Re: Comparing Control Flow Graphs

>Is there any formal notion that captures the "precision" of a control flow
>graph?


>Control Flow Graphs:
>
> x = y x = y
> /\ /\
> / \ / \
> A B A;C1 B;C2
> \ / \ /
> \/ \/
> | |
> C |
> | |
> end end




I don't have a good answer for the main question; but I can tlak about
your examples for a minute. The left-hand case accurately represents the
code. The right-hand case represents the code after duplicating the block
C. The right-hand case permits sharper analysis for forward problems and
may result in better code for C1 and C2 versus C.


I don't really think of one being more precise than the other; the
right-hand case is just the result of a transformation, permittung better
analysis and optimization. Similar things happen with procedure cloning
and loop unrolling. For more examples of the effects of cloning basic
blocks, see the (poorly titled) paper


    title="Avoiding Unconditional Jumps by Code Replication",
    author="Frank Mueller and David B. Whalley",
    pages="322--330",
    journal=sigplan,
    year=1992,
    month=july,
    volume=27,
    number=7,
    note=pldi92




Preston Briggs
--

Return to the comp.compilers page.
Search the comp.compilers archives again.