[LLVMdev] [RFC] LCSSA vs. SSAUpdater ... FIGHT!

[Andrew Trick]

On Feb 7, 2014, at 11:29 AM, Chris Lattner <clattner at apple.com> wrote:

> 
> On Feb 1, 2014, at 4:33 AM, Chandler Carruth <chandlerc at gmail.com> wrote:
> 
>> So, there are two primary ideas behind SSA form management in the loop optimizers of LLVM:
>> 
>> - Require LCSSA form input, leverage its (very powerful) guarantees to simplify maintaining SSA form, and also maintain LCSSA form.
>> 
>> - Don't bother with LCSSA form input, assume the worst, and use powerful incremental SSA formation utilities built on SSAUpdater to form SSA on demand when needed.
>> 
>> (Note, there are plenty of places where SSAUpdater makes sense, so this isn't really about doing away with it at all.)
> 
> It’s worth noting that LCSSA predates SSAUpdater.  If I went back in time and knew what I knew now, I wouldn’t have gone with LCSSA.
> 
> My gripes are three fold: 1) SSAUpdater can handle anything that LCSSA simplifies, 2) that LCSSA is annoying to keep up to date, 3) LCSSA burns compile time optimistically rewriting loop values, which are then later collapsed away even if nothing cares about those values.
> 
> My personal preference would be to get rid of LCSSA completely, but I don’t know how to stage that.

Until recently I felt exactly the same way. I didn’t want LCSSA just as another mechanism for updating SSA.

I’m warming up to having it run during the early loop passes if it
- significantly simplifies the LICM logic
- greatly speeds up SSAUpdater within loops (maybe little net compile-time increase on average?)
- compartmentalizes loop transforms and SSA update so we can debug loop opts one loop at a time

I still don’t particularly like that we force all LLVM clients to perform LCSSA when all they end up doing is rotating and simplifying loops (no LICM/unroll). So it is a tradeoff.

-Andy