2012-04-13 Tom de Vries <tom@codesourcery.com>

[pf3gnuchains/gcc-fork.git] / gcc / README.Portability

diff --git a/gcc/README.Portability b/gcc/README.Portability
index 2724f2a..4101a2f 100644 (file)
--- a/gcc/README.Portability
+++ b/gcc/README.Portability
@@ -10,7 +10,7 @@ This knowledge until know has been sparsely spread around, so I
 thought I'd collect it in one useful place.  Please add and correct
 any problems as you come across them.
 
 thought I'd collect it in one useful place.  Please add and correct
 any problems as you come across them.
 

-I'm going to start from a base of the ISO C89 standard, since that is
+I'm going to start from a base of the ISO C90 standard, since that is

 probably what most people code to naturally.  Obviously using
 constructs introduced after that is not a good idea.
 
 probably what most people code to naturally.  Obviously using
 constructs introduced after that is not a good idea.
 


@@ -51,14 +51,28 @@ foo (bar, )
 needs to be coded in some other way.
 
 
 needs to be coded in some other way.
 
 

-free and realloc
-----------------
+Avoid unnecessary test before free
+----------------------------------

 
 

-Some implementations crash upon attempts to free or realloc the null
-pointer.  Thus if mem might be null, you need to write
+Since SunOS 4 stopped being a reasonable portability target,
+(which happened around 2007) there has been no need to guard
+against "free (NULL)".  Thus, any guard like the following
+constitutes a redundant test:
+
+  if (P)
+    free (P);
+
+It is better to avoid the test.[*]
+Instead, simply free P, regardless of whether it is NULL.
+
+[*] However, if your profiling exposes a test like this in a
+performance-critical loop, say where P is nearly always NULL, and
+the cost of calling free on a NULL pointer would be prohibitively
+high, consider using __builtin_expect, e.g., like this:
+
+  if (__builtin_expect (ptr != NULL, 0))
+    free (ptr);

 
 

-  if (mem)
-    free (mem);

 
 
 Trigraphs
 
 
 Trigraphs


@@ -194,4 +208,3 @@ o Passing incorrect types to fprintf and friends.
 
 o Adding a function declaration for a module declared in another file to
   a .c file instead of to a .h file.
 
 o Adding a function declaration for a module declared in another file to
   a .c file instead of to a .h file.

-