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Re: malloc attributes and realloc


On Fri, 2 Jan 2004, Daniel Berlin wrote:

> 1. heap allocation functions  return pointers that don't point to 
> anything.
> 2. pointer destroying operations destroy pointers irreversibly.
> 
> If #1 does not hold for realloc, it should not be attribute malloc.  
> The fact that it has worked okay so far doesn't mean it will continue 
> to work in the future.  If you change the definition of attribute 
> malloc so that #1 doesn't hold for them, it makes it useless for 
> points-to analysis use in determining heap allocation functions.  Which 
> may or may not be okay by everyone, i don't know.

My current draft documentation change for attribute malloc (for this issue
and PR 3414) is

 The @code{malloc} attribute is used to tell the compiler that a function
-may be treated as if it were the malloc function.  The compiler assumes
-that calls to malloc result in pointers that cannot alias anything.
+may be treated as if any non-@code{NULL} pointer it returns cannot
+alias any other pointer valid when the function returns.
 This will often improve optimization.
+Standard functions with this property include @code{malloc},
+@code{calloc} and @code{realloc}.

Does this description (non-NULL pointers don't alias anything valid when
the allocation function returns) accurately describe what is appropriate
for the points-to analysis, or not?  (And quite what does the malloc
attribute do at present, given the lack of points-to analysis in GCC?)

-- 
Joseph S. Myers
jsm@polyomino.org.uk


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