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Re: malloc attributes and realloc


Joseph S. Myers writes:
> 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}.

This isn't directly relevant to the current thread, but another aspect of
malloc-like functions came up recently: the memory they return is writable.
I.e., if char *p was returned by an attribute(malloc) function, then
  if (*p != 'a')
    *p = 'a';

can be safely optimized into
  *p = 'a';

GCC doesn't currently do this optimization for malloc-returned pointers (it
does for pointers to objects on the stack), but it could.  Would it be worth
adding to the documentation?

-- 
Jonathan Lennox
lennox at cs dot columbia dot edu


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