[llvm] r179957 - SimplifyCFG: If convert single conditional stores

[Arnold Schwaighofer]

Given that we have the optimization implemented in both SimplifyCFG and the Early-If converter now, I believe we can take a pragmatic approach:

* Enable the SimplifyCFG optimization now. This will allow us not to look bad on Phoronix/hmmer and spec/hmmer in llvm 3.3 (20%!). And it seems people agree that this is generally a good thing in the current compilation pipeline.
and
* Add the Early If-converter solution. This is the future proof approach. Once we get rid of the select cannonicalization and enable the early-ifconverter we have this to fall back on.


What do you think?

Best,
Arnold
 

On Apr 24, 2013, at 1:47 PM, Chandler Carruth <chandlerc at google.com> wrote:

> I generally agree with Chris's position in the here-and-now FWIW. We should continue to balance the IR-level if-conversion for canonicalization against the down sides.
> 
> However, I wanted to point out that increasingly I have a different long-term hope for this type of canonicalization that has been heavily influenced by Evan, Andy, Jakob, and Dan Gohman's problems with doing canonicalizations like this at the IR level:
> 
> On Wed, Apr 24, 2013 at 7:37 PM, Chris Lattner <clattner at apple.com> wrote:
> On Apr 24, 2013, at 11:28 AM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
>>> 
>>> Because this is a canonicalization of this sort, it seems clearly good to do on IR, and early.  Doing something like this at the codegen level specifically for micro-architectural reasons could also make sense, but I don't see that eliminating the usefulness of doing it early as well.
>> Introducing a "select" at IR level dose not necessarily means CodeGen convert the "select" with predicated instruction like cmov.
>> cmov is not necessary inexpensive, for example, on Pentium 4, the latency of cmov is about 10+ cycle. 
> 
> Yes, I truly understand that.  My point is that it is still a canonicalization: depending on how the user wrote the code is silly for the opposite reason: if they wrote code with ?: or max on pentium 4, codegen should convert it to an "if" if the branch is biased.
> 
> I feel like we need a better strategy long-term. Increasingly, I'm of the opinion that as we move away from the SelectionDAG's basic block limitations, we should also move away from canonicalizing to the if-converted code. These days I would rather see us add utilities to LLVM to look across basic blocks which form PHI-only CFGs as-if they were straight line code, and to have the canonical form of predicated values be PHI-only CFGs. This has a nice advantage of simplifying the IR model, and being strictly more general than selects. However, it *requires* that we don't have a basic-block granularity hard limitation in passes, and that's just not the world we live in currently. Maybe eventually.
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