pDestPixel; Pixel16 *dataPtr, *transDataPtr;
>  > sint32 startX, endX;
>  > __asm__ __volatile__ (
>  > "push %%ebx     \n\t"  // save reg for PIC!
>  > //code here using %1 but now reg can be rused:
>  > 
>  > "movl %5, %1            \n\t" //no reg left for %5 when gcc should handel it
>  >                               //but now gcc says: 
>  >                               //error: memory input 5 is not directly addressable
>  > "movl (%1,%%ebx,4), %1 \n\t"  //instead of "movl (%5,%%ebx,4), %1 \n\t" 
>  >                               //because m for %5 didn't work here
>  > 
>  > //more code using also ebx and ebp
>  > "pop %%ebx     \n\t" // restore reg for PIC!
>  > :
>  > : "D" (pDestPixel), "d" (endX), "c" (startX), "a" (transDataPtr), "S" (dataPtr), "m" (tileData)
> 
>  > : "cc" //don't tell gcc about what was done to ebx because of PIC!!!
>  > );
>  > 
>  > gcc complains about: error: memory input 5 is not directly addressable
> 
>  > Why is that? How else should I load the tileData pointer from
>  > memory into a register?
> 
> I wouldn't be using the "m" constraint for this.  Pass in the pointer
> as an operand using constraint "r" or "g".  Something like "g"(&tileData);
> 
> Can you post some small sample code?  Make it the shortest possible
> programs that shows your problem.  Make sure all the types it needs
> are defined.

This  is  a problem. My task is to tranlate this inline asm written for MSC to
gcc. It's from a big game (Call to Power II, CTP2). I've attached a file  with
the  function  that  contains  the  code.  But  it  won't compile withou a lot
modifications outside the source tree.

At L131 my translation starts. It has the constrains I got it to compile  with
but  segfaults  then.  I  noticed that using any other var than tileData works
with "m". Could it be a problem for  gcc  to  use  the  double  pointer  here:
Pixel16 **tileData ?

>
>   >  >   >  Also I wonder which -f option that comes with -O2 makes gcc stop
reporting:
>  > >  > error: can't find a register in class 'LEGACY_REGS' while  reloading
'asm'
>  > >
>  > > I guess this is a joke.
>  >
>  > No, it isn't. I wouldn't have asked else wise...
>
> Reload has run out of registers.  It's telling you that it needs
> another one.  A -f option is not going to give reload another
> register.

The  point  is,  if  I  just add -O2 to the g++ cammand, this error disappears
without having changed anything in the code or the number of register the part
uses.
It  seems  that  an  option  in -O2 or above lets g++ be able to cope with the
registers I have left it (ebx and ebp, esp). I found this solution quite often
on the net.

>
>  > Is it the combination of all -f options in -O2? I coudn't find a
>  > single obviouse one in the manpage.  Is it a rule then to include
>  > -O2 or higher if inline asm is used???
>
> No, there's no such rule.  You've run out of registers.
>
> gcc needs one or two spare registers to work with.  The x86 has eight,
> some of which are used for special purposes.  You are using five in
> your asm.  This is a very tight situation.

Compiling all this with -fomit-frame-pointer:

Can gcc not push all registers, and give me all seven without esp?

I  also  tryed  to  use  "R"  instead of "m" for tileData. It compiled without
errors even when I had specified -fPIC. -fPIC seems to use ebx so I pushed and
poped  that  in  my asm code. But when looking at the disassembly I had to see
gcc used ebx for tileData and not ebp. But if I  specified  "b"  it  complains
about:
error: PIC register %ebx clobbered in asm

Is  this a bug in gcc??? Is there a constraint just for ebp so I don't have to
use "R" and hope it gets into ebp?

>
> Andrew.

Thanks Andrew
Lynx

Attachment: asm-test.cpp
Description: Text document