Re: [PATCH for-8.0 15/29] include/qemu/int128: Add vector type to Int128Alias - Philippe Mathieu-Daudé

From: "Philippe Mathieu-Daudé" <philmd@linaro.org>
To: Richard Henderson <richard.henderson@linaro.org>, qemu-devel@nongnu.org
Subject: Re: [PATCH for-8.0 15/29] include/qemu/int128: Add vector type to Int128Alias
Date: Tue, 22 Nov 2022 19:31:08 +0100	[thread overview]
Message-ID: <7e7fc434-b82e-4007-ef5c-bf81beccc4a5@linaro.org> (raw)
In-Reply-To: <7927a474-d542-c84d-5cb7-7ddd5d19d7f4@linaro.org>

On 22/11/22 19:21, Philippe Mathieu-Daudé wrote:
> On 18/11/22 10:47, Richard Henderson wrote:
>> Adding a vector type will make it easier to handle i386
>> have_atomic16 via AVX.
>>
>> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
>> ---
>>   include/qemu/int128.h | 10 +++++-----
>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/qemu/int128.h b/include/qemu/int128.h
>> index f62a46b48c..f29f90e6f4 100644
>> --- a/include/qemu/int128.h
>> +++ b/include/qemu/int128.h
>> @@ -479,16 +479,16 @@ static inline void bswap128s(Int128 *s)
>>   /*
>>    * When compiler supports a 128-bit type, define a combination of
>>    * a possible structure and the native types.  Ease parameter passing
>> - * via use of the transparent union extension.
>> + * via use of the transparent union extension.  Provide a vector type
>> + * for use in atomicity on some hosts.
>>    */
>> -#ifdef CONFIG_INT128
>>   typedef union {
>>       Int128 s;
>> +    uint64_t v __attribute__((vector_size(16)));
>> +#ifdef CONFIG_INT128
>>       __int128_t i;
>>       __uint128_t u;
>> -} Int128Alias __attribute__((transparent_union));
>> -#else
>> -typedef Int128 Int128Alias;
>>   #endif /* CONFIG_INT128 */
>> +} Int128Alias __attribute__((transparent_union));
>>   #endif /* INT128_H */
> 
> This triggers a warning with GCC:

Ah no, looking closer, even configured as ''--cc=gcc-12 --host-cc=gcc-12
--cxx=/bin/false', Clang got selected for ObjC, and this warning comes
from it:

Compiling Objective-C object libcommon.fa.p/ui_cocoa.m.o
In file included from ../../ui/cocoa.m:36:
In file included from include/sysemu/sysemu.h:5:
In file included from include/qemu/timer.h:4:
In file included from include/qemu/bitops.h:16:
In file included from include/qemu/host-utils.h:35:

> include/qemu/int128.h:487:14: warning: alignment of field 'v' (128 bits) 
> does not match the alignment of the first field in transparent union; 
> transparent_union attribute ignored [-Wignored-attributes]
>      uint64_t v __attribute__((vector_size(16)));
>               ^
> include/qemu/int128.h:486:12: note: alignment of first field is 64 bits
>      Int128 s;
>             ^
> Meson:
> 
> Project version: 7.1.91
> C compiler for the host machine: gcc-12 (gcc 12.2.0 "gcc-12 (Homebrew 
> GCC 12.2.0) 12.2.0")
> C linker for the host machine: gcc-12 ld64 819.6
> Host machine cpu family: aarch64
> Host machine cpu: arm64

Objective-C compiler for the host machine: clang (clang 14.0.0)
Objective-C linker for the host machine: clang ld64 819.6

Regards,

Phil.