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Re: [tree-ssa-branch] Optimizing non-SIMPLE trees

On Fri, 09 Aug 2002, Richard Henderson wrote:

> On Fri, Aug 09, 2002 at 10:09:37AM -0400, Diego Novillo wrote:
> > The simplifier refuses to simplify the call to strncmp() because
> > it contains at least one constant argument.  It does this to
> > avoid tricking the code generator into not outputting a builtin
> > call when all the arguments are constant.
> This is bogus.
string-opt-8.c fails for this exact reason.  If we simplify calls
to strncmp, whatever code that converts strncmp into a builtin
call fails to match the builtin and then a libcall is generated.

> > Clearly, the simplifier could be a bit smarter in this case, but
> > the point remains that we will many times refuse to simplify a
> > builtin or other special trees.  The question is now, how do we
> > tell the optimizers to keep their hands off these trees?
> No, this is not clear.  You'd be complicating the optimizers for
> no reason whatsoever.
Just the other day we were talking about trees that would need to
be passed unsimplified.  You have to tell the optimizers that
they cannot look *into* those trees.  MD builtins have this
problem as well, we can't really simplify those trees as we don't
know what the backend might expect.

> The correct solution here is to expand builtin functions *after*
> constant propagation.  Then we get nice builtin expansion when
> the user hasn't written something convenient up front.
Builtin functions are always expanded after constant propagation.
Aren't builtins expanded as part of the tree->RTL expansion?
That's after tree CCP.

> Of course afterwards you get to re-do the propagation, since we
> may well have generated new constants.
Will fold() deal with __builtin_strncmp if all its arguments are


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