Crash under sparc/rtems-4.11 when compiling with FPU
Jiri Gaisler
jiri at gaisler.se
Mon Feb 8 10:13:30 UTC 2016
We have discussed this a number of times. The reason we used
-msoft-float to compile the kernel was to avoid spurious FPU
instructions emitted by the compiler even if float/double is not used in
the C code. If this happens in a task which has not enabled FPU
operations (default), a crash will occur (FPU disabled exception). If
the current compiler version has stopped issuing spurious FPU
instructions, then the -msoft-float option can be removed, but adding
-mhard-float is not needed as this is the default.
Note also that it is probably impossible to mix non-FPU and FPU tasks in
the same kernel, at least when libm is used. Non-FPU tasks would need to
be linked against the soft-float version of libm, while FPU tasks would
need the hard-float version.
Just my two cents ...
Jiri.
On 08/02/16 10:57, Athanasios.Tsiodras at esa.int wrote:
> (slight correction - I pasted the wrong output, the binary works fine :-)
>
> *** BEGIN OF TEST HELLO WORLD ***
> Hello World, 93647.304688
> *** END OF TEST HELLO WORLD ***
>
> I am unaware of the proper etiquette for this, so I better check - am
> I allowed to edit the wiki page at
> https://devel.rtems.org/wiki/TBR/UserManual/Floating_Point_Supportto
> add a section about Leon3 native FPU usage and the need for the patch
> I did in leon3.cfg?
>
> Apologies if I am overstepping - I just don't want others to go
> through the same efforts that I did.
>
> Kind regards,
> Thanassis.
>
> *European Space Agency (via HE Space)*
> Thanassis Tsiodras
> Real-time Embedded Software Engineer
> System, Software and Technology Department
>
> *ESTEC*
> Keplerlaan 1, PO Box 299
> NL-2200 AG Noordwijk, The Netherlands
> Athanasios.Tsiodras at esa.int | www.esa.int
> T +31 71 565 5332
>
>
>
> From: Athanasios.Tsiodras at esa.int
> To: "RTEMS List" <users at rtems.org>
> Date: 08/02/2016 10:50
> Subject: Re: Crash under sparc/rtems-4.11 when compiling with FPU
> Sent by: "users" <users-bounces at rtems.org>
> ------------------------------------------------------------------------
>
>
>
> I verified Sebastian's suggestion - by modifying the leon3.cfg file...
>
> diff --git a/c/src/lib/libbsp/sparc/leon3/make/custom/leon3.cfg
> b/c/src/lib/libbsp/sparc/leon3/make/custom/leon3.cfg
> index 58f2f07..30fc3a4 100644
> --- a/c/src/lib/libbsp/sparc/leon3/make/custom/leon3.cfg
> +++ b/c/src/lib/libbsp/sparc/leon3/make/custom/leon3.cfg
> @@ -9,7 +9,8 @@ RTEMS_CPU_MODEL=leon3
>
> # This contains the compiler options necessary to select the CPU
> model
> # and (hopefully) optimize for it.
> -CPU_CFLAGS = -mcpu=cypress -msoft-float
> +CPU_CFLAGS = -mcpu=cypress -mhard-float
>
> ...the generated binary of hello.exe works fine - I went a step
> further and did a test calculation, too:
>
> diff --git a/testsuites/samples/hello/init.c
> b/testsuites/samples/hello/init.c
> index d8fe450..6ee33f6 100644
> --- a/testsuites/samples/hello/init.c
> +++ b/testsuites/samples/hello/init.c
> @@ -27,8 +27,13 @@ rtems_task Init(
> rtems_task_argument ignored
> )
> {
> + int i = 0;
> + float a=3.14159f, b=1.0f;
> +
> rtems_test_begin();
> - printf( "Hello World\n" );
> + for (i=0; i<10; i++)
> + b*=a;
> + printf( "Hello World, %f\n", b );
> rtems_test_end();
> exit( 0 );
> }
>
> ...which worked fine:
>
> (gdb) tar extended-remote ... ( to grmon)
> (gdb) c
> Continuing.
>
> *** BEGIN OF TEST HELLO WORLD ***
> Hello World, 1.000000
> *** END OF TEST HELLO WORLD ***
>
> ...and I made sure that FPU instructions are inside:
>
> for (i=0; i<10; i++)
> 28: 82 80 7f ff addcc %g1, -1, %g1
> 2c: 12 bf ff ff bne 28 <Init+0x28>
> 30: 91 a2 09 29 fmuls %f8, %f9, %f8
> b*=a;
> printf( "Hello World, %f\n", b );
> 34: 91 a0 19 28 fstod %f8, %f8
>
> So, executive summary.... It seems that the default compilation
> settings of the Leon3 BSP forbid "real FPU" work.
> If one needs to work in native (non-emulated) FPU mode, then the
> leon3.cfg must be patched as I showed above.
>
> In the long run, as Sebastian indicated, a leon3fp (and a
> corresponding leon2fp, I imagine)
> should ideally be added as new BSP targets - and the FPU page
> (_https://devel.rtems.org/wiki/TBR/UserManual/Floating_Point_Support_)
> should probably
> be updated to indicate this.
>
> Thanks to everyone for their help,
> Thanassis.
>
> *
> European Space Agency (via HE Space)*
> Thanassis Tsiodras
> Real-time Embedded Software Engineer
> System, Software and Technology Department
> *
> ESTEC*
> Keplerlaan 1, PO Box 299
> NL-2200 AG Noordwijk, The Netherlands
> Athanasios.Tsiodras at esa.int | _www.esa.int_
> T +31 71 565 5332
>
>
>
> From: "Sebastian Huber" <sebastian.huber at embedded-brains.de>
> To: "RTEMS List" <users at rtems.org>
> Cc: software at gaisler.com
> Date: 07/02/2016 07:40
> Subject: Re: Crash under sparc/rtems-4.11 when compiling with FPU
> Sent by: "users" <users-bounces at rtems.org>
> ------------------------------------------------------------------------
>
>
>
> Hello Athanasios,
>
> it was always a miracle to me how you are supposed to use the
> leon2/leon3 BSPs with a FPU. If you want to use the FPU, then the BSP
> must be built with the -mhard-float option, otherwise the FPU support
> is not enabled in the operating system support. In addition the
> application configuration must be compiled with exactly the same
> compiler flags as the BSP, otherwise it will generate a corrupt
> configuration.
>
> I think we need leon2fp and leon3fp BSPs, which use -mhard-float.
>
> --
> Sebastian Huber, embedded brains GmbH
>
> Address : Dornierstr. 4, D-82178 Puchheim, Germany
> Phone : +49 89 189 47 41-16
> Fax : +49 89 189 47 41-09
> E-Mail : sebastian.huber at embedded-brains.de
> PGP : Public key available on request.
>
> Diese Nachricht ist keine geschäftliche Mitteilung im Sinne des EHUG.
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