eml: speed up common LF-only emails

Emails a *nix MTA are typically LF-only, so we don't need the
complexity of the RE engine when a simple index() works.  We
still need to ensure there's no "\r\n\r\n" before the first
"\n\n", but two calls to index() is still faster than a RE
match.

This gives a 2-5% speedup in some informal tests and saves ~30MB
when scanning a 30MB spam message on newer versions of Perl.
I'll have to diagnose why Perl wastes so much memory doing
RE matches on giant strings, though.

1 files changed, 12 insertions, 4 deletions

diff --git a/lib/PublicInbox/Eml.pm b/lib/PublicInbox/Eml.pm
index 80e7c1af..f022516c 100644
--- a/lib/PublicInbox/Eml.pm
+++ b/lib/PublicInbox/Eml.pm
@@ -71,10 +71,18 @@ sub re_memo ($) {
 # compatible with our uses of Email::MIME
 sub new {
         my $ref = ref($_[1]) ? $_[1] : \(my $cpy = $_[1]);
-        if ($$ref =~ /\r?\n(\r?\n)/s) { # likely
-                # This can modify $$ref in-place and to avoid memcpy/memmove
-                # on a potentially large $$ref.  It does need to make a
-                # copy for $hdr, though.  Idea stolen from Email::Simple
+        # substr() can modify the first arg in-place and to avoid
+        # memcpy/memmove on a potentially large scalar.  It does need
+        # to make a copy for $hdr, though.  Idea stolen from Email::Simple.
+
+        # We also prefer index() on common LFLF emails since it's faster
+        # and re scan can bump RSS by length($$ref) on big strings
+        if (index($$ref, "\r\n") < 0 && (my $pos = index($$ref, "\n\n")) >= 0) {
+                # likely on *nix
+                my $hdr = substr($$ref, 0, $pos + 2, ''); # sv_chop on $$ref
+                chop($hdr); # lower SvCUR
+                bless { hdr => \$hdr, crlf => "\n", bdy => $ref }, __PACKAGE__;
+        } elsif ($$ref =~ /\r?\n(\r?\n)/s) {
                 my $hdr = substr($$ref, 0, $+[0], ''); # sv_chop on $$ref
                 substr($hdr, -(length($1))) = ''; # lower SvCUR
                 bless { hdr => \$hdr, crlf => $1, bdy => $ref }, __PACKAGE__;