Re: [Xen-devel] [PATCH 8/8] x86/efi: Generate uefi_call_wrapper() when compiling with clang

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 10/02/16 13:31, Jan Beulich wrote:
>>>> On 09.02.16 at 21:01, <andrew.cooper3@xxxxxxxxxx> wrote:
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>
>> What is the GCC version check supposed to be achieving here?  GCC has
>> supported this syntax since 3.0
> This is best answered by looking at where we've got these headers
> from - the gnu-efi package. It has ...
>
>> --- a/xen/include/asm-x86/x86_64/efibind.h
>> +++ b/xen/include/asm-x86/x86_64/efibind.h
>> @@ -274,7 +274,7 @@ typedef uint64_t   UINTN;
>>  #endif
>>  #endif
>>  
>> -#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 4)
>> +#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 4) || __clang__
>>  #define uefi_call_wrapper(func, va_num, ...)        func(__VA_ARGS__)
>>  #else
>>  /* for x86_64, EFI_FUNCTION_WRAPPER must be defined */
> /* for x86_64, EFI_FUNCTION_WRAPPER must be defined */
> #if defined(HAVE_USE_MS_ABI)
> #define uefi_call_wrapper(func, va_num, ...) func(__VA_ARGS__)
> #else
> UINTN uefi_call_wrapper(void *func, unsigned long va_num, ...);
> #endif
> #define EFI_FUNCTION __attribute__((ms_abi))
>
> I think this makes clear that the needed feature here is the
> attribute, which certainly wasn't available in older gcc.
>
> With that the question is whether the Clang case won't also need
> a version check, since I can't imagine them having supported this
> prior to gcc introducing it.

Clang has an substantially more sane way than GCC of checking for
individual features.  I will introduce and use the
__has_{attribute,feature}() infrastructure to tests like this.

Experimentally, Clang 3.5 does have ms_abi support

~Andrew

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