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ext/standard: Remove useless _mm_movemask_epi8 check in rot13 #23795

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Merged
LamentXU123 merged 1 commit into from
Sep 29, 2026
Merged

ext/standard: Remove useless _mm_movemask_epi8 check in rot13 #23795

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1 change: 1 addition & 0 deletions UPGRADING
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Original file line number Diff line number Diff line change
Expand Up @@ -79,3 +79,4 @@ PHP 8.7 UPGRADE NOTES
- Standard:
. Improved performance of array_splice() when inserting without removing
elements.
. Improved performance of str_rot13().
24 changes: 8 additions & 16 deletions ext/standard/string.c
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Original file line number Diff line number Diff line change
Expand Up @@ -6054,34 +6054,26 @@ static zend_string *php_str_rot13(zend_string *str)
gt = _mm_cmpgt_epi8(in, a_minus_1);
lt = _mm_cmplt_epi8(in, m_plus_1);
cmp = _mm_and_si128(lt, gt);
if (_mm_movemask_epi8(cmp)) {
cmp = _mm_and_si128(cmp, add);
delta = _mm_or_si128(delta, cmp);
}
cmp = _mm_and_si128(cmp, add);
delta = _mm_or_si128(delta, cmp);

gt = _mm_cmpgt_epi8(in, n_minus_1);
lt = _mm_cmplt_epi8(in, z_plus_1);
cmp = _mm_and_si128(lt, gt);
if (_mm_movemask_epi8(cmp)) {
cmp = _mm_and_si128(cmp, sub);
delta = _mm_or_si128(delta, cmp);
}
cmp = _mm_and_si128(cmp, sub);
delta = _mm_or_si128(delta, cmp);

gt = _mm_cmpgt_epi8(in, A_minus_1);
lt = _mm_cmplt_epi8(in, M_plus_1);
cmp = _mm_and_si128(lt, gt);
if (_mm_movemask_epi8(cmp)) {
cmp = _mm_and_si128(cmp, add);
delta = _mm_or_si128(delta, cmp);
}
cmp = _mm_and_si128(cmp, add);
delta = _mm_or_si128(delta, cmp);

gt = _mm_cmpgt_epi8(in, N_minus_1);
lt = _mm_cmplt_epi8(in, Z_plus_1);
cmp = _mm_and_si128(lt, gt);
if (_mm_movemask_epi8(cmp)) {
cmp = _mm_and_si128(cmp, sub);
delta = _mm_or_si128(delta, cmp);
}
cmp = _mm_and_si128(cmp, sub);
delta = _mm_or_si128(delta, cmp);
Comment on lines 6054 to +6076

@iliaal iliaal Sep 20, 2026

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c | 0x20 maps A-Z onto a-z and moves nothing else into that range (it only touches bit 5, and bytes >= 0x80 stay negative under the signed compare), so the four range tests collapse into one plus a midpoint test:

const __m128i lowbit = _mm_set1_epi8(0x20);
const __m128i a_minus_1 = _mm_set1_epi8('a' - 1);
const __m128i z_plus_1 = _mm_set1_epi8('z' + 1);
const __m128i n = _mm_set1_epi8('n');
const __m128i add = _mm_set1_epi8(13);
const __m128i sub = _mm_set1_epi8(-13);

do {
	__m128i in = _mm_loadu_si128((__m128i *)p);
	__m128i low = _mm_or_si128(in, lowbit);
	__m128i alpha = _mm_and_si128(_mm_cmpgt_epi8(low, a_minus_1), _mm_cmplt_epi8(low, z_plus_1));
	__m128i first = _mm_cmplt_epi8(low, n);
	__m128i delta = _mm_and_si128(_mm_or_si128(_mm_and_si128(first, add), _mm_andnot_si128(first, sub)), alpha);

	_mm_storeu_si128((__m128i *)target, _mm_add_epi8(in, delta));

	p += 16;
	target += 16;
} while (e - p > 15);

gcc 11.4 -O2: the compiled loop body goes from 28 SIMD instructions to 15, and the 4-deep serial OR chain on delta goes away. Best of 7 over 1 MiB against your version: 1.9x on all-lowercase, 1.9x on digits, 2.4x on mixed case. Byte-identical to the scalar fallback for every byte value in every lane.

Separately, for the description: 1.2-1.3x is the floor, not the typical case. Random bytes at 1 MiB measure 7.9x against master here, since the four branches mispredict. Small buffers hide that, the predictor just memorizes a short repeating pattern.

@LamentXU123 LamentXU123 Sep 23, 2026

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Yeah I also confirmed the speed-up on my production server. @iliaal Is it mergeable now?

@iliaal iliaal Sep 29, 2026

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The branch still has the original commit, so the variant isn't in. Fine to merge as is if you'd rather keep this PR to the branch removal; either way please make the UPGRADING line "Improved performance of str_rot13()." like its neighbours.


in = _mm_add_epi8(in, delta);
_mm_storeu_si128((__m128i *)target, in);
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