[PATCH] maint: optimize long-line processing

[Jim Meyering]

FYI, when investigating the big-hole failure,
I noticed that its use of grep took 4.0s of user time and ~7s of system,
but with this change, the user-time component went down to 2.0 seconds
and the 7s stayed the same.

>From 8c74af84958634a22a24dbc7fecf3fafd3f8e4bd Mon Sep 17 00:00:00 2001
From: Jim Meyering <address@hidden>
Date: Mon, 30 Jul 2012 11:39:29 +0200
Subject: [PATCH] maint: optimize long-line processing

* src/main.c (grep): Use memrchr rather than an open-coded loop,
reducing the cost of the replaced code by 50% when processing very
long lines.  If there were a rawmemrchr function (analogous to glibc's
rawmemchr), then the performance improvement would be even greater.
---
 src/main.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/main.c b/src/main.c
index 84066d8..e3b5186 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1200,8 +1200,10 @@ grep (int fd, struct stat const *st)
          the buffer, 0 means there is no incomplete last line).  */
       oldc = beg[-1];
       beg[-1] = eol;
-      for (lim = buflim; lim[-1] != eol; lim--)
-        continue;
+      /* FIXME: use rawmemrchr if/when it exists, since we have ensured
+         that this use of memrchr is guaranteed never to return NULL.  */
+      lim = memrchr (beg - 1, eol, buflim - beg + 1);
+      ++lim;
       beg[-1] = oldc;
       if (lim == beg)
         lim = beg - residue;
--
1.7.12.rc1