newlib: fix fseek optimization with SEEK_CUR

Message ID 20191109162804.1905160-1-bastien.bouclet@gmail.com
State Accepted
Commit 59362c80e3a02c011fd0ef3d7f07a20098d2a9d5
Headers show
Series
  • newlib: fix fseek optimization with SEEK_CUR
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Commit Message

Bastien Bouclet Nov. 9, 2019, 4:28 p.m.
The call to fflush was invalidating the read buffer, preventing relative
seeks to positions that would have been inside the read buffer from
being optimized. The call to srefill would then re-read mostly the same
data that was initially in the read buffer.
---
 newlib/libc/stdio/fseeko.c     | 31 ++++++-------------------------
 newlib/libc/stdio64/fseeko64.c | 31 ++++++-------------------------
 2 files changed, 12 insertions(+), 50 deletions(-)

-- 
2.24.0

Comments

Corinna Vinschen Nov. 13, 2019, 10:15 a.m. | #1
Hi Bastien,

On Nov  9 17:28, Bastien Bouclet wrote:
> The call to fflush was invalidating the read buffer, preventing relative

> seeks to positions that would have been inside the read buffer from

> being optimized. The call to srefill would then re-read mostly the same

> data that was initially in the read buffer.


I checked this against upstream BSD versions.  OpenBSD and NetBSD
operate like our code, including the flush, while FreeBSD uses its
internal ftello and never flushed since the repository import back in
1994.

I'm pretty unsure if we can do this.  Apparently the flush op is only
necessary for streams in append mode.  If at all.

Can we be sure this works as desired on append streams as well?

Also, given that this is changing very basic code, nobody is unaffected.
Any input from other folks?


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer
Red Hat
Corinna Vinschen Nov. 18, 2019, 10:11 a.m. | #2
On Nov  9 17:28, Bastien Bouclet wrote:
> The call to fflush was invalidating the read buffer, preventing relative

> seeks to positions that would have been inside the read buffer from

> being optimized. The call to srefill would then re-read mostly the same

> data that was initially in the read buffer.

> ---

>  newlib/libc/stdio/fseeko.c     | 31 ++++++-------------------------

>  newlib/libc/stdio64/fseeko64.c | 31 ++++++-------------------------

>  2 files changed, 12 insertions(+), 50 deletions(-)


Pushed.


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer
Red Hat

Patch

diff --git a/newlib/libc/stdio/fseeko.c b/newlib/libc/stdio/fseeko.c
index 3e0f9e90b..bbf1af43e 100644
--- a/newlib/libc/stdio/fseeko.c
+++ b/newlib/libc/stdio/fseeko.c
@@ -141,31 +141,12 @@  _fseeko_r (struct _reent *ptr,
   switch (whence)
     {
     case SEEK_CUR:
-      /*
-       * In order to seek relative to the current stream offset,
-       * we have to first find the current stream offset a la
-       * ftell (see ftell for details).
-       */
-      _fflush_r (ptr, fp);   /* may adjust seek offset on append stream */
-      if (fp->_flags & __SOFF)
-	curoff = fp->_offset;
-      else
-	{
-	  curoff = seekfn (ptr, fp->_cookie, (_fpos_t) 0, SEEK_CUR);
-	  if (curoff == -1L)
-	    {
-	      _newlib_flockfile_exit (fp);
-	      return EOF;
-	    }
-	}
-      if (fp->_flags & __SRD)
-	{
-	  curoff -= fp->_r;
-	  if (HASUB (fp))
-	    curoff -= fp->_ur;
-	}
-      else if (fp->_flags & __SWR && fp->_p != NULL)
-	curoff += fp->_p - fp->_bf._base;
+      curoff = _ftello_r(ptr, fp);
+      if (curoff == -1L)
+        {
+          _newlib_flockfile_exit (fp);
+          return EOF;
+        }
 
       offset += curoff;
       whence = SEEK_SET;
diff --git a/newlib/libc/stdio64/fseeko64.c b/newlib/libc/stdio64/fseeko64.c
index 0672086a3..f38005570 100644
--- a/newlib/libc/stdio64/fseeko64.c
+++ b/newlib/libc/stdio64/fseeko64.c
@@ -142,31 +142,12 @@  _fseeko64_r (struct _reent *ptr,
   switch (whence)
     {
     case SEEK_CUR:
-      /*
-       * In order to seek relative to the current stream offset,
-       * we have to first find the current stream offset a la
-       * ftell (see ftell for details).
-       */
-      _fflush_r (ptr, fp);   /* may adjust seek offset on append stream */
-      if (fp->_flags & __SOFF)
-	curoff = fp->_offset;
-      else
-	{
-	  curoff = seekfn (ptr, fp->_cookie, (_fpos64_t) 0, SEEK_CUR);
-	  if (curoff == -1L)
-	    {
-	      _newlib_flockfile_exit(fp);
-	      return EOF;
-	    }
-	}
-      if (fp->_flags & __SRD)
-	{
-	  curoff -= fp->_r;
-	  if (HASUB (fp))
-	    curoff -= fp->_ur;
-	}
-      else if (fp->_flags & __SWR && fp->_p != NULL)
-	curoff += fp->_p - fp->_bf._base;
+      curoff = _ftello64_r(ptr, fp);
+      if (curoff == -1L)
+        {
+          _newlib_flockfile_exit (fp);
+          return EOF;
+        }
 
       offset += curoff;
       whence = SEEK_SET;