[AMDGPU][Scheduler] Refactor ArchVGPR rematerialization during scheduling (#125885)

AMDGPU scheduler's `PreRARematStage` attempts to increase function
occupancy w.r.t. ArchVGPR usage by rematerializing trivial
ArchVGPR-defining instruction next to their single use. It first
collects all eligible trivially rematerializable instructions in the
function, then sinks them one-by-one while recomputing occupancy in all
affected regions each time to determine if and when it has managed to
increase overall occupancy. If it does, changes are committed to the
scheduler's state; otherwise modifications to the IR are reverted and
the scheduling stage gives up.

In both cases, this scheduling stage currently involves repeated queries
for up-to-date occupancy estimates and some state copying to enable
reversal of sinking decisions when occupancy is revealed not to
increase. The current implementation also does not accurately track
register pressure changes in all regions affected by sinking decisions.

This commit refactors this scheduling stage, improving RP tracking and
splitting the stage into two distinct steps to avoid repeated occupancy
queries and IR/state rollbacks.

- Analysis and collection (`canIncreaseOccupancyOrReduceSpill`). The
number of ArchVGPRs to save to reduce spilling or increase function
occupancy by 1 (when there is no spilling) is computed. Then,
instructions eligible for rematerialization are collected, stopping as
soon as enough have been identified to be able to achieve our goal
(according to slightly optimistic heuristics). If there aren't enough of
such instructions, the scheduling stage stops here.
- Rematerialization (`rematerialize`). Instructions collected in the
first step are rematerialized one-by-one. Now we are able to directly
update the scheduler's state since we have already done the occupancy
analysis and know we won't have to rollback any state. Register
pressures for impacted regions are recomputed only once, as opposed to
at every sinking decision.

In the case where the stage attempted to increase occupancy, and if both
rematerializations alone and rescheduling after were unable to improve
occupancy, then all rematerializations are rollbacked.

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineRegisterInfo.cpp b/llvm/lib/CodeGen/MachineRegisterInfo.cpp
index 937f63f..b713525 100644
--- a/llvm/lib/CodeGen/MachineRegisterInfo.cpp
+++ b/llvm/lib/CodeGen/MachineRegisterInfo.cpp
@@ -432,6 +432,11 @@ bool MachineRegisterInfo::hasOneNonDBGUser(Register RegNo) const {
   return hasSingleElement(use_nodbg_instructions(RegNo));
 }
 
+MachineInstr *MachineRegisterInfo::getOneNonDBGUser(Register RegNo) const {
+  auto RegNoDbgUsers = use_nodbg_instructions(RegNo);
+  return hasSingleElement(RegNoDbgUsers) ? &*RegNoDbgUsers.begin() : nullptr;
+}
+
 bool MachineRegisterInfo::hasAtMostUserInstrs(Register Reg,
                                               unsigned MaxUsers) const {
   return hasNItemsOrLess(use_instr_nodbg_begin(Reg), use_instr_nodbg_end(),