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[OpenMP] Fix various alignment issues #142376

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merged 2 commits into from
Jun 25, 2025
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[OpenMP] Fix various alignment issues #142376

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[OpenMP] Fix various alignment issues
rorth Jun 2, 2025
360de46
- Update comment.
rorth Jun 25, 2025
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8 changes: 4 additions & 4 deletions openmp/runtime/src/kmp_alloc.cpp
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Expand Up @@ -70,10 +70,10 @@ static void bectl(kmp_info_t *th, bget_compact_t compact,
/* Buffer allocation size quantum: all buffers allocated are a
multiple of this size. This MUST be a power of two. */

/* On IA-32 architecture with Linux* OS, malloc() does not
ensure 16 byte alignment */
/* On some architectures, malloc() does not ensure 16 byte alignment,
Solaris/sparc and x86 among them. */

#if KMP_ARCH_X86 || !KMP_HAVE_QUAD
#if KMP_ARCH_X86 || KMP_ARCH_SPARC || !KMP_HAVE_QUAD
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Do we want to add anything to the comment above this?

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I found this is complete mess: glibc sysdeps/i386/malloc-alignment. does #define MALLOC_ALIGNMENT 16, contrary to the comment (haven't checked if this has changed historically). Besides, KMP_HAVE_QUAD is a complete misnomer: it's only defined on x86 targets, so should probably be renamed to KMP_X86_HAVE_QUAD to reflect that.

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I've updated this comment now, removing the incorrect Linux reference.

As it turns out, malloc alignment is a total can of worms:

  • In theory, the address returned should be aligned to alignof(max_align_t).
  • However, implementations often differ. E.g. Solaris 32-bit libc malloc only uses 8-byte alignment. At the time this was introduced, __float128 didn't exist on x86 yet and it was considered too risky to raise this later for fear of breaking binary compatiblity. Similarly, SPARC uses 8-byte alignment, too, although one could argue that this should be 16 bytes due to the use of 128-bit long double.
  • Many platforms have different malloc implementions that often differ from the one in libc.
  • The actual alignment cannot be queried at build or runtime, unfortunately.

That said, it might be prudent to switch to one of memalign, posix_memalign, or _aligned_malloc. GCC's libgomp (in libgomp/alloc.c:gomp_aligned_alloc) went that route, e.g.


#define SizeQuant 8
#define AlignType double
Expand Down Expand Up @@ -1861,7 +1861,7 @@ typedef struct kmp_mem_desc { // Memory block descriptor
void *ptr_align; // Pointer to aligned memory, returned
kmp_allocator_t *allocator; // allocator
} kmp_mem_desc_t;
static int alignment = sizeof(void *); // align to pointer size by default
constexpr size_t alignment = SizeQuant;

// external interfaces are wrappers over internal implementation
void *__kmpc_alloc(int gtid, size_t size, omp_allocator_handle_t allocator) {
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2 changes: 1 addition & 1 deletion openmp/runtime/src/kmp_csupport.cpp
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Expand Up @@ -570,7 +570,7 @@ void __kmpc_fork_teams(ident_t *loc, kmp_int32 argc, kmpc_micro microtask,

this_thr->th.th_teams_microtask = NULL;
this_thr->th.th_teams_level = 0;
*(kmp_int64 *)(&this_thr->th.th_teams_size) = 0L;
memset(&this_thr->th.th_teams_size, 0, sizeof(kmp_teams_size_t));
va_end(ap);
#if KMP_STATS_ENABLED
if (previous_state == stats_state_e::SERIAL_REGION) {
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2 changes: 1 addition & 1 deletion openmp/runtime/src/kmp_tasking.cpp
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Expand Up @@ -1528,7 +1528,7 @@ kmp_task_t *__kmp_task_alloc(ident_t *loc_ref, kmp_int32 gtid,
// Calculate shared structure offset including padding after kmp_task_t struct
// to align pointers in shared struct
shareds_offset = sizeof(kmp_taskdata_t) + sizeof_kmp_task_t;
shareds_offset = __kmp_round_up_to_val(shareds_offset, sizeof(void *));
shareds_offset = __kmp_round_up_to_val(shareds_offset, sizeof(kmp_uint64));

// Allocate a kmp_taskdata_t block and a kmp_task_t block.
KA_TRACE(30, ("__kmp_task_alloc: T#%d First malloc size: %ld\n", gtid,
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