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Re: help interpreting gcc 4.1.1 optimisation bug
On Mon, Jun 12, 2006 at 12:32:36PM +0100, Andrew Haley wrote:
> 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.
>
typedef unsigned long UnsignedW;
typedef unsigned char Unsigned8;
typdef std::vector<Unsigned8> Data;
mFront is defined as UnsignedW*, an array of words constituting the
big integer. Having converted the words from host to little endian
with host2little(), I am simply reading (exporting) the bytes out into
a vector using std::vector::assign(InputIter first, InputIter last)
Seems legit, no?
> Andrew.
>