Author: mjg
Date: Wed Jun 6 05:08:05 2018
New Revision: 334702
URL: https://svnweb.freebsd.org/changeset/base/334702
Log:
malloc: elaborate on r334545 due to frequent questions
While here annotate the NULL check as probably true.
Modified:
head/sys/sys/malloc.h
Modified: head/sys/sys/malloc.h
==============================================================================
--- head/sys/sys/malloc.h Wed Jun 6 02:48:09 2018 (r334701)
+++ head/sys/sys/malloc.h Wed Jun 6 05:08:05 2018 (r334702)
@@ -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,
in
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); \
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