[PATCH] ftw.c: Use unlikely for unlikely taken branches such that taken ones are placed closer

[PATCH] ftw.c: Use unlikely for unlikely taken branches such that taken ones are placed closer

[James Tirta Halim]

---
 io/ftw.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/io/ftw.c b/io/ftw.c
index a72c7d5171..94fb2842d2 100644
--- a/io/ftw.c
+++ b/io/ftw.c
@@ -209,7 +209,7 @@ ftw_allocate (struct ftw_data *data, size_t newsize)
   void *newp = realloc (data->dirstreams, data->maxdir
 					  * sizeof (struct dir_data *)
 					  + newsize);
-  if (newp == NULL)
+  if (__glibc_unlikely(newp == NULL))
     return false;
   data->dirstreams = newp;
   data->dirbufsize = newsize;
@@ -255,7 +255,7 @@ static int
 add_object (struct ftw_data *data, struct STRUCT_STAT *st)
 {
   struct known_object *newp = malloc (sizeof (struct known_object));
-  if (newp == NULL)
+  if (__glibc_unlikely(newp == NULL))
     return -1;
   newp->dev = st->st_dev;
   newp->ino = st->st_ino;
@@ -287,7 +287,7 @@ open_dir_stream (int *dfdp, struct ftw_data *data, struct dir_data *dirp)
       size_t bufsize = 1024;
       char *buf = malloc (bufsize);
 
-      if (buf == NULL)
+      if (__glibc_unlikely(buf == NULL))
 	result = -1;
       else
 	{
@@ -298,12 +298,12 @@ open_dir_stream (int *dfdp, struct ftw_data *data, struct dir_data *dirp)
 	  while ((d = __readdir64 (st)) != NULL)
 	    {
 	      size_t this_len = NAMLEN (d);
-	      if (actsize + this_len + 2 >= bufsize)
+	      if (glibc_unlikely(actsize + this_len + 2 >= bufsize))
 		{
 		  char *newp;
 		  bufsize += MAX (1024, 2 * this_len);
 		  newp = (char *) realloc (buf, bufsize);
-		  if (newp == NULL)
+		  if (__glibc_unlikely(newp == NULL))
 		    {
 		      /* No more memory.  */
 		      int save_err = errno;
@@ -325,7 +325,7 @@ open_dir_stream (int *dfdp, struct ftw_data *data, struct dir_data *dirp)
 	  /* Shrink the buffer to what we actually need.  */
 	  void *content = realloc (buf, actsize);
 	  data->dirstreams[data->actdir]->content = content;
-	  if (content == NULL)
+	  if (__glibc_unlikely(content == NULL))
 	    {
 	      int save_err = errno;
 	      free (buf);
-- 
2.42.0

Re: [PATCH] ftw.c: Use unlikely for unlikely taken branches such that taken ones are placed closer

[Xi Ruoyao]

On Tue, 2023-10-03 at 10:40 +0700, James Tirta Halim wrote:
> ---
>  io/ftw.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/io/ftw.c b/io/ftw.c
> index a72c7d5171..94fb2842d2 100644
> --- a/io/ftw.c
> +++ b/io/ftw.c
> @@ -209,7 +209,7 @@ ftw_allocate (struct ftw_data *data, size_t newsize)
>    void *newp = realloc (data->dirstreams, data->maxdir
>  					  * sizeof (struct dir_data *)
>  					  + newsize);
> -  if (newp == NULL)
> +  if (__glibc_unlikely(newp == NULL))

AFAIK the compiler should automatically mark malloc and realloc failures
as unlikely.  On x86_64 GCC 7 predicts the probability of such a failure
1.74%, and GCC 13 predicts it 0.04%.

/* snip */

> @@ -298,12 +298,12 @@ open_dir_stream (int *dfdp, struct ftw_data *data, struct dir_data *dirp)
>  	  while ((d = __readdir64 (st)) != NULL)
>  	    {
>  	      size_t this_len = NAMLEN (d);
> -	      if (actsize + this_len + 2 >= bufsize)
> +	      if (glibc_unlikely(actsize + this_len + 2 >= bufsize))

It seems only this one might be mispredicted by GCC.  But there should
be a white space between "glibc_unlikely" and "(".  And should it be
"__glibc_unlikely" (what's the difference between __glibc_unlikely and
glibc_unlikely)?


-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University

Re: [PATCH] ftw.c: Use unlikely for unlikely taken branches such that taken ones are placed closer

[Andreas Schwab]

On Okt 03 2023, Xi Ruoyao wrote:

> (what's the difference between __glibc_unlikely and glibc_unlikely)?

The difference is that the latter does not exist.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

[PATCH] ftw.c: Use unlikely for unlikely taken branches such that taken ones are placed closer

[Wilco Dijkstra]

Hi,

> @@ -298,12 +298,12 @@ open_dir_stream (int *dfdp, struct ftw_data *data, struct dir_data *dirp)
>  	  while ((d = __readdir64 (st)) != NULL)
>  	    {
>  	      size_t this_len = NAMLEN (d);
> -	      if (actsize + this_len + 2 >= bufsize)
> +	      if (glibc_unlikely(actsize + this_len + 2 >= bufsize))

> It seems only this one might be mispredicted by GCC.  But there should
> be a white space between "glibc_unlikely" and "(".  And should it be
> "__glibc_unlikely"

If GCC's static predictor gets this case wrong, it should be reported as a bug.
An if-statement containing a call should get lower probability in a loop -
the call to realloc implies a rare case to increase buffer space.

Note we should use __glibc_unlikely only in performance critical code, not in
code like this.

Cheers,
Wilco