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/FileUtils.pm: Use Time::HiRes to lookup mtimes on files if available. --- lib/Automake/FileUtils.pm | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-) diff --git a/lib/Automake/FileUtils.pm b/lib/Automake/FileUtils.pm index 848ff22d1761..78e0942e9f53 100644 --- a/lib/Automake/FileUtils.pm +++ b/lib/Automake/FileUtils.pm @@ -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. +# https://github.com/Perl/perl5/issues/17900 +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; + } } -- 2.39.0