| ...Taking as our metric the venerable McCabe score:
|
| v(G) = e - n + 2
|
| where e and n are the number of edges and nodes in the
| control flow graph, and where you are in trouble when
| v(G)>10 in a single module, the simplest patch adds two
| edges and one node, i.e., v'(G)=v(G)+1, and that minimum
| obtains only for patches with no conditional branches in
| the patch....
While I agree with your general point, this particular argument
is a misuse of the McCabe score. Replacing:
X
Y
with
X
goto L
Y
L':
...
L: Y'
goto L'
*at the machine code level* should have absolutely no effect on the
complexity of the algorithm (beyond any delta between Y and Y'). If you
insist on computing your McCabe score from the generated code, and it
gives you a different answer, then the score you are deriving is
meaningless.
The whole point of measurements like McCabe is to measure the complexity
of the algorithm *as seen by a human being*. Automated code transforma-
tions that no human being ever sees should not affect it. Otherwise,
you're going to have to throw out all your optimizing compilers. The
transformation above occurs not just in patching but in other contexts -
e.g., this might be adventageous, with Y and Y' semantically equivalent,
if Y contains a bunch of calls that can't be reached with short
call sequences when at their original location, but can be if they are
relocated to L. Or there might be cache interference effects that are
avoided by relocating.
Roughly similar patterns are used in generating code for loops, where
the surface semantics might require two copies of a test (one at loop
entry, one at the bottom of the loop) but this transformation lets you
get by with a single copy.
As long as the algorithm developer's view is that control flows directly
from X to Y (and there are no incoming edges at Y), this is one node, no
matter how the compiler or patch generator decides to shake and bake it
into memory.
-- Jerry
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