112 lines
3 KiB
Text
112 lines
3 KiB
Text
.bp
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.NH 1
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Inline substitution
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.NH 2
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Introduction
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.PP
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The Inline Substitution technique (IL)
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tries to decrease the overhead associated
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with procedure calls (invocations).
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During a procedure call, several actions
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must be undertaken to set up the right
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environment for the called procedure.
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.[
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johnson calling sequence
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.]
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On return from the procedure, most of these
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effects must be undone.
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This entire process introduces significant
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costs in execution time as well as
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in object code size.
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.PP
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The inline substitution technique replaces
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some of the calls by the modified body of
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the called procedure, hence eliminating
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the overhead.
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Furthermore, as the calling and called procedure
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are now integrated, they can be optimized
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together, using other techniques of the optimizer.
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This often leads to extra opportunities for
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optimization
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.[
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ball predicting effects
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.]
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.[
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carter code generation cacm
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.]
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.[
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scheifler inline cacm
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.]
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.PP
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An inline substitution of a call to a procedure P increases
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the size of the program, unless P is very small or P is
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called only once.
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In the latter case, P can be eliminated.
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In practice, procedures that are called only once occur
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quite frequently, due to the
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introduction of structured programming.
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(Carter
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.[
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carter umi ann arbor
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.]
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states that almost 50% of the Pascal procedures
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he analyzed were called just once).
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.PP
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Scheifler
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.[
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scheifler inline cacm
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.]
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has a more general view of inline substitution.
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In his model, the program under consideration is
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allowed to grow by a certain amount,
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i.e. code size is sacrificed to speed up the program.
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The above two cases are just special cases of
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his model, obtained by setting the size-change to
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(approximately) zero.
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He formulates the substitution problem as follows:
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.IP
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"Given a program, a subset of all invocations,
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a maximum program size, and a maximum procedure size,
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find a sequence of substitutions that minimizes
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the expected execution time."
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.LP
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Scheifler shows that this problem is NP-complete
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.[~[
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aho hopcroft ullman analysis algorithms
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.], chapter 10]
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by reduction to the Knapsack Problem.
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Heuristics will have to be used to find a near-optimal
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solution.
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.PP
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In the following chapters we will extend
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Scheifler's view and adapt it to the EM Global Optimizer.
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We will first describe the transformations that have
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to be applied to the EM text when a call is substituted
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in line.
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Next we will examine in which cases inline substitution
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is not possible or desirable.
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Heuristics will be developed for
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chosing a good sequence of substitutions.
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These heuristics make no demand on the user
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(such as making profiles
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.[
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scheifler inline cacm
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.]
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or giving pragmats
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.[~[
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ichbiah ada military standard
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.], section 6.3.2]),
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although the model could easily be extended
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to use such information.
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Finally, we will discuss the implementation
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of the IL phase of the optimizer.
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.PP
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We will often use the term inline expansion
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as a synonym of inline substitution.
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.sp 0
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The inverse technique of procedure abstraction
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(automatic subroutine generation)
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.[
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shaffer subroutine generation
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.]
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will not be discussed in this report.
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