CODEWITHSUNDEEP
cgccstaticclangintrinsics
jbapple
jbapple

Reputation: 3390

Static vs. external intrinsics

The Clang 6.0.1 avxintrin.h has the declaration:

static __inline __m256i __DEFAULT_FN_ATTRS _mm256_set1_epi32(int)

GCC 5.5 has:

extern __inline __m256i __attribute__((__gnu_inline__, _always_inline__, __artificial__)) _mm256_set1_epi32(int)

Why would one be extern and one static? This is showing up for me in an inline function that calls _mm256_set1_epi32. Clang wants it to be declared static:

#include <immintrin.h>

inline void SimdBlockBloomFilter_make_mask() {
  _mm256_set1_epi32(1);
}

With -Weverything:

warning: static function '_mm256_set1_epi32' is used in an inline
function with external linkage [-Wstatic-in-inline]

This error does not show up when compiling with Clang++.

Upvotes: 3

Views: 201

Answers (1)

Florian Weimer
Florian Weimer

Reputation: 33717

In the GCC version, the gnu_inline attribute is close to the behavior of static inline in C99 and later modes.

The C committee ignored the GNU precedent when redefining the meaning of extern inline: With the GNU compiler, extern inline meant that the compiler should never generate a non-inline copy of the function (even if its address is taken). In C99, extern inline means that a definition in a translation unit completes an inline definition in another translation unit.

The GCC version of <immintrin.h> uses the gnu_inline attribute to get the expected behavior in all compiler modes (C89/C90 and C99 in particular).

Upvotes: 4

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