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, 1 insertions, 0 deletions

diff --git a/bin/sh/jobs.h b/bin/sh/jobs.h
index 5e9d70d..e741b2c 100644
--- a/bin/sh/jobs.h
+++ b/bin/sh/jobs.h
@@ -91,6 +91,7 @@ void setjobctl(int);
 void showjobs(int, int);
 struct job *makejob(union node *, int);
 pid_t forkshell(struct job *, union node *, int);
+pid_t vforkexecshell(struct job *, char **, char **, const char *, int, int []);
 int waitforjob(struct job *, int *);
 int stoppedjobs(void);
 int backgndpidset(void);