[Buildroot] [Bug 10551] New: PowerPC SPE and Musl

[Buildroot] [Bug 10551] New: PowerPC SPE and Musl

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=10551

            Bug ID: 10551
           Summary: PowerPC SPE and Musl
           Product: buildroot
           Version: 2017.08
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P5
         Component: Other
          Assignee: unassigned at buildroot.uclibc.org
          Reporter: sigurd.segtnan at kongsberg.com
                CC: buildroot at uclibc.org
  Target Milestone: ---

Created attachment 7411
  --> https://bugs.busybox.net/attachment.cgi?id=7411&action=edit
diff for package/gcc/gcc.mk (and Config.in in toolchain)

Musl is not supported for PowerPC SPE according to
toolchain/toolchain-buildroot/Config.in line 74: "depens on !BR2_powerpc_SPE #
not supported, build breaks". This breakage may be due the
"--with-long-double-128" in package/gcc/gcc.mk, since we have not experienced
any build breakage when we changed lines 241-245 to

ifeq ($(BR2_powerpc_SPE),y)
ifeq ($(BR2_TOOLCHAIN_USES_MUSL),n)
HOST_GCC_COMMON_CONF_OPTS += \
        --enable-e500_double \
        --with-long-double-128
endif
else
HOST_GCC_COMMON_CONF_OPTS += \
        --enable-e500_double \
        --with-long-double-64 \
        --enable-secureplt
endif

We have used this without problems since buildroot 2016.05 (tested with gcc 6.x
and gcc 7.x).

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[Buildroot] [Bug 10551] PowerPC SPE and Musl

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=10551

--- Comment #1 from Thomas Petazzoni <thomas.petazzoni@free-electrons.com> ---
Thanks for your suggestion. Indeed what you say seems to match the details at
https://wiki.musl-libc.org/faq.html#Q:-My-dynamically-linked-program-crashes-on-PowerPC!.

There is one thing that is unclear to me: is this --with-long-double-{64,128}
option only needed for PowerPC SPE, or for PowerPC in general ? What about
--enable-secureplt ? Is it only for PowerPC SPE, or for PowerPC in general ?

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[Buildroot] [Bug 10551] PowerPC SPE and Musl

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=10551

--- Comment #2 from sigurd.segtnan at kongsberg.com ---
I have asked around, and we used the information from from
https://wiki.musl-libc.org/supported-platforms.html. Since we found something
that worked for the PowerPC we used, we did not investigate further.

Today the relevant part under Supported target architectures (ABIs) is:
* powerpc (needs gcc built with ?enable-secureplt ?with-long-double-64, and
-Wl,?secure-plt to link dynamic binaries.)
* powerpc64

The "-Wl,?secure-plt" is probably for older versions of gcc (secure-plt is
mentioned in the 900-musl-support.patch fotr gcc 4.9.4, and "Pass --secure-plt
to the linker" seems to have been added to gcc in 2015 according to
https://gcc.gnu.org/ml/gcc-patches/2015-10/msg01501.html).

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[Buildroot] [Bug 10551] PowerPC SPE and Musl

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=10551

--- Comment #3 from Thomas Petazzoni <thomas.petazzoni@free-electrons.com> ---
Your patch only makes uses of those options for PowerPC SPE. The musl
documentation doesn't mention this to be SPE specific. This is what confuses
me.

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[Buildroot] [Bug 10551] PowerPC SPE and Musl

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=10551

--- Comment #4 from sigurd.segtnan at kongsberg.com ---
We have tested the patch on Power PC with and without SPE, and it works in both
caes. In view of the original line 74 of buildroot/Config.in ("depens on
!BR2_powerpc_SPE # not supported, build breaks") I suggest that we keep that
patch as is-ish.

The -ish part is the change to Config.in. Instead of commenting out the line, a
link to this discussion might be useful, so that anybody else that have a Power
PC + musl issue can find it.

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[Buildroot] [Bug 10551] PowerPC SPE and Musl

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=10551

Thomas Petazzoni <thomas.petazzoni@free-electrons.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at buildroot.uclibc |vfazio at xes-inc.com
                   |.org                        |

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[Buildroot] [Bug 10551] PowerPC SPE and Musl

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=10551

Thomas Petazzoni <thomas.petazzoni@free-electrons.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
             Status|NEW                         |RESOLVED

--- Comment #5 from Thomas Petazzoni <thomas.petazzoni@free-electrons.com> ---
PowerPC SPE support has been dropped from gcc, so there's no real interest in
fixing that further. See
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=b31d0348ddada49453e3edaaf93a423fdc61dc79.

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[Buildroot] [Bug 10551] PowerPC SPE and Musl

[bugzilla]

https://bugs.busybox.net/show_bug.cgi?id=10551

--- Comment #6 from Pali Rohár <pali@kernel.org> ---
Hello!

Just to let you know, musl libc in this year 2022 in newly released version
1.2.3 added full support for PowerPC SPE FPU. See their release changelog:
https://musl.libc.org/releases.html

Because SPE ABI is same as PowerPC soft-float ABI (floating point numbers are
stored in general purpose registers and SPE instructions uses only those
registers), it was pretty small change.

Here is list of all musl libc PowerPC SPE commits:
https://git.musl-libc.org/cgit/musl/commit/?id=66d1e31292a8c05d172872fe73880ca6d3b68104
https://git.musl-libc.org/cgit/musl/commit/?id=7be59733d71ada3a32a98622507399253f1d5e48
https://git.musl-libc.org/cgit/musl/commit/?id=8274aaaaa1948c50c661aa32e21b3db27a5c0eab

So PowerPC SPE is not dead and still developed in some way.

gcc 8.5.0 is the last gcc version which supports PowerPC SPE ISA.

But using gcc 8.x series with musl SPE is tricky because it has two major
PowerPC bugs which cause crashing of compiled applications.

1. gcc does not set correct musl dynamic linker interpreter (musl SPE uses
/lib/ld-musl-powerpc-sf.so.1)
2. gcc does not pass -Wl,--secure-plt when linking with musl libc as described
at musl faq:
https://wiki.musl-libc.org/faq.html#Q:-My-dynamically-linked-program-crashes-on-PowerPC!

And --enable-secureplt configure flag in gcc 8.x for musl SPE is broken and
does not work.

I helped to prepare gcc 8.x patches for both issues:
https://gitlab.nic.cz/turris/os/build/-/blob/42a1b3544f4712e10e100aad837aeb8f74945f99/patches/openwrt/to-upstream/0102-gcc-ppc-expect-sf-ld-suffix-even-with-spe-enabled.patch
https://gitlab.nic.cz/turris/os/build/-/blob/5d44c2123734174c21e8a28a143a9bec7cc9bf28/patches/openwrt/to-upstream/0103-gcc-ppc-enable_secureplt-yes-for-powerpc-linux-spe.patch

With these gcc patches it is possible to compile gcc (cross)compiler which can
then compile target application for musl libc e500v2 SPE ISA correctly without
crashes. I have verified and it works perfectly!

gcc at build time needs to be configured by flags:

./configure --enable-obsolete --target=powerpc-linux-muslspe --with-cpu=8548
--enable-e500-double

Note that it is needed to disable option --with-long-double-128 (by not
specifying it) because it adds gcc emulation of IBM long double format where
mantissa has 106 bits which musl libc does not support (yet). long double is
basically unimplemented by musl libc.

For e500v1 SPE ISA (same as e500v2 but without DPFP SPE) flags are:

./configure --enable-obsolete --target=powerpc-linux-muslspe --with-cpu=8540

In both cases --enable-obsolete and --target=powerpc*-linux*-musl*spe* (* for
wildcard) are important as this is the way how to build SPE-capable compiler.
Without them you will get just non-SPE compiler. Other flags --with-cpu or
--enable-e500-double (which choose e500v1 vs. e500v2 variant) can be
overwritten at runtime when using compiler by gcc flags.

So to force SPE-capable compiler to produce e500v2 SPE binary:

gcc -mcpu=8548 -mabi=spe -mspe -mfloat-gprs=double

Or for e500v1 SPE binary:

gcc -mcpu=8540 -mabi=spe -mspe -mfloat-gprs=single

To summarize, e500v1 SPE brings: vector 32-bit integer instructions, single
32-bit floating point instructions, vector 32-bit floating point instructions
and e500v2 SPE additionally brings single 64-bit (double) floating point
instructions.


Note that GNU LD linker has two flags related to e500 isa: -me500 and -me500x2.
Do not be confused by e500x2 vs e500v2. e500x2 is the old Motorola name for
e500 core with first version of SPE. Very old name e500x1 was without SPE.
e500x2 was probably later renamed to just e500 and e500x1 separated to e200
family (MPC5500). Later when double precision SPE was introduced, old e500
without DPFP was marked as e500v1 and new SPE with DPFP as e500v2. Hence e500x2
is technically e500v1 (and not v2). But because GNU ld does not distinguish
between e500v1 and e500v2 it has only one -me500 switch, plus old-named
-me500x2 which is just alias to -me500. So you should not use -me500x2 flag for
e500v2 as it is very misleading.

Also gcc cpu name 8540 is misleading as it refers to SoC/processor mpc8540
which was the first released product with e500(v1) core. But in reality
-mcpu=8540 specifies e500(v1) ISA. Same for 8548 which refers to the first
release product with e500v2 core (mpc8548). gcc should have called them e500 or
e500v1 and e500v2.

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