Re: [PATCH RFC 0/3] API for 128-bit IO access

[Robin Murphy <robin.murphy@arm.com>]

On 25/01/18 11:38, Yury Norov wrote:
> On Wed, Jan 24, 2018 at 11:28:55AM +0100, Arnd Bergmann wrote:
>> On Wed, Jan 24, 2018 at 10:05 AM, Yury Norov <ynorov@caviumnetworks.com> wrote:
>>> This series adds API for 128-bit memory IO access and enables it for ARM64.
>>> The original motivation for 128-bit API came from new Cavium network device
>>> driver. The hardware requires 128-bit access to make things work. See
>>> description in patch 3 for details.
>>
>> We might also want to do something similar to the
>> include/linux/io-64-nonatomic-lo-hi.h
>> and hi-lo.h files, to simulate 128-bit access using pairs of 64-bit access on
>> other targets. It's apparently driver specific which half you need to do first
>> to make it work, so we need both.
> 
> OK, will do.
>   
>>> Also, starting from ARMv8.4, stp and ldp instructions become atomic, and
>>> API for 128-bit access would be helpful in core arm64 code.
>>>
>>> This series is RFC. I'd like to collect opinions on idea and implementation
>>> details.
>>> * I didn't implement all 128-bit operations existing for 64-bit variables
>>> and other types (__swab128p etc). Do we need them all right now, or we
>>> can add them when actually needed?
>>
>> I think in this case it's better to do them all at once.
> 
> Ack.
> 
>>> * u128 name is already used in crypto code. So here I use __uint128_t that
>>> comes from GCC for 128-bit types. Should I rename existing type in crypto
>>> and make core code for 128-bit variables consistent with u64, u32 etc? (I
>>> think yes, but would like to ask crypto people for it.)
>>
>> Hmm, that file probably predates the __uint128_t support. My guess would
>> be that the crypto code using it can actually benefit from the new types as
>> well, so maybe move the existing file to include/linux/int128.h and add
>> an #if/#else logic to it so we use 'typedef __uint128_t __u128' if that
>> is available.
> 
> It sounds reasonable. But I worry about testing that changes. Hope,
> crypto community will help with it.
> 
>>> * Some compilers don't support __uint128_t, so I protected all generic code
>>> with config option HAVE_128BIT_ACCESS. I think it's OK, but...
>>
>> That would be nicely solved by using the #if/#else definition above.
>>
>>> * For 128-bit read/write functions I take suffix 'o', which means read/write
>>> the octet of bytes. Is this name OK?
>>
>> Can't think of anything better. It's not an octet though, but 16 bytes
>> ('q' is for quadword, meaning four 16-bit words in Intel terminology).
> 
> Ah, sure. Octet of words. Will change it.
> 
>>> * my mips-linux-gnu-gcc v6.3.0 doesn't support __uint128_t, and I
>>> don't have other BE setup on hand, so BE case is formally not tested.
>>> BE code for arm64 is looking well though.
>>
>> I've run it through my collection of compilers, it seems that most but not
>> all 64-bit targets support it (exceptions appear to be older versions of
>> gcc for s390x and parisc), and none of the 32-bit targets do:
> 
> Thanks for doing this test. Looking at this I realize that this is
> not the architecture feature but compiler feature. So if we add
> 128-bit interface, it would be reasonable to add it for all targets
> that compiled with toolchain supporting 128-bit accesses.
> 
> There's already the option ARCH_SUPPORTS_INT128 that is enabled for
> x86_64 in arch/x86/Kconfig and conditionally enabled for arm64 in
> arch/arm64/Makefile:
> KBUILD_CFLAGS    += $(call cc-ifversion, -ge, 0500, -DCONFIG_ARCH_SUPPORTS_INT128)
> 
> It is used in include/linux/math64.h and in lib/ubsan.c, not so wide.
> 
> So I find things little messed. Crypto code ignores compilers' ability
> to operate with 128-bit numbers. Ubsan and math64 relies on compiler
> version (at least for arm64, and I doubt it would work correctly with clang).
> And now I introduce HAVE_128BIT_ACCESS with the same meaning for memory
> access.
> 
> I think it's time to unify 128-bit code:
>   - enable CONFIG_ARCH_SUPPORTS_INT128 if compiler supports it, ie check
>     it like you do below;
>   - declare u128 as structure or typedef depending on ARCH_SUPPORTS_INT128
>     in generic include/linux/int128.h, as you suggest here;
>   - switch this series to ARCH_SUPPORTS_INT128.
> 
> Does it sound reasonable?

It probably is about time to formalise the current scattered fragments 
of uint_128_t support, but to reiterate Will's comment it is almost 
certainly not worth the effort to implement 'generic' I/O accessors 
which only work under implementation-defined and undiscoverable hardware 
conditions, and will be unusable on the overwhelming majority of 
systems. Just open-code LDP/STP accessors in the one driver which needs 
them and (by definition) only runs on SoCs where they *are* known to 
work correctly.

Robin.

> 
> Yury
> 
>> $ for i in /home/arnd/cross-gcc/bin/*gcc-[3-8]* ; do echo -n $i" " ;
>> echo '__uint128_t v;' | $i -xc -S - -o /dev/null && echo ok ; done
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-4.8.5 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-4.9.3 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-4.9.4 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-5.2.1 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-5.4.1 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-5.5.0 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-6.3.1 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-7.0.0 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-7.0.1 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-7.1.1 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-7.2.1 ok
>> /home/arnd/cross-gcc/bin/aarch64-linux-gcc-8.0.0 ok
>> /home/arnd/cross-gcc/bin/alpha-linux-gcc-4.1.3 ok
>> /home/arnd/cross-gcc/bin/alpha-linux-gcc-4.3.6 ok
>> /home/arnd/cross-gcc/bin/alpha-linux-gcc-4.9.3 ok
>> /home/arnd/cross-gcc/bin/alpha-linux-gcc-7.2.1 ok
>> /home/arnd/cross-gcc/bin/am33_2.0-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/am33_2.0-linux-gcc-5.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/am33_2.0-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/arc-elf-gcc-7.2.1 <stdin>:1:1: error: unknown
>> type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-3.4.6 ok
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.3.6 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.4.7 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.5.4 <stdin>:1:13:
>> error: expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.6.4 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.7.4 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.8.5 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.9.0 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.9.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.9.2 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.9.3 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-4.9.4 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-5.0.0 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-5.1.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-5.2.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-5.3.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-5.4.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-5.5.0 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-6.0.0 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-6.1.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-6.3.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-7.0.0 <stdin>:1:1:
>> error: unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-7.0.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-7.1.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-7.2.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/arm-linux-gnueabi-gcc-8.0.0 <stdin>:1:1:
>> error: unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/bfin-uclinux-gcc-7.0.0
>> bfin-uclinux-gcc-7.0.0: error trying to exec 'cc1': execvp: No such
>> file or directory
>> /home/arnd/cross-gcc/bin/bfin-uclinux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/c6x-elf-gcc-7.2.1 <stdin>:1:1: error: unknown
>> type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/cris-linux-gcc-4.1.3 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/cris-linux-gcc-4.3.6 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/cris-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/cris-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/frv-linux-gcc-4.1.3 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/frv-linux-gcc-4.3.6 <stdin>:1: internal
>> compiler error: in default_secondary_reload, at targhooks.c:618
>> Please submit a full bug report,
>> with preprocessed source if appropriate.
>> See <http://gcc.gnu.org/bugs.html> for instructions.
>> /home/arnd/cross-gcc/bin/frv-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/frv-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/h8300-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/hppa64-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/hppa64-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/hppa-linux-gcc-4.1.3 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/hppa-linux-gcc-4.3.6 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/hppa-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/hppa-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/i386-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/i386-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/ia64-linux-gcc-7.2.1 ok
>> /home/arnd/cross-gcc/bin/m32r-linux-gcc-4.1.3 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/m32r-linux-gcc-4.3.6 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/m32r-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/m32r-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/m68k-linux-gcc-4.1.3 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/m68k-linux-gcc-4.3.6 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/m68k-linux-gcc-6.0.0 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/m68k-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/microblaze-linux-gcc-4.9.3 <stdin>:1:1:
>> error: unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/microblaze-linux-gcc-7.2.1 <stdin>:1:1:
>> error: unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/mips64-linux-gcc-4.9.3 ok
>> /home/arnd/cross-gcc/bin/mips64-linux-gcc-7.2.1 ok
>> /home/arnd/cross-gcc/bin/mips-linux-gcc-4.0.4 <stdin>:1: error: syntax
>> error before 'v'
>> <stdin>:1: warning: data definition has no type or storage class
>> /home/arnd/cross-gcc/bin/mips-linux-gcc-4.1.3 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/mips-linux-gcc-4.3.6 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/mips-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/mips-linux-gcc-7.0.0 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/mips-linux-gcc-7.0.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/mips-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/nios2-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/parisc-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/powerpc64-linux-gcc-4.1.3 ok
>> /home/arnd/cross-gcc/bin/powerpc64-linux-gcc-4.3.6 ok
>> /home/arnd/cross-gcc/bin/powerpc64-linux-gcc-4.9.3 ok
>> /home/arnd/cross-gcc/bin/powerpc64-linux-gcc-7.2.1 ok
>> /home/arnd/cross-gcc/bin/powerpc-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/powerpc-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/riscv32-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/riscv64-linux-gcc-7.2.1 ok
>> /home/arnd/cross-gcc/bin/s390-linux-gcc-4.1.3 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/s390-linux-gcc-4.3.6 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/s390-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/s390-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/sh2-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/sh3-linux-gcc-4.1.3 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/sh3-linux-gcc-4.3.6 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/sh3-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/sh4-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/sh-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/sparc64-linux-gcc-4.9.3 ok
>> /home/arnd/cross-gcc/bin/sparc64-linux-gcc-7.2.1 ok
>> /home/arnd/cross-gcc/bin/sparc-linux-gcc-4.1.3 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/sparc-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/sparc-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/tilegx-linux-gcc-7.2.1 ok
>> /home/arnd/cross-gcc/bin/tilepro-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-3.4.6 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.0.4 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.1.3 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.2.5 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.3.6 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.4.7 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.5.4 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.6.4 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.7.4 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.8.5 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.9.3 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-4.9.4 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-5.4.1 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-5.5.0 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-6.1.1 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-6.3.1 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-7.0.0 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-7.0.1 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-7.1.1 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-7.2.1 ok
>> /home/arnd/cross-gcc/bin/x86_64-linux-gcc-8.0.0 ok
>> /home/arnd/cross-gcc/bin/xtensa-linux-gcc-4.1.3 <stdin>:1: error:
>> expected '=', ',', ';', 'asm' or '__attribute__' before 'v'
>> /home/arnd/cross-gcc/bin/xtensa-linux-gcc-4.9.3 <stdin>:1:1: error:
>> unknown type name '__uint128_t'
>> /home/arnd/cross-gcc/bin/xtensa-linux-gcc-7.2.1 <stdin>:1:1: error:
>> unknown type name '__uint128_t'; did you mean '__int128'?
> 
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