[PATCH] use test -s to speed up check for empty file

[Paolo Bonzini]

This is the source of run-time performance improvements.  It uses
"test -s" to know if there will be output from diff, and skips the
invocation if this is not the case.  I was surprised too by the
difference, but it's there to see.  I abstracted this into a
shell function too.

2007-10-04  Paolo Bonzini  <address@hidden>

        * general.m4 (AT_INIT): Add at_diff_devnull shell function,
        using test -s to avoid useless diff invocations.
        (_AT_CHECK): Use it.
---
 ChangeLog               |    6 ++++++
 lib/autotest/general.m4 |    9 +++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/lib/autotest/general.m4 b/lib/autotest/general.m4
index 3de93df..b87ba96 100644
--- a/lib/autotest/general.m4
+++ b/lib/autotest/general.m4
@@ -256,6 +256,11 @@ at_check_status () {
   esac
 }
 
+at_diff_devnull () {
+  test -s "$[1]" || return 0
+  $at_diff "$at_devnull" "$[1]"
+}
+
 # Load the config file.
 for at_file in atconfig atlocal
 do
@@ -1532,14 +1537,14 @@ m4_case([$4],
        stderr, [echo stderr:; tee stderr <"$at_stderr"],
        ignore, [echo stderr:; cat "$at_stderr"],
        experr, [$at_diff experr "$at_stderr" || at_failed=:],
-       [],     [$at_diff "$at_devnull" "$at_stderr" || at_failed=:],
+        [],     [at_diff_devnull "$at_stderr" || at_failed=:],
        [echo >>"$at_stderr"; 
AS_ECHO(["m4_ifval([$7],[AS_ESCAPE([$4])],[$4])"]) | $at_diff - "$at_stderr" || 
at_failed=:])
 dnl Check stdout.
 m4_case([$3],
        stdout, [echo stdout:; tee stdout <"$at_stdout"],
        ignore, [echo stdout:; cat "$at_stdout"],
        expout, [$at_diff expout "$at_stdout" || at_failed=:],
-       [],     [$at_diff "$at_devnull" "$at_stdout" || at_failed=:],
+        [],     [at_diff_devnull "$at_stdout" || at_failed=:],
        [echo >>"$at_stdout"; 
AS_ECHO(["m4_ifval([$7],[AS_ESCAPE([$3])],[$3])"]) | $at_diff - "$at_stdout" || 
at_failed=:])
 m4_if([$2], [ignore], [at_check_skip], [at_check_status m4_default([$2], 
[0])]) $at_status "$at_srcdir/AT_LINE"
 AS_IF($at_failed, [$5], [$6])
-- 
1.5.2.4