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

[Andrew Trick]

On Apr 24, 2013, at 12:17 PM, Arnold Schwaighofer <aschwaighofer at apple.com> wrote:

> 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?

As long as we don't introduce target heuristics in the early SimplifyCFG passes, it shouldn't be a problem. It is certainly possible for targets to reverse-if-convert if needed.

Long-term I agree with Chandler. I don't like GVN and other high level opts growing dependent on selects that are only formed in a small subset of the cases in which optimization should apply. That is clearly anti-canonical. To be rehashed after ISEL is fixed...

-Andy

> 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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