sh: Use vfork in a few common cases.

This uses vfork() for simple commands and command substitutions containing a
single simple command, invoking an external program under certain conditions
(no redirections or variable assignments, non-interactive shell, no job
control). These restrictions limit the amount of code executed in a vforked
child.

There is a large speedup (for example 35%) in microbenchmarks. The
difference in buildkernel is smaller (for example 0.5%) but still
statistically significant. See
http://lists.freebsd.org/pipermail/freebsd-hackers/2012-January/037581.html
for some numbers.

The use of vfork() can be disabled by setting a variable named
SH_DISABLE_VFORK.

1 files changed, 3 insertions, 0 deletions

diff --git a/bin/sh/var.c b/bin/sh/var.c
index b3bc6f7f..bc00e06 100644
--- a/bin/sh/var.c
+++ b/bin/sh/var.c
@@ -94,6 +94,7 @@ struct var vps2;
 struct var vps4;
 struct var vvers;
 static struct var voptind;
+struct var vdisvfork;
 
 int forcelocal;
 
@@ -125,6 +126,8 @@ static const struct varinit varinit[] = {
 #endif
 	{ &voptind,	0,				"OPTIND=1",
 	  getoptsreset },
+	{ &vdisvfork,	VUNSET,				"SH_DISABLE_VFORK=",
+	  NULL },
 	{ NULL,	0,				NULL,
 	  NULL }
 };