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RE: ARM compiler rewriting code to be longer and slower


Ramana Radhakrishnan writes:
> [Resent because of account funnies. Apologies to those who get this twice]
> 
> Hi,
> 
> > > This problem is reported every once in a while, all targets with
> > small
> > > load-immediate instructions suffer from this, especially since GCC
> > 4.0
> > > (i.e. since tree-ssa).  But it seems there is just not enough
> > interest
> > > in having it fixed somehow, or someone would have taken care of it by
> > > now.
> > >
> > > I've summed up before how the problem _could_ be fixed, but I can't
> > > find where.  So here we go again.
> > >
> > > This could be solved in CSE by extending the notion of "related
> > > expressions" to constants that can be generated from other constants
> > > by a shift. Alternatively, you could create a simple, separate pass
> > > that applies CSE's "related expressions" thing in dominator tree
> > walk.
> > 
> > See http://gcc.gnu.org/ml/gcc-patches/2009-03/msg00158.html for
> > handling
> > something similar when related expressions differ by a small additive
> > constant.  I am planning to finish this and submit it for 4.5.
> 
> Wouldn't doing this in CSE only solve the problem within an extended basic
> block and not necessarily across the program ? Surely you'd want to do it
> globally or am I missing something very basic here ?

No, you're not.  There are plans moving some of what's in CSE to a new LCM
(global) pass.  Also note that for a global a pass you clearly need some more
sophisticated cost model for deciding when CSEing is beneficial.  On a
multi-scalar architecture, instructions synthesizing consts sometimes appear
to be "free" whereas holding a value a in a register for an extended period of
time is not.

Adam


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