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Re: -fstrict-fp-semantics (was Re: numerical instability and estimate-probability)
- From: Brad Lucier <lucier at math dot purdue dot edu>
- To: jh at suse dot cz (Jan Hubicka)
- Cc: lucier at math dot purdue dot edu (Brad Lucier), gcc at gcc dot gnu dot org, jh at suse dot cz, rth at cygnus dot com, mrs at windriver dot com
- Date: Fri, 16 Nov 2001 11:37:03 -0500 (EST)
- Subject: Re: -fstrict-fp-semantics (was Re: numerical instability and estimate-probability)
> > Can we adopt an option, say -fstrict-fp-semantics, that means that fp
> > results are not changed *one bit* by any optimization?
> I am not quite sure how to reach it w/o important penalty in the perofrmance.
I presume that there is so little time in gcc spent in fp that the
performance penalty wouldn't matter.
> Even if we jump into 80bit spills (I was trying to implement it, but never got
> it working, unfortuantely), we still need to disable any load/store propagation
> that may elliminate the truncate.
> This is quite nasty.
I don't believe that this is necessary. If you have an expression that's lifted
above a loop, then lift it as an extended.
> It may be reached by making every FP memory reference
> volatile, but that sounds crazy too.
This is definitely not necessary.
> Also it won't solve the problem with bootstrap, as stage1 compiler may
> be non-gcc one and still may produce different result :(
It adds another iteration, which is unpleasant.
Brad