[llvm] [AMDGPU][CodeGen] Allow remat with multiple users in multiple regions (PR #215598)

[Matt Arsenault via llvm-commits]

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@@ -1593,40 +1590,57 @@ bool PreRARematStage::initGCNSchedStage() {
                [](const MachineInstr *DefMI) { return DefMI->isConvergent(); }))
       continue;
 
-    // We further filter the registers that we can rematerialize based on our
-    // current tracking capabilities in the stage. Users cannot themselves be
-    // marked rematerializable, and no register operand of the defining MI can
-    // be marked rematerializable. We also do not rematerialize an instruction
-    // if it uses registers that aren't available at its use. This ensures that
-    // we are not extending any live range while rematerializing.
-    if (llvm::any_of(Users, [&MarkedRegs](const MachineInstr *UserMI) {
-          assert(UserMI->getNumOperands() > 0 &&
-                 "user must have at least one operand");
-          const MachineOperand &UseMO = UserMI->getOperand(0);
-          return UseMO.isReg() && MarkedRegs.contains(UseMO.getReg());
-        }))
-      continue;
-    MachineInstr *FirstUseMI =
-        CandReg.getRegionUseBounds(UseRegion, *DAG.LIS).first;
-    assert(FirstUseMI && "there must be a user in the region");
-    SlotIndex FirstUseIdx =
-        DAG.LIS->getInstructionIndex(*FirstUseMI).getRegSlot(true);
     SlotIndex RefIdx =
         DAG.LIS->getInstructionIndex(*CandReg.getLastDef()).getRegSlot(true);
+
+    // We further filter the registers that we can rematerialize based on our
+    // current tracking capabilities in the stage. This ensures that we are not
+    // extending any live range while rematerializing, and that rematerializing
+    // any register candidate will not affect other candidates.
+
+    // All register dependencies should not already be considered
+    // rematerializable.
     if (llvm::any_of(CandReg.Dependencies, [&](RegisterIdx DepRegIdx) {
-          const Rematerializer::Reg &DepReg = Remater.getReg(DepRegIdx);
-          Register DepDefReg = DepReg.getDefReg();
-          return MarkedRegs.contains(DepDefReg) ||
-                 !Remater.isRegIdenticalAtUses(DepDefReg, DepReg.Mask, RefIdx,
-                                               {FirstUseIdx});
+          return MarkedRegs.contains(Remater.getReg(DepRegIdx).getDefReg());
         }))
       continue;
-    if (llvm::any_of(Remater.getUnrematableDeps(RegIdx),
-                     [&](const std::pair<Register, LaneBitmask> &RegAndMask) {
-                       const auto &[Reg, Mask] = RegAndMask;
-                       return !Remater.isRegIdenticalAtUses(Reg, Mask, RefIdx,
-                                                            {FirstUseIdx});
-                     }))
+
+    auto IsInvalidUsingRegion =
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arsenm wrote:

This is too big for a lamba 

https://github.com/llvm/llvm-project/pull/215598