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

[Nick Lewycky]

James Molloy wrote:
> Hi Nick,
>
> I'm not an expert on inlining algorithms so please excuse my naivite.
> But usually these "top-down versus bottom-up" arguments (in other
> domains, at least), come to the obvious conclusion that both have merits
> so let's create a hybrid. Why is this not the case here too?

It is. Our current bottom-up inliner is already a hybrid, we sometimes 
decide to inline the caller into the callee instead of inlining the 
callee into the caller.

> As far as I can tell, Yin's pass simply provides a method to add global
> context to local decisions.

I have a strong problem with global metrics. Things like "only allow X% 
code size growth" mean that whether I inline this callsite can depend on 
seemingly unrelated factors like how many other functions are in the 
same module, even outside the call stack at hand. Similarly for other 
things like cutoffs about how many inlinings are to be performed (now it 
depends on traversal order, and if you provide the inliner with a more 
complete program then it may chose to not inline calls it otherwise 
would have). I don't like spooky action at a distance, it's hard to 
predict and hard to debug.

We *do* want more context in the inliner, that's the largest known 
deficiency of our current one. Again, the pass manager rewrite is taking 
place to allow the inliner to call into function analysis passes so that 
we can have more context available when making our inlining decision. 
It's just a long, slow path to getting what we want.

  Algorithms such as a bottom-up inliner
> analyze a callsite and assign it a value. This could be bottom-up or
> top-down, it doesn't really matter. What matters is that eventually, all
> (rational) callsites end up in the same sorted datastructure and are
> addressed in order.
>
> Am I missing something?

The current inliner doesn't assign values across the whole call graph 
then decide where to inline.

Firstly, the local decision (looking at a single caller-callee pair 
through a particular call site) works by attempting to determine how 
much of the callee will be live given the values known at the caller. 
For instance, we will resolve a switch statement to its destination 
block, and potentially eliminate other callees. These simplifications 
would still be possible even if we calculated everything up front.

Secondly, we iterate with the function passes optimizing the new 
function after each inlining is performed. This may eliminate dead code 
(potentially removing call graph edges) and can resolve loads 
(potentially creating new call graph edges as indirect calls are 
resolved to direct calls). Handling the CFG updates is one of the more 
interesting and difficult parts of the inliner, and it's very important 
for getting C++ virtual calls right. This sort of thing can't be 
calculated up front.

Nick

PS. You may have guessed that I'm just plain prejudiced against top-down 
inliners. I am, and I should call that out before going too far down 
into the discussion.

In the past I've seem them used for their ability to game benchmarks 
(that's my side of the story, not theirs). You provide an inliner with 
tweakable knobs that have really messy complicated interactions all 
across the inliner depending on all sorts of things, then you select the 
numbers that happen to give you a 20% speed up on SPEC for no good 
reason, and call it success. Attribute the success to the flexibility 
provided by the design.

> On 6 August 2014 08:54, Nick Lewycky <nicholas at mxc.ca
> <mailto:nicholas at mxc.ca>> wrote:
>
>     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
>     ______________________________ _________________
>     llvm-commits mailing list
>     llvm-commits at cs.uiuc.edu <mailto:llvm-commits at cs.uiuc.edu>
>     http://lists.cs.uiuc.edu/ mailman/listinfo/llvm-commits
>     <http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits>
>
>