[LLHD] Add dedicated Mem2Reg pass #8244
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Implement a dedicated LLHD pass that promotes signal/memory slots to SSA values and forwards drives to probes. The pass is distinct from MLIR's upstream Mem2Reg pass in that it also works for regions where the memory slots are defined outside the region. It also understands `llhd.wait` and `llhd.halt` operations which may suspend execution of a process and therefore allows the values of signals to change before execution resumes. To deal with this, the pass uses a lattice to propagate the need for slot definitions backwards from probes, and to propagate reaching definitions for slots forward from drives and initial probes. Using the lattice, the pass can forward driven values to probes as long as there is no process suspension in between (`llhd.wait`), and it will insert probes immediately after and drives immediately before suspension points. This moves most intra-process data flow into SSA values and block arguments, and retains probes and drives around the suspension point to interact with ops outside the process. This first implementation only supports unconditional blocking drives, and only probes and drives that interact with a `llhd.sig` directly without any field projections or aliasing. The pass is designed to deal with these in the future though: a more detailed aliasing analysis can determine which definitions are invalidated by drives, drives to field projections can be converted into field insertions on the whole definition, and drive conditions can be propagated along definitions to the drives inserted by the pass ahead of suspension points. Fixes #8245 Fixes #8246
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Implement a dedicated LLHD pass that promotes signal/memory slots to SSA values and forwards drives to probes. The pass is distinct from MLIR's upstream Mem2Reg pass in that it also works for regions where the memory slots are defined outside the region. It also understands
llhd.waitandllhd.haltoperations which may suspend execution of a process and therefore allows the values of signals to change before execution resumes. To deal with this, the pass uses a lattice to propagate the need for slot definitions backwards from probes, and to propagate reaching definitions for slots forward from drives and initial probes. Using the lattice, the pass can forward driven values to probes as long as there is no process suspension in between (llhd.wait), and it will insert probes immediately after and drives immediately before suspension points. This moves most intra-process data flow into SSA values and block arguments, and retains probes and drives around the suspension point to interact with ops outside the process.This first implementation only supports unconditional blocking drives, and only probes and drives that interact with a
llhd.sigdirectly without any field projections or aliasing. The pass is designed to deal with these in the future though: a more detailed aliasing analysis can determine which definitions are invalidated by drives, drives to field projections can be converted into field insertions on the whole definition, and drive conditions can be propagated along definitions to the drives inserted by the pass ahead of suspension points.Fixes #8245
Fixes #8246