[PATCH] D15722: [WIP][Polly] SSA Codegen

[Johannes Doerfert via llvm-commits]

> Backtracking sounds like a function to get the generated value on
> demand, and following the control flow upstream by calling itself.
> Sounds like a reasonable idea to avoid generating too many unused
> PHIs.
Yes, but there is little evidence so far that we generate "too many"
unused PHIs with this patch for interesting code.

> But I don't know whether it even could introduce a scaling problem and therefore worth the increased implementation complexity (or maybe it's even simpler?)
I think we can do it with reasonable implementation complexity, however,
you have to consider that we currently (without SSA-codegen) also have a
trade-off "between number of PHIs" and "complexity to place them" even
though it is only implicit. The patch as presented is very good in
terms of complexity to place PHIs but consequently not to smart in
placing them. Alternatively, backtracking would place PHIs smart but
it can cost a lot more to place a PHI in the first place. The overall
complexity of the code is in the worst case always the same [now and with
either SSA-codegen approaches] but the complexity to get to the code is
what is different.

To conclude: I haven't made up my mind what we want and I don't think
you can say "what is best" it in general.

Cheers,
  Johannes
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 213 bytes
Desc: Digital signature
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20160414/27dc03a8/attachment.sig>