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Re: update address taken: don't drop clobbers

On 07/10/14 09:48, Michael Matz wrote:

On Thu, 10 Jul 2014, Richard Biener wrote:

Apart from the out-of-SSA patch you proposed elsewhere a possibility
is to simply never mark undefined SSA names as
SSA_NAME_OCCURS_IN_ABNORMAL_PHI ... (or not mark those
as must-coalesce).

The insight to note is, that undefined SSA names should really be
coalesced with something (otherwise you lost an optimization opportunity),
but it doesn't matter with _what_ each use of the undefined name is
coalesced, you can even identify different uses of them with different SSA
names (e.g. the LHS of each using stmt).  Requires some change in the
order things are done in out-of-ssa.
The last part is what I hinted might be problematical. If some undefined SSA_NAME appears on the RHS of two PHIs and we want to coalesce that undefined SSA_NAME with the LHS of each of those PHIs, then the LHS of those two PHIs must coalesce as well. At least that's my recollection of how all that stuff worked.

It was that realization that made me wonder if we should have a unique SSA_NAME at each undefined use point.


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