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This allows us to pass around the context data that
hipe_regalloc_prepass needs cleanly, without using process dictionary or
parameterised modules (like it was previous to this change).
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This is sound because the liveness data structure only stores liveness
info at basic block boundaries, and the rewrites that happen in
TargetSpecific:check_and_rewrite/2 preserves all existing definitions
and uses, and all new liveness intervals, belonging to newly introduced
temporaries, are always local to a basic block, and thus do not show up
in the liveout or livein sets for the basic block.
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The division into an initial pass that may introduce temps, and
following passes that must not forces us to make the same heuristic
decision during each of these passes. Thus, the splitting heuristic
can't be based on the number of temporaries -- at least without
excluding temporaries above SpillLimit.
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If temps introduced by hipe_regalloc_prepass end up above SpillLimit,
the register allocators will not spill them. This constraint is
unnecessarily limiting the allocators and might theoretically lead to
unallocatable programs (more temps above SpillLimit alive at a time than
there are physical registers).
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As the just_as_good_as assertion was loosened with the `NowRegs >=
CheckRegs` check, it no longer verified that hipe_regalloc_prepass had
not incorrectly labeled a temp as unallocatable. We add that behaviour
back.
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hipe_regalloc_prepass speeds up register allocation by spilling any temp
that is live over a call (which clobbers all register).
In order to detect these, a new function was added to the target
interface; defines_all_alloc/1, that takes an instruction and returns a
boolean.
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