fix opt/8634

[Geoff Keating]

Zdenek Dvorak <rakdver@atrey.karlin.mff.cuni.cz> writes:

> Hello,
> 
> > > I believe purge_addressof costs us almost nothing (I cannot be sure,
> > > as it has no timevar, but the fact that nobody bothered to add one
> > > tells something); is this really the right place where to attempt to
> > > save?
> > 
> > It's got to be about the same cost as purge_builtin_constant_p,
> > as both involve one linear scan of the insn stream.  Which was
> > identified as a definite regression in gcse times, which led to
> > a sequence of patches that avoided running it whenever possible.
> > 
> > If you added a timevar, you'd notice that you've just about
> > exactly doubled the amount of time spent here.
> 
> OK; then it seems that there is something rotten in the very heart of
> gcc.  How can it not be slow when just a simple pass over insns takes
> a non-trivial time?

Because GCC is so fast that a simple pass over insns is expensive
relative to the rest of GCC.

-- 
- Geoffrey Keating <geoffk@geoffk.org>