update address taken: don't drop clobbers

[Jeff Law]

On 10/16/14 05:20, Richard Biener wrote:
>
> It doesn't even know that it has no uses (the variable still needs to be
> written into SSA form).  OTOH it is a missed DSE opportunity before
> update-address-taken?
Perhaps it's a missed DSE prior to update-address-taken, but I suspect 
there aren't many of those in general...  It wouldn't be hard to shove 
in a DSE pass to get some instrumentation across a wider range of code.

>
> As of premature optimization - into-SSA could notice it created SSA
> names with no uses and trigger a fast DCE.
Yea, I'd been pondering that as well.  I'm curious how often we have 
trivially dead code after into-ssa.  If it's most of the time, then 
maybe we just schedule the DCE right after and not even bother to check 
if we've got SSA_NAMEs without any uses.

>
> Btw, I wonder what this odd folding of variadic builtins is about, and why
> it is not done in the stdarg pass (and only there), which would be earlier.
No clue.  I've tried pretty hard not to think about the lowering of 
variadic builtins.  Perhaps the author of that code was trying to keep 
things simple and let the optimizers to the optimization? :-)

jeff