https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
Eric Botcazou changed:
What|Removed |Added
CC||ebotcazou at gcc dot gnu.org
--- Comment
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #19 from mimamer at gmail dot com ---
One last comment on strict aliasing rules: It is ironic that these rules are
supposed to make programs faster, but those developers that really care about
speed are prevented from implementing even
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #18 from mimamer at gmail dot com ---
Ah? I didn't figure that was allowed per strict aliasing rules. But it still
doesn't solve one problem:
inline T *dequeue() {
node *head = anchor.next;
anchor.next = head->next;
head->
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #17 from Andrew Pinski ---
(In reply to mimamer from comment #16)
> Ah ok, but that doesn't solve my problem as I will eventually need to cast a
> node pointer back to T (either at the caller or the callee side). So there
> _is_ no ef
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #16 from mimamer at gmail dot com ---
Ah ok, but that doesn't solve my problem as I will eventually need to cast a
node pointer back to T (either at the caller or the callee side). So there _is_
no efficient solution with strict aliasi
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #15 from Andrew Pinski ---
template class list2
{
public:
struct node {
node*next;
node*prev;
};
nodeanchor;
public:
/* API */
}
struct Obj : list2::node {
/* obj-specific element
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #14 from mimamer at gmail dot com ---
Can you explain what you mean by "use type node inside the struct node"?
Because I still cannot see an (efficient) way for solving this problem.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
Andrew Pinski changed:
What|Removed |Added
Status|WAITING |RESOLVED
Resolution|---
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #12 from Andrew Pinski ---
(In reply to mimamer from comment #11)
> In short form,
>
> template class list2
> {
> public:
> struct node {
> T *next;
> T *prev;
> };
>
> nod
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #11 from mimamer at gmail dot com ---
In short form,
template class list2
{
public:
struct node {
T*next;
T*prev;
};
nodeanchor;
public:
/* API */
}
struct Obj : list2::node {
/*
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
Andrew Pinski changed:
What|Removed |Added
Status|UNCONFIRMED |WAITING
Last reconfirmed|
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #9 from mimamer at gmail dot com ---
Now that I have read enough about aliasing rules I realize that they are
considerably fucked up and that their is no way to efficiently downcast without
violating strict aliasing rules. In explanati
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #8 from mimamer at gmail dot com ---
As concerns -fno-strict-aliasing, I have to put it before -O2. Sorry for the
mixup.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #7 from mimamer at gmail dot com ---
Violation of aliasing or not, the problem arises prior to the downcasting from
list2::node* to T*:
T *head = anchor.next;
anchor.next = head->next;
head->next->prev = (T *)&anchor; // w
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #6 from mimamer at gmail dot com ---
Btw, C++ doesn't give me any aliasing warnings even with -Wstrict-aliasing=1.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #5 from mimamer at gmail dot com ---
T is derived from list2::node, and yes, I'm "up"casting from node to T.
There is no other way around it in the implementation, and if this causes a
problem with aliasing then I don't know what it's
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #4 from mimamer at gmail dot com ---
Indeed, this is alias-related. Two questions:
(1) What is a concrete solution for this problem when -fstrict-aliasing is
being used? How should I change my code? (Sorry, I am no expert on this.)
(
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #3 from Andrew Pinski ---
This seems alias related. Can you make sure you are not violating c/c++ alias
rules. Mainly make sure T and typeof(anchor) can alias.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #2 from mimamer at gmail dot com ---
Turns out the problems arise in different places but only in the
list2::dequeue() function:
inline T *dequeue() {
//asm volatile("":::"memory");
T *head = anchor.next;
anchor.next = hea
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61421
--- Comment #1 from mimamer at gmail dot com ---
A minimalistic version that breaks with all -O2 flags set:
Starting from an empty main_list:
Node *node;
//asm volatile("":::"memory");
while ( (node = main_list.dequeue()) != main_list.end() )
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