This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: update address taken: don't drop clobbers

On Thu, Jul 10, 2014 at 8:22 PM, Jeff Law <> wrote:
> On 07/10/14 09:48, Michael Matz wrote:
>> Hi,
>> 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.

Yes, coalescing doesn't do "live-range splitting" to avoid coalescing the
two PHI results.  But they have to be coalesced anyway.

I still think simply never recording conflicts for undefined SSA names
is a proper "hack" to avoid this issue.

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

That would be unnecessarily expensive.


> jeff

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]