[llvm-dev] Managed Languages BOF @ Dev Meeting

[Chris Lattner via llvm-dev]

> On Oct 18, 2015, at 3:20 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
> 
> 
> 
> Chris Lattner via llvm-dev wrote:
> > I saw this come up in a recent discussion about LLILC.  One simple
> > way to do this is to introduce a new intrinsic, like
> > “llvm.nullcheck(x)” which symbolically represents the null check and
> > bail out in one instruction - instead of the fully elaborated basic
> > block representation (which slows everything down by increasing the
> > height of the CFG).
> 
> Is this compile time slowdown just a generic effect of the fact that a
> lot of things in LLVM have O(f(# BBs)) complexity?  

Yes.

> Or are there
> specific things (like domtree construction) that suffer due to lots of
> basic blocks?

Yes.  :-)

Some things work in time proportional to the depth of the CFG, and each null check adds a level.

> 
> > These null checks can be optimized away through
> > standard techniques, and any remaining ones expanded out to LLVM IR
> > before codegen, or as part of lowering to MachineInstrs.
> 
> That won't be free, since keeping the control flow explicit does give
> us better optimization in cases where we can sink work into the cold
> throwing path, like these:

I don’t claim it to be optimal.  That said, we do sinking at the machine level as well, so we will catch at least some of these cases.

-Chris