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Re: toolchain for powerpc e500v2

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?


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.



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