[Boost-users] [extension]: g++ '-O2' optimization w/ type_map access thru member template function

Subject: [Boost-users] [extension]: g++ '-O2' optimization w/ type_map access thru member template function
From: Jeffrey Chang (jeffrey8chang_at_[hidden])
Date: 2009-04-10 12:59:30

Hi,

I'm a happy user of boost::extension library.  Thanks Jeremy Pack et al. for
this great work, and hope it'll be accepted by Boost soon.   My extension
library is dowloaded from http://boost-extension.blogspot.com/  (August 14,
2008 Link), which works great for me when compiled with "-O0" optimization
flag in g++  (my version: i686-apple-darwin9-g++-4.0.1).

But, when I compile with -O2 and access type_map object through member
template function, g++ seems to optimize away the initialization logic for
'type_map', and I'd get signal assertion of  EXC_BAD_ACCESS.  Here's my test
program:

 65     struct MyDict {
 66         boost::extensions::type_map     mTypeDict;
 67         template< typename T >
 68         T &         refInstance()               { mTypeDict.get<T>(); }
 69     };
 70
 71     typedef std::set<int>   IntSetType;
 72
 73     void test_MyDict_1_mem_data_access() {
 74         MyDict  testObj;
 75         std::cout << "------ 1 " << std:: endl;
 76         IntSetType  &rIntSet = testObj.mTypeDict.get<IntSetType>();
 77         std::cout << "------ 2 " << std:: endl;
 78         std::cout << "rIntSet.size() is " << rIntSet.size() <<
std::endl;
 79         std::cout << "------ 3 " << std:: endl;
 80         rIntSet.insert (123);
 81         std::cout << "------ 4 " << std:: endl;
 82         std::cout << "rIntSet.size() is now " << rIntSet.size() <<
std::endl    ;
 83         std::cout << "------ 5 " << std:: endl;
 84     }
 85
 86     void test_MyDict_2_mem_func_access() {
 87         MyDict  testObj;
 88         std::cout << "====== 1 " << std:: endl;
 89         IntSetType  &rIntSet = testObj.refInstance<IntSetType>();
 90         std::cout << "====== 2 " << std:: endl;
 91         std::cout << "rIntSet.size() is " << rIntSet.size() <<
std::endl;
 92         std::cout << "====== 3 " << std:: endl;
 93         rIntSet.insert (123);
 94         std::cout << "====== 4 " << std:: endl;
 95         std::cout << "rIntSet.size() is now " << rIntSet.size() <<
std::endl    ;
 96         std::cout << "====== 5 " << std:: endl;
 97     }

This test program runs without any problem when compiled with "-O0"; but
with "-O2" compiler flag, "test_MyDIct_1_mem_data_access()" is still run ok
as expect, but "test_MyDict_2_mem_func_access()" halts at line 93.  Here's
the output when run under gdb:

------ 1
------ 2
rIntSet.size() is 0
------ 3
------ 4
rIntSet.size() is now 1
------ 5
====== 1
====== 2
rIntSet.size() is 86
====== 3

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_PROTECTION_FAILURE at address: 0x0000005e
std::_Rb_tree<int, int, std::_Identity<int>, std::less<int>,
std::allocator<int> >::insert_unique (this=0x56, __v=@0xbffff24c) at
testHpcDictByType.c:871
871      _Link_type __x = _M_begin();
(gdb)

So my question is, what's wrong with accessing boost::extension::type_map
through "MyDict::refInstance<T>()" here?  In g++ "-O2" mode, why does the
direct access to "MyDict::mTypeDict" is ok, while "MyDict::refInstance<T>()"
is not ?

I'm not sure if this is a generic g++ optimization problem, or specific to
boost::extension library.  But, for me, it seems to happen only for type_map
data structure, but not others.

Thanks for shedding any light on this.

--- Jeffrey