Re: malloc attributes and realloc

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

Daniel Jacobowitz <drow@mvista.com> writes:

| On Sat, Jan 03, 2004 at 05:40:00AM +0100, Gabriel Dos Reis wrote:
| > Robert Dewar <dewar@gnat.com> writes:
| > 
| > | Gabriel Dos Reis wrote:
| > | 
| > | > | can be safely optimized into
| > | > |   *p = 'a';
| > | > That transformation caanot be applied for pointers returned by
| > | > realloc, in general.
| > | 
| > | Can you explain why?
| > 
| > The reason is obvious.
| > 
| > If p is a non-NULL pointer you get from a realloc, then it is
| > guaranteed that the content of object previously pointed-to is still
| > present at the start of new object.  Therefore, the compiler cannot,
| > in general, replace the test *p != 'a' by true.
| 
| Gaby, you may have missed the original example:
|   if (*p != 'a')
|     *p == 'a';
| 
| Either it's already 'a' or it gets set to 'a'.  Depending on cache
| behaviour and other issues already raised, this is a code-size win and
| can be a performance win.  The only reason not to transform it is if
| the object is in read-only storage, which either C or most useful C
| compilers (I don't know which) allows.

Aha, thanks.  You're indeed correct.

-- Gaby