[PATCHES] A module inliner pass with a greedy call site queue

[Nick Lewycky]

Hal Finkel wrote:
> I'd like you to elaborate on your assertion here, however, that a "topdown inliner is going to work best when you have the whole program." It seems to me that, whole program or not, a top-down inlining approach is exactly what you want to avoid the vector-push_back-cold-path-inlining problem (because, from the caller, you see many calls to push_back, which is small -- because the hot path is small and the cold path has not (yet) been inlined -- and inlines them all, at which point it can make a sensible decision about the cold-path calls).

I don't see that. You get the same information when looking at a pair of 
functions and deciding whether to inline. With the bottom-up walk, we 
analyze the caller and callee in their entirety before deciding whether 
to inline. I assume a top-down inliner would do the same.

If you have a top-down traversal and you don't have the whole program, 
the first problem you have is a whole ton of starting points. At first 
blush bottom up seems to have the same problem, except that they are 
generally very straight-forward leaf functions -- setters and getters or 
little loops to test for a property. Top down you don't yet know what 
you've got, and it has lots of calls that may access arbitrary memory. 
In either case, you apply your metric to inline or not. Then you run the 
function-level passes to perform simplification. Bottom up, you're much 
more likely to get meaningful simplifications -- your getter/setter 
melts away. Top down you may remove some redundant loads or dead stores, 
but you still don't know what's going on because you have these opaque 
not-yet-analyzed callees in the way. If you couldn't analyze the memory 
before, inlining one level away hasn't helped you, and the function size 
has grown. You don't get the simplifications until you go all the way 
down the call stack to the setters and getters etc.

There's a fix for this, and that's to perform a sort of symbolic 
execution and just keep track of what the program has done so far (ie. 
what values registers have taken on so far, which pointers have escaped 
etc.), and make each inlining decision in program execution order. But 
that fix doesn't get you very far if you haven't got a significant chunk 
of program to work with.

Nick