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Fix failure of AVX2-accelerated mb_check_encoding on 32-bit MS Windows #10776

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Fix failure of AVX2-accelerated mb_check_encoding on 32-bit MS Windows #10776

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7 changes: 6 additions & 1 deletion ext/mbstring/mbstring.c
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Original file line number Diff line number Diff line change
Expand Up @@ -5155,7 +5155,12 @@ static bool mb_fast_check_utf8_avx2(zend_string *str)
goto check_operand;
case 7:
case 8:
operand = _mm256_set_epi64x(0, 0, 0, *((int64_t*)p));
/* This was originally: operand = _mm256_set_epi64x(0, 0, 0, *((int64_t*)p));
* However, that caused test failures on 32-bit MS Windows
* (Bad 7/8-byte UTF-8 strings would be wrongly passed through as 'valid')
* It seems this is caused by a bug in MS Visual C++
* Ref: https://stackoverflow.com/questions/37509129/potential-bug-in-visual-studio-c-compiler-or-in-intel-intrinsics-avx2-mm256-s */
operand = _mm256_set_epi32(0, 0, 0, 0, 0, 0, ((int32_t*)p)[1], ((int32_t*)p)[0]);
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same instruction is used in

php-src/ext/hash/xxhash/xxhash.h

Line 3895 in 7638640

{ __m256i const seed = _mm256_set_epi64x((xxh_i64)(0U - seed64), (xxh_i64)seed64, (xxh_i64)(0U - seed64), (xxh_i64)seed64);

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Indeed.

I don't know if the AVX2-accelerated code you are showing from xxhash is used on 32-bit Windows hosts or not. If it is, that indicates that using _mm256_set_epi64x is not necessarily a problem in of itself. Whatever the case, a root-cause analysis cannot be done without access to a test machine, and I don't have one. However, the change in this PR seems to have fixed a problem.

If anyone else has more insight into the issue at hand, I would love to hear their comments.

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Should we use an #if and keep using _mm256_set_epi64x for everything but Windows x86 32-bit?

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seems to be https://stackoverflow.com/questions/37509129/potential-bug-in-visual-studio-c-compiler-or-in-intel-intrinsics-avx2-mm256-s and xxhash should be fixed as well

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@mvorisek Good find, that explains it 🙂

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@mvorisek Great find!!

As a suggestion, I think the first thing would be to add a test case which exposes the problem on 32-bit Windows. Then it will be easy to verify the fix.

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Should we use an #if and keep using _mm256_set_epi64x for everything but Windows x86 32-bit?

I don't think so.

If this particular line was performance-critical, then I would say yes, but I think the performance loss here will be minute. (If the compiler is smart enough to generate optimal code on x86-64, we won't lose anything.) The effect will only be on 7 and 8 byte UTF-8 strings, and for those, I expect the overhead of entering/exiting a function from PHP code will dominate this tiny bit of overhead.

Having a lot of #ifs in C code is a bad thing. I'm sure you know all the reasons why that is so.

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@iluuu1994 Please note, I would be happy to be proved wrong on the above statement.

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I wouldn't consider an #if an issue, but I'm fine either way 🙂

goto check_operand;
case 9:
operand = _mm256_set_m128i(_mm_setzero_si128(), _mm_srli_si128(_mm_loadu_si128((__m128i*)(p - 6)), 6));
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1 change: 0 additions & 1 deletion ext/mbstring/tests/utf_encodings.phpt
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Expand Up @@ -5,7 +5,6 @@ mbstring
--SKIPIF--
<?php
if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
if (substr(PHP_OS, 0, 3) === 'WIN' && PHP_INT_SIZE === 4) die("skip not for Windows x86");
?>
--FILE--
<?php
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