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Use alignment builtins for ZEND_MM_ALIGNED macros #21913

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Use alignment builtins for ZEND_MM_ALIGNED macros #21913

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3c2446d
Preserve pointer type in ZEND_MM_ALIGNED_BASE
tobil4sk Apr 29, 2026
eb4b0d7
Use alignment builtins for ZEND_MM_ALIGNED macros
tobil4sk Nov 5, 2025
526ee4f
Adjust ZEND_MM_ALIGNED_OFFSET argument name
tobil4sk May 1, 2026
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23 changes: 18 additions & 5 deletions Zend/zend_alloc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -175,12 +175,25 @@ static size_t _real_page_size = ZEND_MM_PAGE_SIZE;
typedef uint32_t zend_mm_page_info; /* 4-byte integer */
typedef zend_ulong zend_mm_bitset; /* 4-byte or 8-byte integer */

#define ZEND_MM_ALIGNED_OFFSET(size, alignment) \
(((size_t)(size)) & ((alignment) - 1))
#define ZEND_MM_ALIGNED_BASE(size, alignment) \
(((size_t)(size)) & ~((alignment) - 1))
#define ZEND_MM_SIZE_TO_NUM(size, alignment) \
#ifdef PHP_HAVE_BUILTIN_ALIGN_DOWN
# define ZEND_MM_ALIGNED_BASE(ptr, alignment) \
__builtin_align_down((ptr), (alignment))
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@iluuu1994 iluuu1994 May 1, 2026

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Thinking about this a bit more, I'm not sure we want the result of ZEND_MM_ALIGNED_BASE to be of the type of ptr, as the base has a different type in every case. I guess we could cast to void* before passing to __builtin_align_down? Then the result is void*, which would force coercion to an appropriate type. WDYT?

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@tobil4sk tobil4sk May 1, 2026

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I think there would be benefit in ZEND_MM_ALIGNED_BASE mapping directly to the builtin.

That being said, currently the two implementations return different types (uintptr_t vs T*). I guess it might be more consistent to ensure the result for both is void*.

# define ZEND_MM_ALIGNED_OFFSET(ptr, alignment) \
(((size_t)(ptr)) - (size_t)ZEND_MM_ALIGNED_BASE(ptr, alignment))
#else
# define ZEND_MM_ALIGNED_BASE(ptr, alignment) \
(((uintptr_t)(ptr)) & ~((alignment) - 1))
# define ZEND_MM_ALIGNED_OFFSET(ptr, alignment) \
(((size_t)(ptr)) & ((alignment) - 1))
#endif

#ifdef PHP_HAVE_BUILTIN_ALIGN_UP
# define ZEND_MM_SIZE_TO_NUM(size, alignment) \
(__builtin_align_up((size), (alignment)) / (alignment))
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@iluuu1994 iluuu1994 May 1, 2026

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Is this usage of __builtin_align_up() actually correct when the input is size_t rather than some pointer type?

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@tobil4sk tobil4sk May 1, 2026

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Yes, this one actually only works with size_t (or another integer type) because of the / operator which does not accept a pointer operand. I think this is OK because the macro name explicitly states that it operates on a size, not a pointer.

#else
# define ZEND_MM_SIZE_TO_NUM(size, alignment) \
(((size_t)(size) + ((alignment) - 1)) / (alignment))
#endif

#define ZEND_MM_BITSET_LEN (sizeof(zend_mm_bitset) * 8) /* 32 or 64 */
#define ZEND_MM_PAGE_MAP_LEN (ZEND_MM_PAGES / ZEND_MM_BITSET_LEN) /* 16 or 8 */
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2 changes: 2 additions & 0 deletions build/php.m4
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Expand Up @@ -2380,6 +2380,8 @@ AC_CACHE_CHECK([for $1], [php_var],
[__builtin_ssubll_overflow], [long long tmpvar; return $1(3, 7, &tmpvar);],
[__builtin_unreachable], [$1();],
[__builtin_usub_overflow], [unsigned int tmpvar; return $1(3, 7, &tmpvar);],
[__builtin_align_down], [return $1(1, 2) == 0 ? 1 : 0;],
[__builtin_align_up], [return $1(1, 2) == 2 ? 1 : 0;],
[
m4_warn([syntax], [Unsupported builtin '$1', the test may fail.])
$1();
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2 changes: 2 additions & 0 deletions configure.ac
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Expand Up @@ -490,6 +490,8 @@ PHP_CHECK_BUILTIN([__builtin_ssubl_overflow])
PHP_CHECK_BUILTIN([__builtin_ssubll_overflow])
PHP_CHECK_BUILTIN([__builtin_unreachable])
PHP_CHECK_BUILTIN([__builtin_usub_overflow])
PHP_CHECK_BUILTIN([__builtin_align_down])
PHP_CHECK_BUILTIN([__builtin_align_up])

dnl Check AVX512
PHP_CHECK_AVX512_SUPPORTS
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2 changes: 2 additions & 0 deletions win32/build/config.w32
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Expand Up @@ -370,6 +370,8 @@ if (VS_TOOLSET) {
AC_DEFINE("PHP_HAVE_BUILTIN_SSUBLL_OVERFLOW", 1, "Define to 1 if the compiler supports '__builtin_ssubll_overflow'.");
AC_DEFINE("PHP_HAVE_BUILTIN_SMULL_OVERFLOW", 1, "Define to 1 if the compiler supports '__builtin_smull_overflow '.");
AC_DEFINE("PHP_HAVE_BUILTIN_SMULLL_OVERFLOW", 1, "Define to 1 if the compiler supports '__builtin_smulll_overflow'.");
AC_DEFINE("PHP_HAVE_BUILTIN_ALIGN_DOWN", 1, "Define to 1 if the compiler supports '__builtin_align_down'.");
AC_DEFINE("PHP_HAVE_BUILTIN_ALIGN_UP", 1, "Define to 1 if the compiler supports '__builtin_align_up'.");

if (PHP_SANITIZER == "yes") {
if (COMPILER_NUMERIC_VERSION < 500) {
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