malloc: try to use builtins for zeroing at the callsite

Plenty of allocation sites pass M_ZERO and sizes which are small and known
at compilation time. Handling them internally in malloc loses this information
and results in avoidable calls to memset.

Instead, let the compiler take the advantage of it whenever possible.

Discussed with:	jeff

sys/contrib/zstd/lib/freebsd/stdlib.h

@@ -35,6 +35,7 @@
 
 MALLOC_DECLARE(M_ZSTD);
 
+#undef malloc
 #define	malloc(x)	(malloc)((x), M_ZSTD, M_WAITOK)
 #define	free(x)		(free)((x), M_ZSTD)
 #define	calloc(a, b)	(mallocarray)((a), (b), M_ZSTD, M_WAITOK | M_ZERO)

sys/kern/kern_malloc.c

@@ -549,7 +549,7 @@ malloc_dbg(caddr_t *vap, size_t *sizep, struct malloc_type *mtp,
  *	the allocation fails.
  */
 void *
-malloc(size_t size, struct malloc_type *mtp, int flags)
+(malloc)(size_t size, struct malloc_type *mtp, int flags)
 {
 	int indx;
 	caddr_t va;

sys/netinet/libalias/alias_mod.h

@@ -41,6 +41,7 @@ MALLOC_DECLARE(M_ALIAS);
 
 /* Use kernel allocator. */
 #if defined(_SYS_MALLOC_H_)
+#undef malloc
 #define	malloc(x)	malloc(x, M_ALIAS, M_NOWAIT|M_ZERO)
 #define	calloc(n, x)	mallocarray((n), (x), M_ALIAS, M_NOWAIT|M_ZERO)
 #define	free(x)		free(x, M_ALIAS)

sys/sys/malloc.h

@@ -38,6 +38,9 @@
 #define	_SYS_MALLOC_H_
 
 #include <sys/param.h>
+#ifdef _KERNEL
+#include <sys/systm.h>
+#endif
 #include <sys/queue.h>
 #include <sys/_lock.h>
 #include <sys/_mutex.h>
@@ -183,6 +186,22 @@ 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);
+#ifdef _KERNEL
+#define	malloc(size, type, flags) ({					\
+	void *_malloc_item;						\
+	size_t _size = (size);						\
+	if (__builtin_constant_p(size) && __builtin_constant_p(flags) &&\
+	    ((flags) & M_ZERO)) {					\
+		_malloc_item = malloc(_size, type, (flags) &~ M_ZERO);	\
+		if (((flags) & M_WAITOK) || _malloc_item != NULL)	\
+			bzero(_malloc_item, _size);			\
+	} else {							\
+		_malloc_item = malloc(_size, type, flags);		\
+	}								\
+	_malloc_item;							\
+})
+#endif
+
 void	*malloc_domain(size_t size, struct malloc_type *type, int domain,
 	    int flags) __malloc_like __result_use_check __alloc_size(1);
 void	*mallocarray(size_t nmemb, size_t size, struct malloc_type *type,