Abstract

this paper was presented at the 1992 SIGPLAN Conference on Programming Language Design and Implementation. Permission to copy without fee all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage, the ACM copyright notice and the title of the publication and its date appear, and notice is given that copying is by permission of the Association for Computing Machinery. To copy otherwise, or to republish, requires a fee and/or specific permission. 2 \Delta upon good local allocation by eliminating unnecessary loads at the entrance to a basic block and by eliminating unnecessary stores at the exit from a basic block. An initial load of v is unnecessary if all predecessors exit with v in some register, and a terminal store of w is unnecessary if w is dead or all succeeding loads of w can be replaced with a reference to w's register. Of course we do not initially know which values will ultimately be allocated to registers, so we estimate each value's chances of ultimate register residence. At the basic block level, we know values loaded locally into a register and not overwritten have a 100% chance of exiting in a register. The likelihood of other values residing in a register on exit depends on their likelihood of residing in a register upon entrance, the number of unused registers in a block, and the pattern of local register usage. The likelihood that a value will be in a register upon entrance to a block depends on the likelihood it will exit in a register from all predecessor blocks. During global register allocation, we will model the competition between register candidates with estimates of the likelihood of register residence based on the demand for registers. Each instruction that requires a target ...

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