Re: toolchain for powerpc e500v2

["Titus von Boxberg" <titus at v9g dot de>]

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.

>
> 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).

Regards
Titus



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