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Re: toolchain for powerpc e500v2

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?


The SPE floating point support is not standard PPC hardware floating point, and uses totally different instructions. 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.

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.

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.

Personally, I disagree with your aim of avoiding compiler switches for the target - I consider compiler switches in the makefile as part of the source code, and would rather state them explicitly there than rely on a custom-built compiler. But that's a matter of choice.

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