Don't use plain "ret" instructions at targets of jump instructions,

since the branch caches on at least Athlon XP through Athlon 64 CPU's
don't understand such instructions and guarantee a cache miss taking
at least 10 cycles.  Use the documented workaround "ret $0" instead
("nop; ret" also works, but "ret $0" is probably faster on old CPUs).

Normal code (even asm code) doesn't branch to "ret", since there is
usually some cleanup to do, but the __mcount, .mcount and .mexitcount
entry points were optimized too well to have the minimum number of
instructions (3 instructions each if profiling is not enabled) and
they did this.  I didn't see a significant number of cache misses for
.mexitcount, but for the shared "ret" for __mcount and .mcount I
observed cache misses costing 26 cycles each.  For a send(2) syscall
that makes about 70 function calls, the cost of these cache misses
alone increased the syscall time from about 4000 cycles to about 7000
cycles.  4000 is for a profiling (GUPROF) kernel with profiling disabled;
after this fix, configuring profiling only costs about 600 cycles in the
4000, which is consistent with almost perfect branch prediction in the
mcounting calls.

sys/amd64/amd64/prof_machdep.c

@@ -135,7 +135,7 @@ __mcount:							\n\
 	popq	%rdx						\n\
 	popq	%rax						\n\
 .mcount_exit:							\n\
-	ret							\n\
+	ret	$0						\n\
 ");
 #else /* !__GNUCLIKE_ASM */
 #error "this file needs to be ported to your compiler"
@@ -187,7 +187,7 @@ GMON_PROF_HIRES	=	4					\n\
 	popq	%rdx						\n\
 	popq	%rax						\n\
 .mexitcount_exit:						\n\
-	ret							\n\
+	ret	$0						\n\
 ");
 #endif /* __GNUCLIKE_ASM */
 

sys/i386/isa/prof_machdep.c

@@ -113,7 +113,7 @@ __mcount:							\n\
 	addl	$8,%esp						\n\
 	popfl							\n\
 .mcount_exit:							\n\
-	ret							\n\
+	ret	$0						\n\
 ");
 #else /* !__GNUCLIKE_ASM */
 #error "this file needs to be ported to your compiler"
@@ -157,7 +157,7 @@ GMON_PROF_HIRES	=	4					\n\
 	popl	%eax						\n\
 	popl	%edx						\n\
 .mexitcount_exit:						\n\
-	ret							\n\
+	ret	$0						\n\
 ");
 #endif /* __GNUCLIKE_ASM */