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add comments about TLS

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daanx 2024-12-26 10:24:20 -08:00
parent 47b5f48b29
commit 783dcb4129
1 changed files with 22 additions and 14 deletions
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36
include/mimalloc/prim.h
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@ -119,17 +119,15 @@ void _mi_prim_thread_associate_default_heap(mi_heap_t* heap);
//-------------------------------------------------------------------
// Thread id: `_mi_prim_thread_id()`
//
// Getting the thread id should be performant as it is called in the
// fast path of `_mi_free` and we specialize for various platforms as
// inlined definitions. Regular code should call `init.c:_mi_thread_id()`.
// We only require _mi_prim_thread_id() to return a unique id
// for each thread (unequal to zero).
//-------------------------------------------------------------------
//-------------------------------------------------------------------
// Access to TLS (thread local storage) slots.
// We need fast access to both a unique thread id (in `free.c:mi_free`) and
// to a thread-local heap pointer (in `alloc.c:mi_malloc`).
// To achieve this we use specialized code for various platforms.
//-------------------------------------------------------------------
// On some libc + platform combinations we can directly access a thread-local storage (TLS) slot.
// The TLS layout depends on both the OS and libc implementation so we use specific tests for each main platform.
// If you test on another platform and it works please send a PR :-)
@ -210,7 +208,7 @@ static inline void mi_prim_tls_slot_set(size_t slot, void* value) mi_attr_noexce
#elif _WIN32 && MI_WIN_USE_FIXED_TLS && !defined(MI_WIN_USE_FLS)
// On windows we can store the thread-local heap at a fixed TLS slot to avoid
// thread-local initialization checks in the fast path. This use a fixed location
// thread-local initialization checks in the fast path. This uses a fixed location
// in the TCB though (last user-reserved slot by default) which may clash with other applications.
#define MI_HAS_TLS_SLOT 2 // 2 = we can reliable initialize the slot (saving a test on each malloc)
@ -241,6 +239,18 @@ static inline void mi_prim_tls_slot_set(size_t slot, void* value) mi_attr_noexce
#endif
//-------------------------------------------------------------------
// Get a fast unique thread id.
//
// Getting the thread id should be performant as it is called in the
// fast path of `_mi_free` and we specialize for various platforms as
// inlined definitions. Regular code should call `init.c:_mi_thread_id()`.
// We only require _mi_prim_thread_id() to return a unique id
// for each thread (unequal to zero).
//-------------------------------------------------------------------
// Do we have __builtin_thread_pointer? This would be the preferred way to get a unique thread id
// but unfortunately, it seems we cannot test for this reliably at this time (see issue #883)
// Nevertheless, it seems needed on older graviton platforms (see issue #851).
@ -311,7 +321,8 @@ static inline mi_threadid_t _mi_prim_thread_id(void) mi_attr_noexcept {
/* ----------------------------------------------------------------------------------------
The thread local default heap: `_mi_prim_get_default_heap()`
Get the thread local default heap: `_mi_prim_get_default_heap()`
This is inlined here as it is on the fast path for allocation functions.
On most platforms (Windows, Linux, FreeBSD, NetBSD, etc), this just returns a
@ -404,7 +415,4 @@ static inline mi_heap_t* mi_prim_get_default_heap(void) {
#endif // mi_prim_get_default_heap()
#endif // MIMALLOC_PRIM_H