[GHC] #10766: user manual: INLINE's interaction with optimization levels is not clear

[GHC]

#10766: user manual: INLINE's interaction with optimization levels is not clear
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        Reporter:  osa1              |                   Owner:
            Type:  task              |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Documentation     |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Description changed by osa1:

Old description:

> user manual: INLINE's interaction with optimization levels is not clear
>
> From the user manual it's not possible to say how should INLINE and
> actual inlining behavior should interact with optimization levels.
>
> What the user manuals says:
>
> - Without INLINE GHC tries to inline depending on size of definitions and
> current optimization level(user manual says -O is needed, but can we
> assume that for any inlining to be done we need at least -O? Because I
> have an example in which inlining is happening even with -O0).
>
> - With INLINE GHC tries hard to inline. But it doesn't say which
> optimization level is needed.
>
> I think we should add word or two about this. Does anyone know the answer
> here?
>
> My guess is: Even though INLINE makes sure GHC saves RHS of the
> definition in .hi files to be able to inline later, optimization levels
> still have an effect on deciding whether to inline or not. Because some
> INLINEd RHSs may be too big in which case it may decide not to inline
> with -O1 but inline with -O2 etc.
>
> Furthermore, without -O, GHC doesn't mark definitions as INLINE. But if
> we have INLINEd definitions and compiling a code with -O0, it may still
> inline those definitions, because there are two different mechanisms to
> decide whether to mark as INLINE and whether to inline the RHS, and while
> without -O GHC doesn't mark anything as INLINE, it can still inline
> previously marked definitions.

New description:

 From the user manual it's not possible to say how should INLINE and actual
 inlining behavior should interact with optimization levels.

 What the user manuals says:

 - Without INLINE GHC tries to inline depending on size of definitions and
 current optimization level(user manual says -O is needed, but can we
 assume that for any inlining to be done we need at least -O? Because I
 have an example in which inlining is happening even with -O0).

 - With INLINE GHC tries hard to inline. But it doesn't say which
 optimization level is needed.

 I think we should add word or two about this. Does anyone know the answer
 here?

 My guess is: Even though INLINE makes sure GHC saves RHS of the definition
 in .hi files to be able to inline later, optimization levels still have an
 effect on deciding whether to inline or not. Because some INLINEd RHSs may
 be too big in which case it may decide not to inline with -O1 but inline
 with -O2 etc.

 Furthermore, without -O, GHC doesn't mark definitions as INLINE. But if we
 have INLINEd definitions and compiling a code with -O0, it may still
 inline those definitions, because there are two different mechanisms to
 decide whether to mark as INLINE and whether to inline the RHS, and while
 without -O GHC doesn't mark anything as INLINE, it can still inline
 previously marked definitions.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10766#comment:1>
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