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Force -O2 on GCC + AVX2, document split load

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GCC for AVX2 goes overboard on the unrolling with -O3, causing slower
code than MSVC and Clang.

We can override that with a pragma that forces GCC to use -O2 instead.

Note that GCC still generates the best scalar and SSE2 code with -O3.

I also mentioned the fact that GCC will split _mm256_loadu_si256 into
two instructions on a generic+avx2 target (which is an optimization that
only applies to the non-AVX2 Sandy and Ivy Bridge chips), and provide
the recommended flags.
This commit is contained in:
easyaspi314 (Devin) 2020-02-21 10:05:58 -05:00
parent 777ec6529a
commit 5309e282ce
1 changed files with 37 additions and 1 deletions
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38
xxh3.h
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@ -151,6 +151,37 @@
# endif
#endif
/*
* UGLY HACK:
* GCC usually generates the best code with -O3 for xxHash,
* except for AVX2 where it is overzealous in its unrolling
* resulting in code roughly 3/4 the speed of Clang.
*
* There are other issues, such as GCC splitting _mm256_loadu_si256
* into _mm_loadu_si128 + _mm256_inserti128_si256 which is an
* optimization which only applies to Sandy and Ivy Bridge... which
* don't even support AVX2.
*
* That is why when compiling the AVX2 version, it is recommended
* to use either
* -O2 -mavx2 -march=haswell
* or
* -O2 -mavx2 -mno-avx256-split-unaligned-load
* for decent performance, or just use Clang instead.
*
* Fortunately, we can control the first one with a pragma
* that forces GCC into -O2, but the other one we can't without
* "failed to inline always inline function due to target mismatch"
* warnings.
*/
#if XXH_VECTOR == XXH_AVX2 /* AVX2 */ \
&& defined(__GNUC__) && !defined(__clang__) /* GCC, not Clang */ \
&& defined(__OPTIMIZE__) && !defined(__OPTIMIZE_SIZE__) /* respect -O0 and -Os */
# pragma GCC push_options
# pragma GCC optimize("-O2")
#endif
#if XXH_VECTOR == XXH_NEON
/*
* NEON's setup for vmlal_u32 is a little more complicated than it is on
@ -1699,6 +1730,11 @@ XXH128_hashFromCanonical(const XXH128_canonical_t* src)
return h;
}
/* Pop our optimization override from above */
#if XXH_VECTOR == XXH_AVX2 /* AVX2 */ \
&& defined(__GNUC__) && !defined(__clang__) /* GCC, not Clang */ \
&& defined(__OPTIMIZE__) && !defined(__OPTIMIZE_SIZE__) /* respect -O0 and -Os */
# pragma GCC pop_options
#endif
#endif /* XXH3_H_1397135465 */