[Bug c++/36685] clarify/diagnose use of weak constant

[Bug c++/36685] clarify/diagnose use of weak constant

[foom at fuhm dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36685

James Y Knight <foom at fuhm dot net> changed:

           What    |Removed                     |Added
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                 CC|                            |foom at fuhm dot net

--- Comment #4 from James Y Knight <foom at fuhm dot net> ---
The interaction of __attribute__((weak)) and const int was recently explored in
Clang in this thread:
https://discourse.llvm.org/t/weak-attribute-semantics-on-const-variables/62311

Strangely, it appears that Clang and GCC have opposite odd behavior currently:
GCC appears to evaluate weak constants _only_ in constant evaluation (so long
as

[Bug c++/36685] clarify/diagnose use of weak constant

[foom at fuhm dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36685

--- Comment #5 from James Y Knight <foom at fuhm dot net> ---
(oops submitted commit by mistake, continuing...)

Strangely, it appears that Clang and GCC both have odd behaviors currently, but
somewhat opposites:

- GCC appears to evaluate weak constants _only_ in frontend constant
evaluation. It does not treat them as known during backend optimizations.
(However, GCC does not allow you to use such a variable in a context that is
required to be constant by language rules).

- Clang evaluates weak constants _only_ in backend optimizations, and never in
frontend constant evaluation.


Clang will be changing to stop optimizing based on weak constant value, so that
they are consistently treated as unknown values.

I'd suggest that GCC probably ought to do the same (by no longer constant
evaluating such variables).

[Bug c++/36685] New: clarify/diagnose use of weak constant

[algrant at acm dot org]

Consider this:

  __attribute__((weak)) extern int const K = 7;
  int f(void) { return K; }
  int a[K];

Assuming the combination of 'weak' and 'const' is legitimate
(and it seems useful enough to allow), the compiler should avoid
the constant-promotion optimization when generating f(), and
should fault a[].


-- 
           Summary: clarify/diagnose use of weak constant
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: algrant at acm dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36685

[Bug c++/36685] clarify/diagnose use of weak constant

[algrant at acm dot org]

------- Comment #1 from algrant at acm dot org  2008-07-01 10:21 -------
Similar argument applies to __attribute__((visibility("default"))) .


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36685

[Bug c++/36685] clarify/diagnose use of weak constant

[joseph at codesourcery dot com]

------- Comment #2 from joseph at codesourcery dot com  2008-07-01 11:35 -------
Subject: Re:   New: clarify/diagnose use of weak constant

See the inconclusive discussion at 
<http://gcc.gnu.org/ml/gcc-patches/2008-05/subjects.html#00451> (the 
original message was the previous month), and bug 35501.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36685