[PATCH] Speed up sprintf

[PATCH] Speed up sprintf

[Jakub Jelinek]

Hi!

My http://sources.redhat.com/ml/libc-hacker/2003-05/msg00032.html
change apparently slowed down sprintf quite a lot on longer strings, if
buffer passed to sprintf is located in the first half of address space.

The problem is that the fix from last year for sprintf now sets write_end
to ~0L and if write_ptr is small enough, (ssize_t) (write_end - write_ptr)
is negative.  This results in overflow being called for every single
character instead of doing a mempcpy.
The following patch seems to fix it, now sprintf at least on the sample
testcase I have on IA-64 is 30% faster than sprintf in glibc 2.2.4
(as opposed to being almost 4 times slower).

2004-05-13  Jakub Jelinek  <jakub@redhat.com>

	* libio/genops.c (_IO_default_xsputn): Avoid one overflow per char if
	count is negative, yet write_ptr < write_end.
	(_IO_default_xsgetn): Avoid one underflow per char if count is
	negative, yet read_ptr < read_end.

--- libc/libio/genops.c.jj	2004-02-12 11:52:09.000000000 +0100
+++ libc/libio/genops.c	2004-05-13 13:02:15.715861960 +0200
@@ -447,10 +447,10 @@ _IO_default_xsputn (f, data, n)
   for (;;)
     {
       /* Space available. */
-      _IO_ssize_t count = f->_IO_write_end - f->_IO_write_ptr;
-      if (count > 0)
+      if (f->_IO_write_ptr < f->_IO_write_end)
 	{
-	  if ((_IO_size_t) count > more)
+	  _IO_size_t count = f->_IO_write_end - f->_IO_write_ptr;
+	  if (count > more)
 	    count = more;
 	  if (count > 20)
 	    {
@@ -462,9 +462,7 @@ _IO_default_xsputn (f, data, n)
 #endif
 	      s += count;
             }
-	  else if (count <= 0)
-	    count = 0;
-	  else
+	  else if (count)
 	    {
 	      char *p = f->_IO_write_ptr;
 	      _IO_ssize_t i;
@@ -504,10 +502,10 @@ _IO_default_xsgetn (fp, data, n)
   for (;;)
     {
       /* Data available. */
-      _IO_ssize_t count = fp->_IO_read_end - fp->_IO_read_ptr;
-      if (count > 0)
+      if (fp->_IO_read_ptr < fp->_IO_read_end)
 	{
-	  if ((_IO_size_t) count > more)
+	  _IO_size_t count = fp->_IO_read_end - fp->_IO_read_ptr;
+	  if (count > more)
 	    count = more;
 	  if (count > 20)
 	    {
@@ -519,9 +517,7 @@ _IO_default_xsgetn (fp, data, n)
 #endif
 	      fp->_IO_read_ptr += count;
 	    }
-	  else if (count <= 0)
-	    count = 0;
-	  else
+	  else if (count)
 	    {
 	      char *p = fp->_IO_read_ptr;
 	      int i = (int) count;

	Jakub

Re: [PATCH] Speed up sprintf

[Ulrich Drepper]

Applied.

-- 
➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖