[Bug c++/54893] New: unable to access volatile variable within relaxed transaction

[Bug c++/54893] New: unable to access volatile variable within relaxed transaction

[spear at cse dot lehigh.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54893

             Bug #: 54893
           Summary: unable to access volatile variable within relaxed
                    transaction
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: spear@cse.lehigh.edu


Attempts to access a volatile variable in gcc or g++ fails.

Sample code:
    #include <stdio.h>
    volatile int test_var = 0;
    int main()
    {
      __transaction_relaxed {
        test_var++;
      }
      printf("Changed variable value to %d\n", test_var);
    }

Compilation:
    g++ test.c -o test -fgnu-tm

Error:
    test.c:6:15: error: invalid volatile use of 'test_var' inside transaction

gcc version info:
    g++ (GCC) 4.7.1
    Copyright (C) 2012 Free Software Foundation, Inc.
    This is free software; see the source for copying conditions.  There is NO
    warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

[Bug c++/54893] unable to access volatile variable within relaxed transaction

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54893

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pmarlier at gcc dot gnu.org

--- Comment #1 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-10-11 10:40:53 UTC ---
Please see bug 46654.  This error was added on purpose.  Has the specification
changed and volatiles are now allowed inside transactions?

[Bug c++/54893] unable to access volatile variable within relaxed transaction

[spear at cse dot lehigh.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54893

--- Comment #2 from Mike Spear <spear at cse dot lehigh.edu> 2012-10-11 13:21:19 UTC ---
There is a key difference here.  The transaction in my example is /relaxed/. 
According to the specification, there are no restrictions on what a relaxed
transaction can do.  Relaxed transactions can call unsafe functions, do I/O,
acquire locks, execute inline asm, etc.  And access to C++11 atomics and C11
volatiles is definitely legal in relaxed transactions.

If I understand the GCC TM implementation correctly, immediately before the
attempt to access a volatile within the relaxed transaction, the transaction
will need to become serial irrevocable, after which point it can continue to
access and directly modify volatile variables.

Note that a workaround for the time being is to move the volatile access into a
function, to *not* mark the function [[transaction_callable]], and then to call
the function instead of accessing the volatile directly.  This will achieve the
exact same behavior as is expected, though with the overhead of a function call
on each volatile access within a relaxed transaction.  However, longer term
this is definitely a bug in the current implementation.

[Bug c++/54893] unable to access volatile variable within relaxed transaction

[torvald at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54893

--- Comment #3 from torvald at gcc dot gnu.org 2012-10-11 19:54:11 UTC ---
I agree with Michael.  Accesses to volative vars are disallowed in safe code,
but relaxed transactions can run unsafe code (after going irrevocable).  The
test case in bug 46654 is an atomic transaction.

[Bug c++/54893] unable to access volatile variable within relaxed transaction

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54893

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-10-11
         AssignedTo|unassigned at gcc dot       |aldyh at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

--- Comment #4 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-10-11 20:29:13 UTC ---
Created attachment 28427
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28427
untested patch

Testing...

[Bug middle-end/54893] unable to access volatile variable within relaxed transaction

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54893

--- Comment #5 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-10-17 21:18:21 UTC ---
Author: aldyh
Date: Wed Oct 17 21:18:16 2012
New Revision: 192549

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=192549
Log:
    PR middle-end/54893
        * trans-mem.c (diagnose_tm_1_op): Allow volatiles inside relaxed
        transactions.

Added:
    trunk/gcc/testsuite/c-c++-common/tm/pr54893.c
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/trans-mem.c

[Bug middle-end/54893] unable to access volatile variable within relaxed transaction

[aldyh at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54893

Aldy Hernandez <aldyh at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #6 from Aldy Hernandez <aldyh at gcc dot gnu.org> 2012-10-17 21:19:29 UTC ---
fixed on mainline