[llvm] [AMPGPU] Emit s_singleuse_vdst instructions when a register is used multiple times in the same instruction. (PR #89601)

[Jay Foad via llvm-commits]

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@@ -83,37 +83,42 @@ class AMDGPUInsertSingleUseVDST : public MachineFunctionPass {
         // instruction to be marked as a single use producer.
         bool AllProducerOperandsAreSingleUse = true;
 
-        for (const auto &Operand : MI.operands()) {
-          if (!Operand.isReg())
-            continue;
-          const auto Reg = Operand.getReg();
-
-          // Count the number of times each register is read.
-          if (Operand.readsReg())
-            for (const MCRegUnit &Unit : TRI->regunits(Reg))
-              RegisterUseCount[Unit]++;
-
-          // Do not attempt to optimise across exec mask changes.
-          if (MI.modifiesRegister(AMDGPU::EXEC, TRI)) {
-            for (auto &UsedReg : RegisterUseCount)
-              UsedReg.second = 2;
-          }
+        // Gather a list of Registers used before updating use counts to avoid
+        // double counting registers that appear multiple times in a single
+        // MachineInstr.
+        SmallVector<MCRegUnit> RegistersUsed;
 
-          // If we are at the point where the register first became live,
-          // check if the operands are single use.
-          if (!MI.modifiesRegister(Reg, TRI))
-            continue;
+        for (const auto &Operand : MI.all_defs()) {
+          const auto Reg = Operand.getReg();
 
           const auto RegUnits = TRI->regunits(Reg);
-          if (any_of(RegUnits, [&RegisterUseCount](const MCRegUnit &Unit) {
+          if (any_of(RegUnits, [&RegisterUseCount](const MCRegUnit Unit) {
                 return RegisterUseCount[Unit] > 1;
               }))
             AllProducerOperandsAreSingleUse = false;
 
           // Reset uses count when a register is no longer live.
-          for (const MCRegUnit &Unit : RegUnits)
+          for (const MCRegUnit Unit : RegUnits)
             RegisterUseCount.erase(Unit);
         }
+
+        for (const auto &Operand : MI.all_uses()) {
+          const auto Reg = Operand.getReg();
+
+          // Count the number of times each register is read.
+          for (const MCRegUnit Unit : TRI->regunits(Reg)) {
+            if (!llvm::is_contained(RegistersUsed, Unit))
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jayfoad wrote:

Don't need the `llvm::`

https://github.com/llvm/llvm-project/pull/89601