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[bug#60807] [PATCH 1/2] mtime: use Time::HiRes::stat when available for

From: Jacob Bachmeyer
Subject: [bug#60807] [PATCH 1/2] mtime: use Time::HiRes::stat when available for subsecond resolution
Date: Sat, 14 Jan 2023 21:27:35 -0600
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20090807 SeaMonkey/1.1.17 Mnenhy/

Mike Frysinger wrote:
Perl's builtin stat function returns timestamps that have 1 second
resolution.  This can lead automake needlessly regenerating files
because it compares timestamps as "older than or equal to" rather
than only "older than".  This is perfectly reasonable as we have
no way of knowing what file is older if they have the same value
which means we must be pessimistic & assume an upate is required.

However, given modern systems that are quite fast and can easily
generate many files in less than an second, we end up doing a lot
of extra work.

Until Perl gets around to figuring out how to support subsecond
timestamp resolution, optionally import the Time::HiRes module and
use its stat function instead.  If it's available, great, if not,
then we're no worse off than we are today.

Performance-wise, at least by using the testsuite, there doesn't
seem to be any measurable difference.

* lib/Automake/ Use Time::HiRes to lookup mtimes on
files if available.
 lib/Automake/ | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/lib/Automake/ b/lib/Automake/
index 848ff22d1761..78e0942e9f53 100644
--- a/lib/Automake/
+++ b/lib/Automake/
@@ -42,6 +42,11 @@ use Exporter;
 use File::stat;
 use IO::File;
+# Perl's builtin stat does not provide sub-second resolution. Use Time::HiRes
+# if it's available instead.  Hopefully one day perl will update.
+my $have_time_hires = eval { require Time::HiRes; };
 use Automake::Channels;
 use Automake::ChannelDefs;
@@ -115,10 +120,18 @@ sub mtime ($)
   return 0
     if $file eq '-' || ! -f $file;
- my $stat = stat ($file)
-    or fatal "cannot stat $file: $!";
-  return $stat->mtime;
+  if ($have_time_hires)
+    {
+      my @stat = Time::HiRes::stat ($file)
+       or fatal "cannot stat $file: $!";
+      return $stat[9];
+    }
+  else
+    {
+      my $stat = stat ($file)
+       or fatal "cannot stat $file: $!";
+      return $stat->mtime;
+    }

If you change that variable to a constant, you can eliminate the runtime overhead entirely, since Perl optimizes if(1) and if(0) and folds constants at compile time.

Something like:

   use constant HAVE_Time_HiRes => eval { require Time::HiRes; };


   if (HAVE_Time_HiRes)

If you do this, Perl will inline the block actually used and elide the branch at runtime. This is generally useful for any test that can only go one way in a specific run of the program.

-- Jacob

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