[PATCH] [llgo] irgen: generate switch instructions

[Philip Reames]

On 01/08/2015 01:15 AM, Chandler Carruth wrote:
>
> On Wed, Jan 7, 2015 at 11:05 AM, Peter Collingbourne <peter at pcc.me.uk 
> <mailto:peter at pcc.me.uk>> wrote:
>
>     > I don't know if this will ever be necessary, but another option is for llgo to grow its own IR on
>     top of go/ssa. I'd rather not go there for this, though.
>
>
>     Agreed. FWIW, the long term direction I'd like to see is
>     attempting to extend go/ssa with what we need (in this particular
>     case, a native switch instruction).
>
>     > I did look to see if there was an existing pass, but again I
>     couldn't see anything that looked relevant
>
>
>     If you grep for
>     `SimplifyCFGOpt::FoldValueComparisonIntoPredecessors` you should
>     find something.
>
>     > Geo-mean improvement of 0.49% (0.23% - 0.75% @ 95% CI)
>
>
>     Great! I'm a little surprised that we can do better than LLVM on
>     its own, but maybe I don't understand what the simplifycfg code is
>     doing.
>
>
> Just a peanut gallery comment, but I'm pretty disappointed that LLVM 
> can't turn if-chains into switches for you. I think not dealing with 
> switches sounds like a really reasonable simplification for a frontend 
> to desire. The optimizer should fix this code, and should both be 
> faster and more powerful in doing so.
I agree here.
>
> Any interest in trying to make LLVM's switch formation powerful enough 
> to catch the cases you care about? Or at least reducing the test case 
> we missed? If we can do that, it seems like it would remove the 
> ugliness from the approach by letting you generate "naive" switches 
> with high confidence that they optimize appropriately?
This is close enough to my own interests that if you can create a small 
test case, I'll take a serious look at fixing it.  I'm not going to go 
too far, but if it ends up being a simple fix, I'd like to have this 
work.  Please file a bug with your IR fragment.
>
>
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20150108/e2219a81/attachment.html>