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malloc: elaborate on r334545 due to frequent questions

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While here annotate the NULL check as probably true.
This commit is contained in:
Mateusz Guzik 2018-06-06 05:08:05 +00:00
parent b2ca2e50b9
commit ffc3ab5d39
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=334702
1 changed files with 37 additions and 1 deletions
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38
sys/sys/malloc.h
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@ -186,6 +186,41 @@ void free(void *addr, struct malloc_type *type);
void free_domain(void *addr, struct malloc_type *type);
void *malloc(size_t size, struct malloc_type *type, int flags) __malloc_like
__result_use_check __alloc_size(1);
/*
* Try to optimize malloc(..., ..., M_ZERO) allocations by doing zeroing in
* place if the size is known at compilation time.
*
* Passing the flag down requires malloc to blindly zero the entire object.
* In practice a lot of the zeroing can be avoided if most of the object
* gets explicitly initialized after the allocation. Letting the compiler
* zero in place gives it the opportunity to take advantage of this state.
*
* Note that the operation is only applicable if both flags and size are
* known at compilation time. If M_ZERO is passed but M_WAITOK is not, the
* allocation can fail and a NULL check is needed. However, if M_WAITOK is
* passed we know the allocation must succeed and the check can be elided.
*
* _malloc_item = malloc(_size, type, (flags) &~ M_ZERO);
* if (((flags) & M_WAITOK) != 0 || _malloc_item != NULL)
* bzero(_malloc_item, _size);
*
* If the flag is set, the compiler knows the left side is always true,
* therefore the entire statement is true and the callsite is:
*
* _malloc_item = malloc(_size, type, (flags) &~ M_ZERO);
* bzero(_malloc_item, _size);
*
* If the flag is not set, the compiler knows the left size is always false
* and the NULL check is needed, therefore the callsite is:
*
* _malloc_item = malloc(_size, type, (flags) &~ M_ZERO);
* if (_malloc_item != NULL)
* bzero(_malloc_item, _size);
*
* The implementation is a macro because of what appears to be a clang 6 bug:
* an inline function variant ended up being compiled to a mere malloc call
* regardless of argument. gcc generates expected code (like the above).
*/
#ifdef _KERNEL
#define malloc(size, type, flags) ({ \
void *_malloc_item; \
@ -193,7 +228,8 @@ void *malloc(size_t size, struct malloc_type *type, int flags) __malloc_like
if (__builtin_constant_p(size) && __builtin_constant_p(flags) &&\
((flags) & M_ZERO) != 0) { \
_malloc_item = malloc(_size, type, (flags) &~ M_ZERO); \
if (((flags) & M_WAITOK) != 0 || _malloc_item != NULL) \
if (((flags) & M_WAITOK) != 0 || \
__predict_true(_malloc_item != NULL)) \
bzero(_malloc_item, _size); \
} else { \
_malloc_item = malloc(_size, type, flags); \