Richard Guenther wrote:
On Mon, Jan 12, 2009 at 12:03 AM, Raoul Gough <raoulgo...@yahoo.co.uk> wrote:
Richard Guenther wrote:
[snip]
This is also invalid. You have to use placement new to change the dynamic
type of memory.
Yes, I guess that makes sense. I've modified the example like this:
#include <new>
void** global_free_list = 0;
inline void operator delete(void* p2) throw()
{
// Save "next" pointer in re-used client storage
new (p2) (void *) (global_free_list);
global_free_list = static_cast<void **>(p2);
}
double foo(double* p1)
{
double result = *p1;
delete p1;
return result;
}
[snip]
So how is this example looking now? Does the alias analysis mean that g++
will never reorder the read and write via p1 and p2?
Yes, as that would now be an invalid thing to do. Note that for C
there is no way to do "placement new", but the memory model of C
only has static typing, not the notion of a dynamic type. Which is
why some people (including me) say you cannot do a C conforming
implementation of malloc that ever re-uses memory.
I don't really see which part of the alias analysis shows that g++ won't
reorder the accesses via the double* and the void**. Is it because the
intersection of the "may aliases" lists is non-empty?
----8<-------- start
Symbol memory tags
SMT.7, UID D.1868, void *, is addressable, is global, score: 556356,
direct reads: 0, direct writes: 0, indirect reads: 0, indirect writes:
1, call clobbered (stored in global, is global var), may aliases: {
global_free_list }
SMT.8, UID D.1869, double, is addressable, is global, score: 320002,
direct reads: 0, direct writes: 0, indirect reads: 1, indirect writes:
0, call clobbered (is global var, is incoming pointer), may aliases: {
global_free_list }
[...]
Name memory tags
NMT.9, UID D.1870, double, is addressable, is global, score: 8, direct
reads: 1, direct writes: 0, indirect reads: 0, indirect writes: 0, call
clobbered (is global var, is incoming pointer), may aliases: {
global_free_list SMT.8 }
NMT.10, UID D.1871, void *, is addressable, is global, score: 16, direct
reads: 0, direct writes: 1, indirect reads: 0, indirect writes: 0, call
clobbered (stored in global, is global var), may aliases: {
global_free_list SMT.7 }
----8<-------- end
By the way, if I change the example very slightly, so it uses char* for
p1, the aliasing information is somewhat different. In particular, it
includes the void* in the may-aliases list for the char.
double bar(char* p1)
{
double result = *p1;
delete p1;
return result;
}
Alias information for double bar(char*)
[...]
Symbol memory tags
SMT.22, UID D.1898, void *, is addressable, is global, score: 876358,
direct reads: 0, direct writes: 0, indirect reads: 1, indirect writes:
1, call clobbered (stored in global, is global var, is incoming
pointer), may aliases: { global_free_list }
SMT.23, UID D.1899, char, is addressable, is global, score: 320002,
direct reads: 0, direct writes: 0, indirect reads: 1, indirect writes:
0, call clobbered (is global var, is incoming pointer), may aliases: {
global_free_list SMT.22 }
[...]
Name memory tags
NMT.24, UID D.1900, char, is addressable, is global, score: 8, direct
reads: 1, direct writes: 0, indirect reads: 0, indirect writes: 0, call
clobbered (is global var, is incoming pointer), may aliases: {
global_free_list SMT.22 SMT.23 }
NMT.25, UID D.1901, void *, is addressable, is global, score: 16, direct
reads: 0, direct writes: 1, indirect reads: 0, indirect writes: 0, call
clobbered (stored in global, is global var), may aliases: {
global_free_list SMT.22 }
So I guess I'm just trying to understand how the alias data structures
are used. In the one case (with a char*) it's pretty easy I guess, but I
don't really understand how it represents the potential aliasing in the
void** versus double* example.
--
Thanks,
Raoul Gough.
