Inlining

Inlining is the most important compiler optimisation pass as it enables most other optimisation opportunities. The pass is simple, saturated names are replaced with their definitions, the details are complicated. The compiler must make judgements as to whether inlining a function will lead to further optimisations, if not then it is easy to increase the code size needlessly.

Getting Started

• ​Secrets of the GHC inliner -- quite an old paper but a great description of the main ideas
• ​GHC User Guide -- Provides a description of INLINE, INLINABLE and NOINLINE pragmas.

Relevant Tickets

There are lots of old relevant tickets related to inlining. Perfect for a keen newcomer!

#3073

Avoid reconstructing dictionaries in recursive instance methods

#3755

Improve join point inlining

#3781

Improve inlining for local functions

#4960

Better inlining test in CoreUnfold

#5834

Allow both INLINE and INLINABLE for the same function

#5928

INLINABLE fails to specialize in presence of simple wrapper

#7109

Inlining depends on datatype size, even with INLINE pragmas

#7114

Cannot recover (good) inlining behaviour from 7.0.2 in 7.4.1

#7283

Specialise INLINE functions

#7511

Room for GHC runtime improvement >~5%, inlining related

#7803

Superclass methods are left unspecialized

#7829

make better/more robust loopbreaker choices

#8099

Alternate syntax for indicating when a function is "fully applied" for purposes of inlining

#8589

Bad choice of loop breaker with INLINABLE/INLINE

#8662

GHC does not inline cheap inner loop when used in two places

#8668

SPECIALIZE silently fails to apply

#8774

Transitivity of Auto-Specialization

#8814

7.8 optimizes attoparsec improperly

#9020

Massive blowup of code size on trivial program

#9320

Inlining regression/strangeness in 7.8

#9370

unfolding info as seen when building a module depends on flags in a previously-compiled module

#9418

Warnings about "INLINE binder is (non-rule) loop breaker"

#9701

GADTs not specialized properly

#9798

Frustrating behaviour of the INLINE pragma

#10371

GHC fails to inline and specialize a function

#10421

exponential blowup in inlining (without INLINE pragmas)

#10710

More self-explanatory pragmas for inlining phase control

#10766

user manual: INLINE's interaction with optimization levels is not clear

#11068

Make Generic/Generic1 methods inlinable

#11263

"Simplifier ticks exhausted" that resolves with fsimpl-tick-factor=200

#12274

GHC panic: simplifier ticks exhausted

#12454

Cross-module specialisation of recursive functions

#12463

SPECIALIZABLE pragma?

#12747

INLINE vs NOINLINE vs <nothing> give three different results; two would be better

#13016

SPECIALIZE INLINE doesn't necessarily inline specializations of a recursive function

#13851

Change in specialisation(?) behaviour since 8.0.2 causes 6x slowdown

#14211

Compiler is unable to INLINE as well as the programmer can manually

#14275

Large Haskell value unexpectedly gets an unfolding

Relevant Wiki Pages

• Commentary/Compiler/DesugaringInstances -- About how default methods can lead to poor inliner performance due to recursion