--- Additional Comments From hugh at mimosa dot com 2005-07-17 08:04
---
There is a thread on the gcc at gcc.gnu.org mailing list discussing this. A
possible fix to GCC4.x has been posted in that thread:
http://gcc.gnu.org/ml/gcc/2005-07/msg00699.html
--
--
What|Removed |Added
AssignedTo|unassigned at gcc dot gnu |rth at gcc dot gnu dot org
|dot org |
Status|UNCONFIRMED
--- Additional Comments From hugh at mimosa dot com 2005-07-16 16:18
---
Here is some C Lawyering from Henry Spencer. I asked him to look at and comment
on this bug. With his permission, I'm quoting his response here.
There is little room for compiler writers to maneuver here, unless
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-16 16:35 ---
Subject: Re: gcc -O2 discards cast to volatile
hugh at mimosa dot com [EMAIL PROTECTED] writes:
[...]
| If GCC (a) wants to be C99-conforming, and (b) wants to provide useful
| semantics for
neroden at gcc dot gnu dot org [EMAIL PROTECTED] writes:
[...]
| of an arbitrary memory address. If the underlying object has
| to be volatile, how do we make it volatile? Can we simply create a new
| underlying object by throwing in a temporary pointer variable? Is
something
| like
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-15 07:51 ---
Subject: Re: gcc -O2 discards cast to volatile
neroden at gcc dot gnu dot org [EMAIL PROTECTED] writes:
[...]
| of an arbitrary memory address. If the underlying object has
| to be volatile,
--- Additional Comments From neroden at gcc dot gnu dot org 2005-07-15
05:50 ---
OK, so in X.org, the key area where this is hit is the definition of
MMIO_IN8 in compiler.h. For alpha, sparc, and powerpc, inline volatile
ASM is used instead (much better in some ways).
--
What|Removed |Added
CC||james at juranfamily dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22278
--- Additional Comments From gcc2eran at tromer dot org 2005-07-03 05:09
---
(In reply to comment #32)
| I meant that we were mostly concerned about what the standard says about the
| effect of casting (say) int* into volatile int*, but the other directly is
| simply undefined.
gdr at integrable-solutions dot net [EMAIL PROTECTED] writes:
| | Still, consider the following variant:
| |
| | void quux(int *bar) {
| | *(volatile int*)bar = 42;
| | }
| |
| | volatile int foo;
| | quux((int*)foo);
| |
| | This time there is no attempt [...] to refer to an
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-03 05:11 ---
Subject: Re: gcc -O2 discards cast to volatile
gdr at integrable-solutions dot net [EMAIL PROTECTED] writes:
| | Still, consider the following variant:
| |
| | void quux(int *bar) {
| |
--- Additional Comments From gcc2eran at tromer dot org 2005-07-03 06:59
---
(In reply to comment #34)
OK, that was just a demonstration of of the problem you pointed out in comments
9 and 12 and example (a) of comment 16. In this case, your analysis (be
conservative) and my reading of
--- Additional Comments From schlie at comcast dot net 2005-07-03 07:09
---
(In reply to comment #35)
Subject: Re: gcc -O2 discards cast to volatile
I apologize for interjecting, but wanted to verify my perception the
conclusions reached; are they essentially?:
- although an object
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-03 07:27 ---
Subject: Re: gcc -O2 discards cast to volatile
schlie at comcast dot net [EMAIL PROTECTED] writes:
| (In reply to comment #35)
| Subject: Re: gcc -O2 discards cast to volatile
|
| I apologize for
--- Additional Comments From schlie at comcast dot net 2005-07-03 07:54
---
(In reply to comment #39)
Subject: Re: gcc -O2 discards cast to volatile
So unfortunately, again the question which comes to mind is what
should happen when a program specifies an undefined behavior?
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-07-02
16:48 ---
See PR 21568 and http://gcc.gnu.org/ml/gcc/2005-05/msg00085.html.
This is allowed by the C and C++ standards.
*** This bug has been marked as a duplicate of 21568 ***
--
What|Removed
--- Additional Comments From olivier dot baudron at m4x dot org 2005-07-02
16:59 ---
I am not sure this is the same problem.
Besides, the following code is *not* optimized away with -O2:
void test (volatile char *addr) {
*addr;
}
--
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-07-02
17:03 ---
Did you read the message which I referenced? it is still not a bug if you read
the message.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22278
--- Additional Comments From falk at debian dot org 2005-07-02 17:33
---
I see a difference here in that gcc doesn't know whether the referenced
object is declared to be volatile, it isn't visible as in PR 21568.
IMHO we actually have a bug here; I don't see anything in the standard
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-07-02
17:42 ---
Since the orginal pointer is not violatile the cast will not change any thing
since the compiler can
deduce it is not violatile.
From C99, 5.1.2.3 P3:
In the abstract machine, all expressions are
--- Additional Comments From falk at debian dot org 2005-07-02 17:53
---
(In reply to comment #5)
Since the orginal pointer is not violatile the cast will not change any thing
since the compiler can
deduce it is not violatile.
It could deduce that if it was invalid to form a
--- Additional Comments From gcc2eran at tromer dot org 2005-07-02 22:07
---
Prior versions of gcc did respect casts to pointer-to-volatile and did not
optimize away the access. I've seen a lot of code that relies on that, and which
would thus be broken by gcc 4.x (in a subtle and
pinskia at gcc dot gnu dot org [EMAIL PROTECTED] writes:
| See PR 21568 and http://gcc.gnu.org/ml/gcc/2005-05/msg00085.html.
Both those issues about completely *different* from the issue
submitted in this PR. In the above cases, the accessed object actually
is NOT volatile. This is not the
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-02 22:08 ---
Subject: Re: gcc -O2 discards cast to volatile
pinskia at gcc dot gnu dot org [EMAIL PROTECTED] writes:
| See PR 21568 and http://gcc.gnu.org/ml/gcc/2005-05/msg00085.html.
Both those issues about
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-02 22:10 ---
Subject: Re: gcc -O2 discards cast to volatile
pinskia at gcc dot gnu dot org [EMAIL PROTECTED] writes:
| Did you read the message which I referenced?
Not just because people disagrees with your
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-02 22:10 ---
Subject: Re: gcc -O2 discards cast to volatile
falk at debian dot org [EMAIL PROTECTED] writes:
| I see a difference here in that gcc doesn't know whether the referenced
| object is declared to be
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-02 22:11 ---
Subject: Re: gcc -O2 discards cast to volatile
pinskia at gcc dot gnu dot org [EMAIL PROTECTED] writes:
| Since the orginal pointer is not violatile
How do you know that? Just looking at function
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-02 22:12 ---
Subject: Re: gcc -O2 discards cast to volatile
falk at debian dot org [EMAIL PROTECTED] writes:
| According to Joseph Myers, the question is whether this counts as access to
| a volatile object,
--- Additional Comments From falk at debian dot org 2005-07-02 22:19
---
(In reply to comment #8)
I can't imagine a programmer casting into a pointer to volatile without really
meaning it, so if the behavior is not defined by the standard then both
compatibility and the principle of
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-07-02
22:20 ---
(In reply to comment #13)
It is also wrong-code.
This is the opposite view of what JSM said on IRC. Take it up with him if you
believe otherwise.
Basically this is all implemention defined and since
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-02 22:20 ---
Subject: Re: gcc -O2 discards cast to volatile
gcc2eran at tromer dot org [EMAIL PROTECTED] writes:
| Prior versions of gcc did respect casts to pointer-to-volatile and did not
| optimize away the
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-02 22:39 ---
Subject: Re: gcc -O2 discards cast to volatile
pinskia at gcc dot gnu dot org [EMAIL PROTECTED] writes:
| (In reply to comment #13)
|
| It is also wrong-code.
|
| This is the opposite view of
--- Additional Comments From pinskia at gcc dot gnu dot org 2005-07-02
22:49 ---
For all useful purposes, it helps remembering that GCC is not an
academic exercise in C standard reading.
Ok, then all of your comments about extensions to C++ should be thrown out the
window since I
--- Additional Comments From gcc2eran at tromer dot org 2005-07-02 23:30
---
(In reply to comment #17)
Furthermore, the fundamental issue is whether this
*(volatile int*) (int*) (foo);
(or equivalent) where foo has been defined volatile int should be
treated differently from
--- Additional Comments From falk at debian dot org 2005-07-02 23:45
---
(In reply to comment #19)
How about this?
int foo;
*(volatile int*) (foo);
In other words, why should the compiler bother at all with the qualifiers of
what the pointer really points to?
Because
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-02 23:54 ---
Subject: Re: gcc -O2 discards cast to volatile
pinskia at gcc dot gnu dot org [EMAIL PROTECTED] writes:
| For all useful purposes, it helps remembering that GCC is not an
| academic exercise in C
gcc2eran at tromer dot org [EMAIL PROTECTED] writes:
| --- Additional Comments From gcc2eran at tromer dot org 2005-07-02 23:30
---
| (In reply to comment #17)
| Furthermore, the fundamental issue is whether this
|
| *(volatile int*) (int*) (foo);
|
| (or equivalent) where foo
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-03 00:03 ---
Subject: Re: gcc -O2 discards cast to volatile
gcc2eran at tromer dot org [EMAIL PROTECTED] writes:
| --- Additional Comments From gcc2eran at tromer dot org 2005-07-02 23:30
---
| (In
--- Additional Comments From gcc2eran at tromer dot org 2005-07-03 01:19
---
(In reply to comment #22)
| int foo;
| *(volatile int*) (foo);
It was included in my previous message.
Then it's still eluding me, since your foo (the object?) was volatile.
| In other words,
--- Additional Comments From joseph at codesourcery dot com 2005-07-03
01:27 ---
Subject: Re: gcc -O2 discards cast to volatile
On Sun, 3 Jul 2005, gcc2eran at tromer dot org wrote:
Of course, but please forgive my ignorance -- where does does the standard
prescribe these
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-03 01:42 ---
Subject: Re: gcc -O2 discards cast to volatile
gcc2eran at tromer dot org [EMAIL PROTECTED] writes:
| but that is a fundamental departure from the language semantics.
| Replace volatile with const
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-03 01:43 ---
Subject: Re: gcc -O2 discards cast to volatile
gcc2eran at tromer dot org [EMAIL PROTECTED] writes:
| (In reply to comment #22)
| | int foo;
| | *(volatile int*) (foo);
|
| It was
--- Additional Comments From gcc2eran at tromer dot org 2005-07-03 01:46
---
(In reply to comment #24)
Thanks!
None of the quotes and references in comment 23 changed in N1124.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22278
--- Additional Comments From hugh at mimosa dot com 2005-07-03 01:53
---
[ see https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=162274 ]
I feel that he gcc documentation promises that there will be an access. The
documentation can change, but it is a contract between the
--- Additional Comments From gcc2eran at tromer dot org 2005-07-03 02:30
---
(In reply to comment #25)
If when the compiler sees const T* p, it assumes that *p is
effectively const, then it would miscompile
int foo(int* p, const int* q) {
int r = *q;
*p = r * r;
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-03 02:54 ---
Subject: Re: gcc -O2 discards cast to volatile
gcc2eran at tromer dot org [EMAIL PROTECTED] writes:
| [#5] If an attempt is made to modify an object defined with
| a
--- Additional Comments From gcc2eran at tromer dot org 2005-07-03 04:14
---
(In reply to comment #30)
| OK. Then the volatile-stripping direction can be handled arbitrarily.
I do not understand that comment.
I meant that we were mostly concerned about what the standard says about
gcc2eran at tromer dot org [EMAIL PROTECTED] writes:
| (In reply to comment #30)
| | OK. Then the volatile-stripping direction can be handled arbitrarily.
|
| I do not understand that comment.
|
| I meant that we were mostly concerned about what the standard says about the
| effect of
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-03 04:43 ---
Subject: Re: gcc -O2 discards cast to volatile
gcc2eran at tromer dot org [EMAIL PROTECTED] writes:
| (In reply to comment #30)
| | OK. Then the volatile-stripping direction can be handled
--- Additional Comments From gdr at integrable-solutions dot net
2005-07-03 05:18 ---
Subject: Re: gcc -O2 discards cast to volatile
gcc2eran at tromer dot org [EMAIL PROTECTED] writes:
| --- Additional Comments From gcc2eran at tromer dot org 2005-07-03 05:09
---
| (In
--- Additional Comments From hugh at mimosa dot com 2005-07-03 05:40
---
Simple rule about const and volatile qualifiers (not restrict): the program can
manipulate pointer values with and without their qualifiers. But when the
program accesses an object that is a volatile object (i.e.
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