136 lines
3.1 KiB
Text
136 lines
3.1 KiB
Text
.bp
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.NH 1
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Cross jumping
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.NH 2
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Introduction
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.PP
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The "Cross Jumping" optimization technique (CJ)
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.[
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wulf design optimizing compiler
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.]
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is basically a space optimization technique. It looks for pairs of
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basic blocks (B1,B2), for which:
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.DS
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SUCC(B1) = SUCC(B2) = {S}
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.DE
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(So B1 and B2 both have one and the same successor).
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If the last few non-branch instructions are the same for B1 and B2,
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one such sequence can be eliminated.
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.DS
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Pascal:
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if cond then
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S1
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S3
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else
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S2
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S3
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(pseudo) EM:
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TEST COND TEST COND
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BNE *1 BNE *1
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S1 S1
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S3 ---> BRA *2
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BRA *2 1:
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1: S2
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S2 2:
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S3 S3
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2:
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Fig. 9.1 An example of Cross Jumping
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.DE
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As the basic blocks have the same successor,
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at least one of them ends in an unconditional branch instruction (BRA).
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Hence no extra branch instruction is ever needed, just the target
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of an existing branch needs to be changed; neither the program size
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nor the execution time will ever increase.
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In general, the execution time will remain the same, unless
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further optimizations can be applied because of this optimization.
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.PP
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This optimization is particularly effective,
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because it cannot always be done by the programmer at the source level,
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as demonstrated by the Fig. 8.2.
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.DS
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Pascal:
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if cond then
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x := f(4)
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else
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x := g(5)
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EM:
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... ...
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LOC 4 LOC 5
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CAL F CAL G
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ASP 2 ASP 2
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LFR 2 LFR 2
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STL X STL X
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Fig. 9.2 Effectiveness of Cross Jumping
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.DE
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At the source level there is no common tail,
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but at the EM level there is a common tail.
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.NH 2
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Implementation
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.PP
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The implementation of cross jumping is rather straightforward.
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The technique is applied to one procedure at a time.
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The control flow graph of the procedure
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is scanned for pairs of basic blocks
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with the same (single) successor and with common tails.
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Note that there may be more than two such blocks (e.g. as the result
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of a case statement).
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This is dealt with by repeating the entire process until no
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further optimizations can de done for the current procedure.
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.sp
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If a suitable pair of basic blocks has been found, the control flow
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graph must be altered. One of the basic
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blocks must be split into two.
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The control flow graphs before and after the optimization are shown
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in Fig. 9.3 and Fig. 9.4.
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.DS
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-------- --------
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| S1 | | S2 |
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| S3 | | S3 |
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-------- --------
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|------------------|--------------------|
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v
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Fig. 9.3 CFG before optimization
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.DE
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.DS
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-------- --------
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| S1 | | S2 |
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-------- --------
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|--------------------<------------------|
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v
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--------
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| S3 |
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--------
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v
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Fig. 9.4 CFG after optimization
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.DE
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Some attributes of the three resulting blocks (such as immediate dominator)
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are updated.
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.PP
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In some cases, cross jumping might split the computation of an expression
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into two, by inserting a branch somewhere in the middle.
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Most code generators will generate very poor assembly code when
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presented with such EM code.
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Therefor, cross jumping is not performed in these cases.
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