|Version 1 (modified by nfrisby, 3 years ago) (diff)|
This pages serves as a public log what I did for my GHC internship from 21 Jan 2013 to 12 April 2013.
general core knowledge
- Max's page about code generation: really good.
- document ticky profiling
- Core -> STG -> CMM and _what you can learn by looking at each one_
Late Lambda Float
LLF = Late Lambda Float
As the GHC optimization papers explain, it is an early design decision to *not* perform lambda lifting. My initial project was to investigate the effects of aggressively floating lambdas to the top-level at the end of the core2core pipeline.
- The main reason to not perform lambda lifting is that abstracting over free variables loses information and thereby inhibits *downstream* optimization.
- Doing it *late* (ie just before CorePrep) circumvents this issue.
- The original conjecture was that doing it would save allocation: a dynamically allocated closure becomes a static top-level function.
- Max Bolingbroke did a quick implementation of this idea some years ago (~mid 2000s), but it seems it was abandoned. I don't know why.
We decided to implement LLF by re-using most of the FloatOut machinery.
FloatOut is structured in three phases.
- Annotate all expressions with their free variables.
- Consume those annotations while annotating each binder with the target "level" (essentially a depth wrt value lambdas) to which we want to float it.
- Consume those annotations while actually relocating the bindings.
We wholesale re-use the third phase (compiler/simplCore/FloatOut) with no changes, add logic to the middle phase, and enrich the first phase with more analyses.
Most of my changes were
- Adding flags (compiler/main/DynFlags compiler/simplCore/CoreMonad compiler/simplCore/SimplCore)
- Implementing the LLF logic in the first two FloatOut phases (compiler/simplCore/SetLevels)
- Adding LLF to the core2core pipeline (compiler/simplCore/SimplCore)
In order to minimize factors, I decided to float only lambdas during LLF. Thus there is no need to perform FloatIn afterwards: all of our floats are to the top-level, so there will be nothing to FloatIn.
We placed LLF as the last pass before CorePrep. After experimentation, we decided to follow it with a simplifier pass.
The basic shape of things:
outer = CTX[let f x = RHS[x] in BODY[f]]
where outer is a top-level binding. LLF transforms this to:
poly_f FVS x = RHS[x] outer = CTX[BODY[f FVS]]
wbere FVS are the free variables of RHS[x]. We'll use a, b, c, ... for particular variables in FVS.
The poly prefix is vestigial: in the past, floated bindings could never cross lambdas, so the abstracted variables were only type variables. Hence the machinery that adds the new parameters was only ever adding type parameters; it was creating polymorphic functions. This scheme was not updated even when the machinery was enriched to also abstract over values.
- join points
- Note [join point abstraction]
Discovered Detriments of LLF
These are the various negative consequences that we discovered on the way. We discuss mitigation below.
- Unapplied occurrences of f in BODY results in the creation of PAPs, which increases allocation. For example: map f xs becomes map (f a b c) xs. Max had identified this issue earlier.
- Abstracting over a known function might change a fast entry call in RHS to a slow entry call. For example, if CTX binds a to a lambda, that information is lost in the right-hand side of poly_f. This can increase runtime.
- Replacing a floated binder's occurrence (ie f becomes f a b c) can add free variables to a thunk's closure, which increases allocation.
- TODO putStr (eg sphere)
Mitigating PAP Creation
Preserving Fast Entries
Mitigating Thunk Growth
- easier: if f occurs inside of a thunk in BODY, then limit its free variables.
- harder: approximate the maximum number of free variables that floating f would add to a thunk in BODY, and limit that.
Discovered Benefits of LLF
We didn't see as much decrease in allocation as we would have liked.
- nice demonstration of the basic effect in puzzle
- #7663 - simulates having the inliner discount for free variables like it discounts for parameters
- Floating functions to the top-level creates more opportunities for the simplifier.
CTX[case a of [p1 -> let f x = ... in case a of ...]]
The let prevents the cases from being merged. Since LLF is so aggressive, it floats f when it otherwise wouldn't be.
Thunk Join Points
We discovered that the worker-wrapper was removing the void argument from join points (eg knights and mandel2). This ultimately resulted in LLF *increasing* allocation. A thunk was let-no-escape before LLF but not after, since it occurred free in the right-hand side of a floated binding and hence now occurred (escapingly) as an argument.
SPJ was expecting no such non-lambda join points to exist. We identified where it was happening (WwLib.mkWorkerArgs) and switched it off.
TODO the result?