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Re: help interpreting gcc 4.1.1 optimisation bug


andrew@walrond.org writes:
 > I want a reduced test case for this problem for bugzilla, but don't
 > really know the exact cause. I _think_ code is being improperly
 > optimized away, but I don't know. This function is part of a
 > BigInteger library, and works perfectly upto and including -O2, but
 > produces bogus results at -O3, unless I uncomment the debug_msg()
 > line, which just dumps the contents of 'i' to the screen. I'm
 > guessing that without it, the compiler fails to notice that i's
 > contents are being accessed via the _front8/_back8 pointers, and
 > throws the preceeding 'i' related code away. Heres the code:
 > 
 >     void Integer::export_bytes(Data& _buffer, PortMode _mode) const
 >     {
 >         debug_scope("export_bytes(Data&)");
 > 
 >         Integer i(*this,1);
 > 
 >         if (_mode==SIGNED) {
 >             if (*mBack & TOP_BIT)
 >                 i.grow(1,0);
 >             if (sign()<0)
 >                 i.twoscomplement();
 >         }
 > 
 >         for (UnsignedW* p=i.mFront; p<=i.mBack; p++)
 >             *p = host2little(*p);
 > 
 >         Unsigned8* _front8 = reinterpret_cast<Unsigned8*>(i.mFront);
 >         Unsigned8* _back8 = _front8+(i.mSize*WORD_BYTES)-1;
 >         if (_mode==UNSIGNED)
 >             while (_back8!=_front8 and *_back8==0)
 >                 _back8--;
 >         else if (mSign==1)
 >             while (_back8!=_front8 and *_back8==0 and
 >                    !(*(_back8-1)&0x80))
 >                 _back8--;
 >         else
 >             while (_back8!=_front8 and *_back8==0xff and
 >                    *(_back8-1)&0x80)
 >                 _back8--;
 > 
 >         //debug_msg(String("Force i\n")+i.dump());
 > 
 >         _buffer.assign(_front8, _back8+1);
 >     }
 > 
 > Any thoughts on how to proceed?

I think your code is broken.  A reinterpret_cast from foo* to bar* is
only well-defined if foo and bar are compatible types or one of
{foo,bar} is type char.  I can't be sure from your code, but I'm
guessing that i.mFront is not declared as a pointer to Unsigned8, but
as a pointer to an incompatible type UnsignedW.

Andrew.


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