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Lifespans

Optimizers may be interested in the characteristic of the barrier blocks for making a decision. The variables have a lifespan in the code blocks, denoted by properties beginLifespan,endLifespan. The beginLifespan denotes the intruction where it receives its first value, the endLifespan the last instruction in which it was used as operand or target.

If, however, the last use lies within a BARRIER block, we can not be sure about its end of life status, because a block redo may implictly revive it. For these situations we associate the endLifespan with the block exit.

In many cases, we have to determine if the lifespan interferes with a optimization decision being prepared. The lifespan is calculated once at the beginning of the optimizer sequence. It should either be maintained to reflect the most accurate situation while optimizing the code base. In particular, it means that any move/remove/addition of a MAL instruction calls for either a recalculation or further propagation. Unclear what will be the best strategy. For the time being we just recalc.

See is all arguments mentioned in the instruction at point pc are still visible at instruction qc and have not been updated in the mean time. Take into account that variables may be declared inside a block. This can be calculated using the BARRIER/CATCH and EXIT pairs.

The safety property should be relatively easy to determine for each MAL function. This calls for accessing the function MAL block and to inspect the arguments of the signature.

Any instruction may block identification of a common subexpression. It suffices to stumble upon an unsafe function whose parameter lists has a non-empty intersection with the targeted instruction. To illustrate, consider the sequence

     L1 := f(A,B,C);
     ...
     G1 := g(D,E,F);
     ...
     l2:= f(A,B,C);
     ...
     L2:= h()

The instruction G1:=g(D,E,F) is blocking if G1 is an alias for {A,B,C}. Alternatively, function g() may be unsafe and {D,E,F} has a non-empty intersection with {A,B,C}. An alias can only be used later on for readonly (and not be used for a function with side effects).