Dead code fragments are recognized by assignments to variables whose value is not consumed any more. It can be detected by marking all variables used as arguments as being relevant. In parallel, we build a list of instructions that should appear in the final result. The new code block is then built in one scan, discarding the superflous instructions.
Instructions that produce side effects to the environment, e.g., printing and BAT updates, should be taken into account. Such (possibly recursive) functions should be marked with a property (unsafe). Likewise, instructions marked as control flow instructions should be retained.
An illustrative example is the following MAL snippet:
V7 := bat.new(:oid,:int); V10 := bat.new(:int,:oid); V16 := algebra.markH(V7); V17 := algebra.join(V16,V7); V19 := bat.new(:oid,:int); V22 := bat.new(:oid,:int); V23 := algebra.join(V16,V22); io.print("done"); optimizer.deadCodeRemoval();
The dead code removal trims this program to the following short block:
A refinement of the dead code comes from using arguments that ceased
to exist due to actions taken by an optimizer. For example, in the
snippet below the
pushranges optimizer may conclude that variable
V31 becomes empty and simply injects a 'dead' variable by dropping
the assignment statement. This makes other code dead as well.
V30 := algebra.select( V7, 10,100); V31 := algebra.select(V30,-1,5); V32 := aggr.sum(V31); io.print(V32);