GHC.Types.Tickish

Documentation

data GenTickish pass

Constructors

ProfNote

An {--} profiling annotation, either automatically added by the desugarer as a result of -auto-all, or added by the user.

Fields

• profNoteCC :: CostCentre

  the cost centre

• profNoteCount :: !Bool

  bump the entry count?

• profNoteScope :: !Bool

  scopes over the enclosed expression (i.e. not just a tick)

HpcTick

A "tick" used by HPC to track the execution of each subexpression in the original source code.

Fields

• tickModule :: Module
• tickId :: !Int

Breakpoint

A breakpoint for the GHCi debugger. This behaves like an HPC tick, but has a list of free variables which will be available for inspection in GHCi when the program stops at the breakpoint.

NB. we must take account of these Ids when (a) counting free variables, and (b) substituting (don't substitute for them)

Fields

• breakpointExt :: XBreakpoint pass
• breakpointId :: !Int
• breakpointFVs :: [XTickishId pass]

  the order of this list is important: it matches the order of the lists in the appropriate entry in ModBreaks.

  Careful about substitution! See Note [substTickish] in GHC.Core.Subst.

SourceNote

A source note.

Source notes are pure annotations: Their presence should neither influence compilation nor execution. The semantics are given by causality: The presence of a source note means that a local change in the referenced source code span will possibly provoke the generated code to change. On the flip-side, the functionality of annotated code *must* be invariant against changes to all source code *except* the spans referenced in the source notes (see "Causality of optimized Haskell" paper for details).

Therefore extending the scope of any given source note is always valid. Note that it is still undesirable though, as this reduces their usefulness for debugging and profiling. Therefore we will generally try only to make use of this property where it is necessary to enable optimizations.

Fields

• sourceSpan :: RealSrcSpan

  Source covered

• sourceName :: String

  Name for source location (uses same names as CCs)

Instances

Outputable (XTickishId pass) => Outputable (GenTickish pass)

Defined in GHC.Core.Ppr

Methods

ppr :: GenTickish pass -> SDoc

type CoreTickish = GenTickish 'TickishPassCore

type StgTickish = GenTickish 'TickishPassStg

type CmmTickish = GenTickish 'TickishPassCmm

Tickish in Cmm context (annotations only)

type family XTickishId (pass :: TickishPass)

tickishCounts :: GenTickish pass -> Bool

A "counting tick" (where tickishCounts is True) is one that counts evaluations in some way. We cannot discard a counting tick, and the compiler should preserve the number of counting ticks as far as possible.

However, we still allow the simplifier to increase or decrease sharing, so in practice the actual number of ticks may vary, except that we never change the value from zero to non-zero or vice versa.

data TickishScoping

Specifies the scoping behaviour of ticks. This governs the behaviour of ticks that care about the covered code and the cost associated with it. Important for ticks relating to profiling.

Constructors

NoScope

No scoping: The tick does not care about what code it covers. Transformations can freely move code inside as well as outside without any additional annotation obligations

SoftScope

Soft scoping: We want all code that is covered to stay covered. Note that this scope type does not forbid transformations from happening, as long as all results of the transformations are still covered by this tick or a copy of it. For example

let x = tick... (let y = foo in bar) in baz ===> let x = tick... bar; y = tick... foo in baz

Is a valid transformation as far as "bar" and "foo" is concerned, because both still are scoped over by the tick.

Note though that one might object to the "let" not being covered by the tick any more. However, we are generally lax with this - constant costs don't matter too much, and given that the "let" was effectively merged we can view it as having lost its identity anyway.

Also note that this scoping behaviour allows floating a tick "upwards" in pretty much any situation. For example:

case foo of x -> tick... bar ==> tick... case foo of x -> bar

While this is always legal, we want to make a best effort to only make us of this where it exposes transformation opportunities.

CostCentreScope

Cost centre scoping: We don't want any costs to move to other cost-centre stacks. This means we not only want no code or cost to get moved out of their cost centres, but we also object to code getting associated with new cost-centre ticks - or changing the order in which they get applied.

A rule of thumb is that we don't want any code to gain new annotations. However, there are notable exceptions, for example:

let f = y -> foo in tick... ... (f x) ... ==> tick... ... foo[x/y] ...

In-lining lambdas like this is always legal, because inlining a function does not change the cost-centre stack when the function is called.

Instances

Eq TickishScoping

Defined in GHC.Types.Tickish

Methods

(==) :: TickishScoping -> TickishScoping -> Bool

(/=) :: TickishScoping -> TickishScoping -> Bool

tickishScoped :: GenTickish pass -> TickishScoping

Returns the intended scoping rule for a Tickish

tickishScopesLike :: GenTickish pass -> TickishScoping -> Bool

Returns whether the tick scoping rule is at least as permissive as the given scoping rule.

tickishFloatable :: GenTickish pass -> Bool

Returns True for ticks that can be floated upwards easily even where it might change execution counts, such as:

Just (tick... foo) ==> tick... (Just foo)

This is a combination of tickishSoftScope and tickishCounts. Note that in principle splittable ticks can become floatable using mkNoTick -- even though there's currently no tickish for which that is the case.

tickishCanSplit :: GenTickish pass -> Bool

Returns True for a tick that is both counting and scoping and can be split into its (tick, scope) parts using mkNoScope and mkNoTick respectively.

mkNoCount :: GenTickish pass -> GenTickish pass

mkNoScope :: GenTickish pass -> GenTickish pass

tickishIsCode :: GenTickish pass -> Bool

Return True if this source annotation compiles to some backend code. Without this flag, the tickish is seen as a simple annotation that does not have any associated evaluation code.

What this means that we are allowed to disregard the tick if doing so means that we can skip generating any code in the first place. A typical example is top-level bindings:

foo = tick... y -> ... ==> foo = y -> tick... ...

Here there is just no operational difference between the first and the second version. Therefore code generation should simply translate the code as if it found the latter.

data TickishPlacement

Governs the kind of expression that the tick gets placed on when annotating for example using mkTick. If we find that we want to put a tickish on an expression ruled out here, we try to float it inwards until we find a suitable expression.

Constructors

PlaceRuntime

Place ticks exactly on run-time expressions. We can still move the tick through pure compile-time constructs such as other ticks, casts or type lambdas. This is the most restrictive placement rule for ticks, as all tickishs have in common that they want to track runtime processes. The only legal placement rule for counting ticks.

PlaceNonLam

As PlaceRuntime, but we float the tick through all lambdas. This makes sense where there is little difference between annotating the lambda and annotating the lambda's code.

PlaceCostCentre

In addition to floating through lambdas, cost-centre style tickishs can also be moved from constructors, non-function variables and literals. For example:

let x = scc... C (scc... y) (scc... 3) in ...

Neither the constructor application, the variable or the literal are likely to have any cost worth mentioning. And even if y names a thunk, the call would not care about the evaluation context. Therefore removing all annotations in the above example is safe.

Instances

Eq TickishPlacement

Defined in GHC.Types.Tickish

Methods

(==) :: TickishPlacement -> TickishPlacement -> Bool

(/=) :: TickishPlacement -> TickishPlacement -> Bool

tickishPlace :: GenTickish pass -> TickishPlacement

Placement behaviour we want for the ticks

tickishContains :: Eq (GenTickish pass) => GenTickish pass -> GenTickish pass -> Bool

Returns whether one tick "contains" the other one, therefore making the second tick redundant.