jemalloc: set LG_VADDR to 64 on amd64

and allow to revert it back to 48 with WITHOUT_JEMALLOC_LG_VADDR_WIDE build
option.

Reviewed by:	andrew, emaste
Sponsored by:	Advanced Micro Devices (AMD)
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D46686

contrib/jemalloc/include/jemalloc/jemalloc_FreeBSD.h

@@ -39,7 +39,11 @@
 #  define JEMALLOC_TLS_MODEL	__attribute__((tls_model("initial-exec")))
 #endif
 #ifdef __amd64__
+#ifdef _USE_LG_VADDR_WIDE
+#  define LG_VADDR		64
+#else
 #  define LG_VADDR		48
+#endif
 #  define LG_SIZEOF_PTR		3
 #  define JEMALLOC_TLS_MODEL	__attribute__((tls_model("initial-exec")))
 #endif

lib/libc/stdlib/malloc/jemalloc/Makefile.inc

@@ -5,6 +5,9 @@ JEMALLOCSRCS:= jemalloc.c arena.c background_thread.c base.c bin.c bitmap.c \
 	test_hooks.c ticker.c tsd.c witness.c
 
 CFLAGS+=-I${SRCTOP}/contrib/jemalloc/include
+.if ${MK_JEMALLOC_LG_VADDR_WIDE} != no
+CFLAGS+=-D_USE_LG_VADDR_WIDE
+.endif
 
 .for src in ${JEMALLOCSRCS}
 MISRCS+=jemalloc_${src}

share/mk/src.opts.mk

@@ -116,6 +116,7 @@ __DEFAULT_YES_OPTIONS = \
     IPFW \
     ISCSI \
     JAIL \
+    JEMALLOC_LG_VADDR_WIDE \
     KDUMP \
     KVM \
     LDNS \

tools/build/options/WITHOUT_JEMALLOC_LG_VADDR_WIDE

@@ -0,0 +1,5 @@
+Disallow programs to use more than 48 address bits on amd64.
+Incompatible with LA57 mode.
+Enabling this option might result in a slight reduction in memory
+consumption for jemalloc metadata, but also requires disabling LA57
+(if hardware supports it).