[cfe-dev] __attribute__((enable_if(...))) for de-macroifying the builtin headers

[Sean Silva]

On Wed, Apr 15, 2015 at 6:46 PM, Nick Lewycky <nlewycky at google.com> wrote:

> On 15 April 2015 at 10:27, Sean Silva <chisophugis at gmail.com> wrote:
>
>> In the post-commit of r231792, I suggested the idea of using
>> __attribute__((enable_if(...))) for avoiding the mess of the macros in the
>> builtin headers. AFAIK, the macros are currently used to make sure that the
>> "immediates" are constant expressions, piggybacking on the constant
>> expression check in the __builtin_* call.
>>
>> I've attached a file with a proof-of-concept for using
>> __attribute__((enable_if(...))) for this purpose. I originally though using
>> __builtin_constant_p in the enable_if, but that turns out to not be
>> necessary (see the docs:
>> http://clang.llvm.org/docs/AttributeReference.html#enable-if ; the
>> enable_if condition fails for non-constant expressions anyway). The core is:
>>
>> // Current builtin headers:
>> //
>> //#define _mm256_insertf128_si256(V1, V2, M) __extension__ ({ \
>> //  (__m256i)__builtin_shufflevector( \
>> //    (__v4di)(V1), \
>> //    (__v4di)_mm256_castsi128_si256((__m128i)(V2)), \
>> //    (((M) & 1) ? 0 : 4), \
>> //    (((M) & 1) ? 1 : 5), \
>> //    (((M) & 1) ? 4 : 2), \
>> //    (((M) & 1) ? 5 : 3) );})
>>
>> // A bit cleaner.
>> static __inline __attribute__((__always_inline__, __nodebug__))
>> __m256i _mm256_insertf128_si256(__m256i __a, __m128i __b, int __imm8)
>> __attribute__((enable_if(__imm8, "'__imm8' must be a constant")))
>> {
>>   if (__imm8 & 1)
>>     return __builtin_shufflevector(__a, _mm256_castsi128_si256(__b), 0,
>> 1, 4, 5);
>>   else
>>     return __builtin_shufflevector(__a, _mm256_castsi128_si256(__b), 4,
>> 5, 2, 3);
>> }
>>
>
> I think:
>
> static __inline __attribute__((__always_inline__, __nodebug__))
> __m256i _mm256_insertf128_si256(__m256i __a, __m128i __b, int __imm8)
> __attribute__((enable_if(__imm8 & 1 == 1, "'__imm8' must be a constant")))
> {
>   return __builtin_shufflevector(__a, _mm256_castsi128_si256(__b), 0, 1,
> 4, 5);
> }
>
> static __inline __attribute__((__always_inline__, __nodebug__))
> __m256i _mm256_insertf128_si256(__m256i __a, __m128i __b, int __imm8)
> __attribute__((enable_if(__imm8 & 1 == 0, "'__imm8' must be a constant")))
> {
>   return __builtin_shufflevector(__a, _mm256_castsi128_si256(__b), 4, 5,
> 2, 3);
> }
>

Unfortunately this approach doesn't scale to some of the more nontrivial
ones like _mm256_alignr_epi8 in http://reviews.llvm.org/D8301 .

>
> Nick, are you okay using enable_if for this? It's sort of a hack but if we
>> are going to be carrying this attribute around forever (has it reached that
>> level of compatibility guarantee yet?), we might as well use it to solve
>> this problem for us.
>>
>
> Attribute enable_if is here to stay, though it may change meaning in
> corner cases (notably multiple enable_if's on a single function). Using it
> in the compiler's own header files is fine. Does anyone ever try to take
> the address of _mm256_insertf128_si256 (I assume not if it's a macro)?
>

Yeah, I think taking the address of these is a "don't do that" sort of
thing. "static inline always_inline"

What changes do you foresee in the case of multiple enable_if's? The
current behavior (in my empirical testing; haven't used the source) seems
to be a good fit for this use case.

-- Sean Silva
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