4.10. Optimisation (code improvement)

The -O* options specify convenient “packages” of optimisation flags; the -f* options described later on specify individual optimisations to be turned on/off; the -m* options specify machine-specific optimisations to be turned on/off.

4.10.1. -O*: convenient “packages” of optimisation flags.

There are many options that affect the quality of code produced by GHC. Most people only have a general goal, something like “Compile quickly” or “Make my program run like greased lightning.” The following “packages” of optimisations (or lack thereof) should suffice.

Note that higher optimisation levels cause more cross-module optimisation to be performed, which can have an impact on how much of your program needs to be recompiled when you change something. This is one reason to stick to no-optimisation when developing code.

No -O*-type option specified:

This is taken to mean: “Please compile quickly; I'm not over-bothered about compiled-code quality.” So, for example: ghc -c Foo.hs


Means “turn off all optimisation”, reverting to the same settings as if no -O options had been specified. Saying -O0 can be useful if eg. make has inserted a -O on the command line already.

-O or -O1:

Means: “Generate good-quality code without taking too long about it.” Thus, for example: ghc -c -O Main.lhs


Means: “Apply every non-dangerous optimisation, even if it means significantly longer compile times.”

The avoided “dangerous” optimisations are those that can make runtime or space worse if you're unlucky. They are normally turned on or off individually.

At the moment, -O2 is unlikely to produce better code than -O.

We don't use a -O* flag for day-to-day work. We use -O to get respectable speed; e.g., when we want to measure something. When we want to go for broke, we tend to use -O2 (and we go for lots of coffee breaks).

The easiest way to see what -O (etc.) “really mean” is to run with -v, then stand back in amazement.

4.10.2. -f*: platform-independent flags

These flags turn on and off individual optimisations. They are normally set via the -O options described above, and as such, you shouldn't need to set any of them explicitly (indeed, doing so could lead to unexpected results). A flag -fwombat can be negated by saying -fno-wombat. The flags below are off by default, except where noted below. See Section 4.20.15, “Individual optimisations” for a compact list.


Part of Data Parallel Haskell (DPH).

Off by default. Enable the vectorisation avoidance optimisation. This optimisation only works when used in combination with the -fvectorise transformation.

While vectorisation of code using DPH is often a big win, it can also produce worse results for some kinds of code. This optimisation modifies the vectorisation transformation to try to determine if a function would be better of unvectorised and if so, do just that.


On by default. Merge immediately-nested case expressions that scrutinse the same variable. Example

  case x of
     Red -> e1
     _   -> case x of 
              Blue -> e2
              Green -> e3
  case x of
     Red -> e1
     Blue -> e2
     Green -> e2


On by default.. Enables the common-sub-expression elimination optimisation. Switching this off can be useful if you have some unsafePerformIO expressions that you don't want commoned-up.


A very experimental flag that makes dictionary-valued expressions seem cheap to the optimiser.


On by default. Eta-expand let-bindings to increase their arity.


On by default. Eta-reduce lambda expressions, if doing so gets rid of a whole group of lambdas.


Usually GHC black-holes a thunk only when it switches threads. This flag makes it do so as soon as the thunk is entered. See Haskell on a shared-memory multiprocessor.


When this option is given, intermediate floating point values can have a greater precision/range than the final type. Generally this is a good thing, but some programs may rely on the exact precision/range of Float/Double values and should not use this option for their compilation.

Note that the 32-bit x86 native code generator only supports excess-precision mode, so neither -fexcess-precision nor -fno-excess-precision has any effect. This is a known bug, see Section 14.2.1, “Bugs in GHC”.


An experimental flag to expose all unfoldings, even for very large or recursive functions. This allows for all functions to be inlined while usually GHC would avoid inlining larger functions.


On by default. Float let-bindings inwards, nearer their binding site. See Let-floating: moving bindings to give faster programs (ICFP'96).

This optimisation moves let bindings closer to their use site. The benefit here is that this may avoid unnecessary allocation if the branch the let is now on is never executed. It also enables other optimisation passes to work more effectively as they have more information locally.

This optimisation isn't always beneficial though (so GHC applies some heuristics to decide when to apply it). The details get complicated but a simple example is that it is often beneficial to move let bindings outwards so that multiple let bindings can be grouped into a larger single let binding, effectively batching their allocation and helping the garbage collector and allocator.


On by default. Run the full laziness optimisation (also known as let-floating), which floats let-bindings outside enclosing lambdas, in the hope they will be thereby be computed less often. See Let-floating: moving bindings to give faster programs (ICFP'96). Full laziness increases sharing, which can lead to increased memory residency.

NOTE: GHC doesn't implement complete full-laziness. When optimisation in on, and -fno-full-laziness is not given, some transformations that increase sharing are performed, such as extracting repeated computations from a loop. These are the same transformations that a fully lazy implementation would do, the difference is that GHC doesn't consistently apply full-laziness, so don't rely on it.


Worker-wrapper removes unused arguments, but usually we do not remove them all, lest it turn a function closure into a thunk, thereby perhaps creating a space leak and/or disrupting inlining. This flag allows worker/wrapper to remove all value lambdas. Off by default.


Causes GHC to ignore uses of the function Exception.assert in source code (in other words, rewriting Exception.assert p e to e (see Section 7.19, “Assertions ”). This flag is turned on by -O.


Tells GHC to ignore all inessential information when reading interface files. That is, even if M.hi contains unfolding or strictness information for a function, GHC will ignore that information.


Off by default.Run demand analysis again, at the end of the simplification pipeline. We found some opportunities for discovering strictness that were not visible earlier; and optimisations like -fspec-constr can create functions with unused arguments which are eliminated by late demand analysis. Improvements are modest, but so is the cost. See notes on the Trac wiki page.


Off by default, but enabled by -O2. Turn on the liberate-case transformation. This unrolls recursive function once in its own RHS, to avoid repeated case analysis of free variables. It's a bit like the call-pattern specialiser (-fspec-constr) but for free variables rather than arguments.


Set the size threshold for the liberate-case transformation.


The type checker sometimes displays a fragment of the type environment in error messages, but only up to some maximum number, set by this flag. The default is 6. Turning it off with -fno-max-relevant-bindings gives an unlimited number. Syntactically top-level bindings are also usually excluded (since they may be numerous), but -fno-max-relevant-bindings includes them too.


Turn off the "state hack" whereby any lambda with a State# token as argument is considered to be single-entry, hence it is considered OK to inline things inside it. This can improve performance of IO and ST monad code, but it runs the risk of reducing sharing.


Tells GHC to omit all inessential information from the interface file generated for the module being compiled (say M). This means that a module importing M will see only the types of the functions that M exports, but not their unfoldings, strictness info, etc. Hence, for example, no function exported by M will be inlined into an importing module. The benefit is that modules that import M will need to be recompiled less often (only when M's exports change their type, not when they change their implementation).


On by default. Tells GHC to omit heap checks when no allocation is being performed. While this improves binary sizes by about 5%, it also means that threads run in tight non-allocating loops will not get preempted in a timely fashion. If it is important to always be able to interrupt such threads, you should turn this optimization off. Consider also recompiling all libraries with this optimization turned off, if you need to guarantee interruptibility.


Make GHC be more precise about its treatment of bottom (but see also -fno-state-hack). In particular, stop GHC eta-expanding through a case expression, which is good for performance, but bad if you are using seq on partial applications.


Off by default, but enabled by -O2. Only applies in combination with the native code generator. Use the graph colouring register allocator for register allocation in the native code generator. By default, GHC uses a simpler, faster linear register allocator. The downside being that the linear register allocator usually generates worse code.


Off by default, only applies in combination with the native code generator. Use the iterative coalescing graph colouring register allocator for register allocation in the native code generator. This is the same register allocator as the -freg-graph one but also enables iterative coalescing during register allocation.


GHC's optimiser can diverge if you write rewrite rules ( Section 7.21, “Rewrite rules ”) that don't terminate, or (less satisfactorily) if you code up recursion through data types (Section 14.2.1, “Bugs in GHC”). To avoid making the compiler fall into an infinite loop, the optimiser carries a "tick count" and stops inlining and applying rewrite rules when this count is exceeded. The limit is set as a multiple of the program size, so bigger programs get more ticks. The -fsimpl-tick-factor flag lets you change the multiplier. The default is 100; numbers larger than 100 give more ticks, and numbers smaller than 100 give fewer.

If the tick-count expires, GHC summarises what simplifier steps it has done; you can use -fddump-simpl-stats to generate a much more detailed list. Usually that identifies the loop quite accurately, because some numbers are very large.


(Default: 45) Governs the maximum size that GHC will allow a function unfolding to be. (An unfolding has a “size” that reflects the cost in terms of “code bloat” of expanding (aka inlining) that unfolding at a call site. A bigger function would be assigned a bigger cost.)

Consequences: (a) nothing larger than this will be inlined (unless it has an INLINE pragma); (b) nothing larger than this will be spewed into an interface file.

Increasing this figure is more likely to result in longer compile times than faster code. The -funfolding-use-threshold is more useful.


(Default: 8) This is the magic cut-off figure for unfolding (aka inlining): below this size, a function definition will be unfolded at the call-site, any bigger and it won't. The size computed for a function depends on two things: the actual size of the expression minus any discounts that apply (see -funfolding-con-discount).

The difference between this and -funfolding-creation-threshold is that this one determines if a function definition will be inlined at a call site. The other option determines if a function definition will be kept around at all for potential inlining.


Part of Data Parallel Haskell (DPH).

Off by default. Enable the vectorisation optimisation transformation. This optimisation transforms the nested data parallelism code of programs using DPH into flat data parallelism. Flat data parallel programs should have better load balancing, enable SIMD parallelism and friendlier cache behaviour.


Off by default, but enabled by -O2. Turn on call-pattern specialisation; see Call-pattern specialisation for Haskell programs.

This optimisation specializes recursive functions according to their argument "shapes". This is best explained by example so consider:

last :: [a] -> a
last [] = error "last"
last (x : []) = x
last (x : xs) = last xs

In this code, once we pass the initial check for an empty list we know that in the recursive case this pattern match is redundant. As such -fspec-constr will transform the above code to:

last :: [a] -> a
last []       = error "last"
last (x : xs) = last' x xs
      last' x []       = x
      last' x (y : ys) = last' y ys

As well avoid unnecessary pattern matching it also helps avoid unnecessary allocation. This applies when a argument is strict in the recursive call to itself but not on the initial entry. As strict recursive branch of the function is created similar to the above example.

It is also possible for library writers to instruct GHC to perform call-pattern specialisation extremely aggressively. This is necessary for some highly optimized libraries, where we may want to specialize regardless of the number of specialisations, or the size of the code. As an example, consider a simplified use-case from the vector library:

import GHC.Types (SPEC(..))

foldl :: (a -> b -> a) -> a -> Stream b -> a
{-# INLINE foldl #-}
foldl f z (Stream step s _) = foldl_loop SPEC z s
    foldl_loop !sPEC z s = case step s of
                            Yield x s' -> foldl_loop sPEC (f z x) s'
                            Skip       -> foldl_loop sPEC z s'
                            Done       -> z

Here, after GHC inlines the body of foldl to a call site, it will perform call-pattern specialization very aggressively on foldl_loop due to the use of SPEC in the argument of the loop body. SPEC from GHC.Types is specifically recognized by the compiler.

(NB: it is extremely important you use seq or a bang pattern on the SPEC argument!)

In particular, after inlining this will expose f to the loop body directly, allowing heavy specialisation over the recursive cases.


On by default. Specialise each type-class-overloaded function defined in this module for the types at which it is called in this module. Also specialise imported functions that have an INLINABLE pragma (Section, “INLINABLE pragma”) for the types at which they are called in this module.


Turn on the static argument transformation, which turns a recursive function into a non-recursive one with a local recursive loop. See Chapter 7 of Andre Santos's PhD thesis


On by default.. Switch on the strictness analyser. There is a very old paper about GHC's strictness analyser, Measuring the effectiveness of a simple strictness analyser, but the current one is quite a bit different.

The strictness analyser figures out when arguments and variables in a function can be treated 'strictly' (that is they are always evaluated in the function at some point). This allow GHC to apply certain optimisations such as unboxing that otherwise don't apply as they change the semantics of the program when applied to lazy arguments.


This option causes all constructor fields which are marked strict (i.e. “!”) to be unpacked if possible. It is equivalent to adding an UNPACK pragma to every strict constructor field (see Section 7.20.11, “UNPACK pragma”).

This option is a bit of a sledgehammer: it might sometimes make things worse. Selectively unboxing fields by using UNPACK pragmas might be better. An alternative is to use -funbox-strict-fields to turn on unboxing by default but disable it for certain constructor fields using the NOUNPACK pragma (see Section 7.20.12, “NOUNPACK pragma”).


On by default.. This option causes all constructor fields which are marked strict (i.e. “!”) and which representation is smaller or equal to the size of a pointer to be unpacked, if possible. It is equivalent to adding an UNPACK pragma (see Section 7.20.11, “UNPACK pragma”) to every strict constructor field that fulfils the size restriction.

For example, the constructor fields in the following data types

data A = A !Int
data B = B !A
newtype C = C B
data D = D !C

would all be represented by a single Int# (see Section 7.2, “Unboxed types and primitive operations”) value with -funbox-small-strict-fields enabled.

This option is less of a sledgehammer than -funbox-strict-fields: it should rarely make things worse. If you use -funbox-small-strict-fields to turn on unboxing by default you can disable it for certain constructor fields using the NOUNPACK pragma (see Section 7.20.12, “NOUNPACK pragma”).

Note that for consistency Double, Word64, and Int64 constructor fields are unpacked on 32-bit platforms, even though they are technically larger than a pointer on those platforms.