Re: List of optimizations enabled by -O1 seems incorrect

[Nikos Chantziaras <realnc at arcor dot de>]

On 08/13/2009 09:06 PM, Andrew Haley wrote:
> Nikos Chantziaras wrote:
> > I'm trying to discover which GCC optimization is responsive for a
> > runtime error reported by Valgrind:
> >
> >    "Conditional jump or move depends on uninitialised value(s)"
> >
> > This pops up when compiling with -O1 and above.  The code in question is
> > C++ and is of this form:
> >
> >    if (!foo&&  !bar)
> >
> > The uninitialized variable is 'bar'.  However, it shouldn't be evaluated
> > at all due to short-circuit if rules.  Of course the optimizer is free
> > to evaluate it anyway if there are no side-effects when doing so.
> >
> > So just out of interest, I'm trying to find out which optimization is
> > responsible for this.  I looked up the GCC info pages and I compiled
> > with -O1 and then disabled the optimizations enabled by -O1 one by one,
> > in hope to hit the one that triggers the error, until all of them were
> > disabled in the end:
> >  [...]
> >
> > However, the error still appears which makes me conclude that the above
> > list of -O1 optimizations is not complete.  Which ones am I missing?
>
> Not all optimizations are switchable.  If you want to know the complete
> set of switches, compile with -fverbose-asm and look at the .s file.

Thanks for the tip.  Looking at the generated assembly files, I finally 
solved the "mystery": GCC uses SSE/SSE2 instructions to fetch both 
variables in one CPU instruction.  It's faster but circumvents 
short-circuit "if" rules and Valgrind prints an error.