gcc 4.5 powerpc wants r31

gcc 4.5 powerpc wants r31

[Fabian Cenedese]

Hi

Using a cross compiled gcc for PowerPC-CPUs: powerpc-eabi-gcc.exe --version
powerpc-eabi-gcc.exe (crosstool-NG-1.8.2) 4.5.2

I'm having a problem compiling some inline assembler code. This
works with older compilers but 4.5 doesn't like it.

register uint32 uFlags    asm("r20") = 0;
asm volatile ("mr		31,%0": : "r" (uFlags) : "31");

Compiling unoptimized I get

file.cpp:3271:1: error: 31 cannot be used in asm here

Compiling with -O2 works.

Seems like this problem is already old (2006):
https://bugs.gentoo.org/show_bug.cgi?format=multiple;id=133948
http://www.stackless.com/pipermail/stackless/2006-August/002415.html

Is there a solution apart from -fomit-frame-pointer or "don't do this"?

Thanks

bye  Fabi

Re: gcc 4.5 powerpc wants r31

[Ian Lance Taylor]

Fabian Cenedese <Cenedese@indel.ch> writes:

> Using a cross compiled gcc for PowerPC-CPUs: powerpc-eabi-gcc.exe --version
> powerpc-eabi-gcc.exe (crosstool-NG-1.8.2) 4.5.2
>
> I'm having a problem compiling some inline assembler code. This
> works with older compilers but 4.5 doesn't like it.
>
> register uint32 uFlags    asm("r20") = 0;
> asm volatile ("mr		31,%0": : "r" (uFlags) : "31");
>
> Compiling unoptimized I get
>
> file.cpp:3271:1: error: 31 cannot be used in asm here
>
> Compiling with -O2 works.
>
> Seems like this problem is already old (2006):
> https://bugs.gentoo.org/show_bug.cgi?format=multiple;id=133948
> http://www.stackless.com/pipermail/stackless/2006-August/002415.html
>
> Is there a solution apart from -fomit-frame-pointer or "don't do this"?

No.  If gcc is using the frame pointer, then it can't work correctly if
you clobber the frame pointer.  It's a feature that gcc is telling you
this.

Ian

Re: gcc 4.5 powerpc wants r31

[Fabian Cenedese]

At 06:38 14.09.2011 -0700, Ian Lance Taylor wrote:
>Fabian Cenedese <Cenedese@indel.ch> writes:
>> register uint32 uFlags    asm("r20") = 0;
>> asm volatile ("mr             31,%0": : "r" (uFlags) : "31");
>>
>> file.cpp:3271:1: error: 31 cannot be used in asm here
>>
>> Is there a solution apart from -fomit-frame-pointer or "don't do this"?
>
>No.  If gcc is using the frame pointer, then it can't work correctly if
>you clobber the frame pointer.  It's a feature that gcc is telling you
>this.

This code is part of an embedded operating system. So of course
we'd like full control over all registers. After this register setting
is an asm jump to some other code, so r31 is not really used
for the C-code anymore.

I found that it works if I remove the clobber. That is good enough
as there's no more C-code that could look at the clobber. 

Thanks

bye  Fabi