Re: toolchain for powerpc e500v2

[David Brown <david at westcontrol dot com>]

On 07/02/2011 12:14, Titus von Boxberg wrote:
> Am Mo, 7.02.2011, 11:16 schrieb David Brown:
> > On 06/02/2011 12:48, Titus von Boxberg wrote:
> > > Hi all,
> > >
> > > I'm trying to use ct-ng (of approx. May 2010) to generate
> > > cross gcc-4.5.1, glibc-2.9/linux for a powerpc e500v2 / SPE.
> > >
> > > Setting floating point to "hardware" results in a build
> > > error in glibc where the assembler complains while compiling
> > > an FP file about an unmatched constraint
> > > (I could try to get the original message if it helps).
> > >
> > > The tool chain gets built when I set floating point support to "software".
> > > However, the resulting compiler does not emit hardware FP instructions
> > > unless I give -mhard-float on the command line.
> > >
> > > Is my assumption correct that setting floating point to "hardware" turns on
> > > code generation for the standard PowerPC FPU which this target lacks?
> > >
> > > I'd like to avoid having a compiler which needs target specific flags
> > > on the command line, thus:
> > > Does anyone know which options to turn on to get a compiler that
> > > emits correct SPE FP instructions without explicitly being told so
> > > by -mhard-float?
> > >
> > > Regards
> > > Titus
> >
> >
> > If you try to use
> > -mhard-float, either as a command-line switch or by default in the
> > build, things are going to go badly wrong, as it will generate PPC
> > floating point instructions that are not implemented on the e500v2.
> That's wrong. The compiler configured for Soft FP / SPE generates the correct
> instructions (but only with -mhard-float), and the code is working as well.
> The tool chain build for hard fp indeed goes wrong. That's my problem.

OK, if you say so - I haven't tried building from source, and it was 
something like a year and a half ago that I tried the CodeSourcery 
toolchain for the e500.  It could well be that SPE floating point was 
activated slightly differently then, and has since then been better 
integrated as a variant of hardware floating point.

> > I would look at the CodeSourcery toolchain.  They have support for SPE
> > floating point in current releases, which you can get as source or
> > pre-build binary.  I don't know whether this support has made it into
> > the mainline FSF trees as yet (call me lazy, but I've just used the
> > binaries from CodeSourcery), but if not then I'm sure it will
> > eventually.  It requires particular flags to enable the SPE floating
> > point - it may be possible to build your own version that uses it by
> > default.
> Thanks for the hint!
>
> > Remember that the SPE only supports single-precision floating point - be
> > very careful to avoid doubles in your code (or use flags to force 32-bit
> > "doubles").  It's easy to write "x = y * 1.5" instead of "x = y * 1.5f",
> > and give your cpu a great deal more work.
> This SPE supports double. I think it is enabled by -mfloat-gprs=double
> (or maybe also by default by the compiler configuration option --enable-e500_double,
> but I did not verify this yet).

Looking more carefully, I see you are correct.  I have only used the 
e500v1 core, which has only single-precision floating point SPE support.

mvh.,

David


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