[llvm] [AMDGPU] New AMDGPUInsertSingleUseVDST pass (PR #72388)

[Carl Ritson via llvm-commits]

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+//===- AMDGPUInsertSingleUseVDST.cpp - Insert s_singleuse_vdst instructions ==//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// Insert s_singleuse_vdst instructions on GFX11.5+ to mark regions of VALU
+/// instructions that produce single-use VGPR values. If the value is forwarded
+/// to the consumer instruction prior to VGPR writeback, the hardware can
+/// then skip (kill) the VGPR write.
+//
+//===----------------------------------------------------------------------===//
+
+#include "AMDGPU.h"
+#include "GCNSubtarget.h"
+#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
+#include "SIInstrInfo.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineOperand.h"
+#include "llvm/CodeGen/Register.h"
+#include "llvm/CodeGen/TargetSubtargetInfo.h"
+#include "llvm/IR/DebugLoc.h"
+#include "llvm/MC/MCRegister.h"
+#include "llvm/Pass.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "amdgpu-insert-single-use-vdst"
+
+namespace {
+class AMDGPUInsertSingleUseVDST : public MachineFunctionPass {
+private:
+  const SIInstrInfo *SII;
+
+public:
+  static char ID;
+
+  AMDGPUInsertSingleUseVDST() : MachineFunctionPass(ID) {}
+
+  void emitSingleUseVDST(MachineInstr &MI) const {
+    BuildMI(*MI.getParent(), MI, DebugLoc(), SII->get(AMDGPU::S_SINGLEUSE_VDST))
+        .addImm(0x1);
+  }
+
+  bool runOnMachineFunction(MachineFunction &MF) override {
+    const auto &ST = MF.getSubtarget<GCNSubtarget>();
+    if (!ST.hasVGPRSingleUseHintInsts())
+      return false;
+
+    SII = ST.getInstrInfo();
+    const auto *TRI = MF.getSubtarget().getRegisterInfo();
+    bool InstructionEmitted = false;
+
+    for (MachineBasicBlock &MBB : MF) {
+      DenseMap<MCPhysReg, unsigned> RegisterUseCount; // TODO: MCRegUnits
+
+      // Handle boundaries at the end of basic block separately to avoid
+      // false positives. If they are live at the end of a basic block then
+      // assume it has more uses later on.
+      for (const auto &Liveouts : MBB.liveouts())
+        RegisterUseCount[Liveouts.PhysReg] = 2;
+
+      for (MachineInstr &MI : reverse(MBB.instrs())) {
+        // All registers in all operands need to be single use for an
+        // 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())
+            RegisterUseCount[Reg]++;
+
+          // Do not attempt to optimise across exec mask changes.
+          if (MI.modifiesRegister(AMDGPU::EXEC, TRI)) {
+            for (auto &UsedReg : RegisterUseCount)
+              UsedReg.second = 2;
+          }
+
+          // 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;
+          if (RegisterUseCount[Reg] > 1)
+            AllProducerOperandsAreSingleUse = false;
+          // Reset uses count when a register is no longer live.
+          RegisterUseCount.erase(Reg);
+        }
+        if (AllProducerOperandsAreSingleUse && SIInstrInfo::isVALU(MI)) {
+          // TODO: Replace with candidate logging for instruction grouping
+          // later.
+          emitSingleUseVDST(MI);
----------------
perlfu wrote:

Does this work with bundles -- do that need a test?

https://github.com/llvm/llvm-project/pull/72388