scripts: add stackdelta script

This adds a simple perl script for reading two files as produced by
the stackusage script and computing the changes in stack usage. For
example:

$ scripts/stackusage -o /tmp/old.su CC=gcc-4.7 -j8 fs/ext4/
$ scripts/stackusage -o /tmp/new.su CC=gcc-5.0 -j8 fs/ext4/
$ scripts/stackdelta /tmp/{old,new}.su | sort -k5,5g

shows that gcc 5.0 generally produces less stack-hungry code than gcc
4.7. Obviously, the script can also be used for measuring the effect
of commits, .config tweaks or whatnot.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Michal Marek <mmarek@suse.com>

diff --git a/scripts/stackdelta b/scripts/stackdelta
new file mode 100755 (executable)
index 0000000..48eabf2
--- /dev/null
+++ b/scripts/stackdelta
@@ -0,0 +1,59 @@
+#!/usr/bin/perl
+
+# Read two files produced by the stackusage script, and show the
+# delta between them.
+#
+# Currently, only shows changes for functions listed in both files. We
+# could add an option to show also functions which have vanished or
+# appeared (which would often be due to gcc making other inlining
+# decisions).
+#
+# Another possible option would be a minimum absolute value for the
+# delta.
+#
+# A third possibility is for sorting by delta, but that can be
+# achieved by piping to sort -k5,5g.
+
+sub read_stack_usage_file {
+    my %su;
+    my $f = shift;
+    open(my $fh, '<', $f)
+       or die "cannot open $f: $!";
+    while (<$fh>) {
+       chomp;
+       my ($file, $func, $size, $type) = split;
+       # Old versions of gcc (at least 4.7) have an annoying quirk in
+       # that a (static) function whose name has been changed into
+       # for example ext4_find_unwritten_pgoff.isra.11 will show up
+       # in the .su file with a name of just "11". Since such a
+       # numeric suffix is likely to change across different
+       # commits/compilers/.configs or whatever else we're trying to
+       # tweak, we can't really track those functions, so we just
+       # silently skip them.
+       #
+       # Newer gcc (at least 5.0) report the full name, so again,
+       # since the suffix is likely to change, we strip it.
+       next if $func =~ m/^[0-9]+$/;
+       $func =~ s/\..*$//;
+       # Line numbers are likely to change; strip those.
+       $file =~ s/:[0-9]+$//;
+       $su{"${file}\t${func}"} = {size => $size, type => $type};
+    }
+    close($fh);
+    return \%su;
+}
+
+@ARGV == 2
+    or die "usage: $0 <old> <new>";
+
+my $old = read_stack_usage_file($ARGV[0]);
+my $new = read_stack_usage_file($ARGV[1]);
+my @common = sort grep {exists $new->{$_}} keys %$old;
+for (@common) {
+    my $x = $old->{$_}{size};
+    my $y = $new->{$_}{size};
+    my $delta = $y - $x;
+    if ($delta) {
+       printf "%s\t%d\t%d\t%+d\n", $_, $x, $y, $delta;
+    }
+}