Loop unrolling and asm

Loop unrolling and asm

[Christian Iseli]

Hi folks,

While trying out egcs-1.0.2-prerelease, I compiled the
new XFree86-3.3.2 on a Linux 2.1.88 (RH 5.0) PPro box.
binutils-2.8.(...).21 is installed.

I used -O2 -march=i686 -funroll-loops to compile XFree.
I encountered a small problem where a loop containing asm
was unrolled.  The asm included labels (.label00, .label01, ...)
and the assembler died because of multiple definitions of
the labels.

The problem, of course, went away when I recompiled the incriminated
file without loop unrolling.

XFree seems to run fine.

Now the question: how do you go about including an asm with labels
in a loop that will be unrolled?

					Christian

Re: Loop unrolling and asm

[Richard Henderson]

On Tue, Mar 10, 1998 at 10:38:32AM +0100, Christian Iseli wrote:
> Now the question: how do you go about including an asm with labels
> in a loop that will be unrolled?

Use local labels if your assembler supports them (jcc 1f; blah; 1:).
Otherwise you are out o luck.


r~

Re: Loop unrolling and asm

[Jeffrey A Law]

  In message < 199803100938.KAA18441@lslsun17.epfl.ch >you write:
  > Now the question: how do you go about including an asm with labels
  > in a loop that will be unrolled?
Use the "forward/backward" label feature of gas.  Of course it
probably won't work with other assemblers.

The code will look soemthing like this
   jmp 0f
   [ ... blah blah blah ... ]
0:
   [ ... blah blah blah ... ]
   jmp 0b

jeff

Re: Loop unrolling and asm

[Joern Rennecke]

> Hi folks,
> 
> While trying out egcs-1.0.2-prerelease, I compiled the
> new XFree86-3.3.2 on a Linux 2.1.88 (RH 5.0) PPro box.
> binutils-2.8.(...).21 is installed.
> 
> I used -O2 -march=i686 -funroll-loops to compile XFree.
> I encountered a small problem where a loop containing asm
> was unrolled.  The asm included labels (.label00, .label01, ...)
> and the assembler died because of multiple definitions of
> the labels.
> 
> The problem, of course, went away when I recompiled the incriminated
> file without loop unrolling.
> 
> XFree seems to run fine.
> 
> Now the question: how do you go about including an asm with labels
> in a loop that will be unrolled?

There are two ways to do this:

- local labels
  Not all assemblers support these, but at least gnu as does.  You define
  a local label as <digit>: , and reference it as <digit>b for a backwards
  reference, or <digit>f for a forwards reference.  E.g.:
  0:
   add -1,r0
   bne 0b

- using "%=" in the assembler template.  This will expand to a number
  which is unique to the template as seen by final_scan_insn.  That is,
  for each copy that final_scan_insn created, you will get a different
  number.

Re: Loop unrolling and asm

[H.J. Lu]

> 
> Hi folks,
> 
> While trying out egcs-1.0.2-prerelease, I compiled the
> new XFree86-3.3.2 on a Linux 2.1.88 (RH 5.0) PPro box.
> binutils-2.8.(...).21 is installed.
> 
> I used -O2 -march=i686 -funroll-loops to compile XFree.
> I encountered a small problem where a loop containing asm
> was unrolled.  The asm included labels (.label00, .label01, ...)
> and the assembler died because of multiple definitions of
> the labels.
> 
> The problem, of course, went away when I recompiled the incriminated
> file without loop unrolling.
> 
> XFree seems to run fine.
> 
> Now the question: how do you go about including an asm with labels
> in a loop that will be unrolled?
> 

Use 0f, 1f, .... 9f, 0b, .... 9b instead of .label00 in asm. But
At one time, I needed more than 10 lables backward/forward. I had
to disable inline. I think it is limited by binutils. Can we change
binutils to accept more than 10 local lables? 


-- 
H.J. Lu (hjl@gnu.org)

Re: Loop unrolling and asm

[Jim Wilson]

	Now the question: how do you go about including an asm with labels
	in a loop that will be unrolled?

Try using %= to get a unique number.

Or try using labels that don't have to be unique.  GNU as supports local
symbol names which do not have to be unique.   You define them using a number
from 0 to 9 and then you refer to them using 0b or 0f to indicate whether you
want the first 0 label searching backwards, or the first 0 label searching
forwards.

Jim

Re: Loop unrolling and asm

[Richard Henderson]

On Tue, Mar 10, 1998 at 07:15:06AM -0800, H.J. Lu wrote:
> Use 0f, 1f, .... 9f, 0b, .... 9b instead of .label00 in asm. But
> At one time, I needed more than 10 lables backward/forward. I had
> to disable inline. I think it is limited by binutils. Can we change
> binutils to accept more than 10 local lables? 

You must not have tried it -- it has done so for a long long time.


r~