[AMDGPU] Mitigate GFX12 VALU read SGPR hazard

Any SGPR read by a VALU can potentially obscure SALU writes to the
same register.  Insert s_wait_alu instructions to mitigate the
hazard on affected paths.

Compute a global cache of SGPRs with any VALU reads and use this to
avoid inserting mitigation for SGPRs never accessed by VALUs.

To avoid excessive search when compile time is priority implement
secondary mode where all SALU writes are mitigated.

Author: perlfu

llvm/lib/Target/AMDGPU/GCNHazardRecognizer.cpp

@@ -14,6 +14,7 @@
 #include "GCNSubtarget.h"
 #include "MCTargetDesc/AMDGPUMCTargetDesc.h"
 #include "SIMachineFunctionInfo.h"
+#include "llvm/ADT/PostOrderIterator.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/ScheduleDAG.h"
@@ -44,22 +45,22 @@ static cl::opt<unsigned, false, MFMAPaddingRatioParser>
                      cl::desc("Fill a percentage of the latency between "
                               "neighboring MFMA with s_nops."));
 
+static cl::opt<unsigned> MaxExhaustiveHazardSearch(
+    "amdgpu-max-exhaustive-hazard-search", cl::init(128), cl::Hidden,
+    cl::desc("Maximum function size for exhausive hazard search"));
+
 //===----------------------------------------------------------------------===//
 // Hazard Recognizer Implementation
 //===----------------------------------------------------------------------===//
 
 static bool shouldRunLdsBranchVmemWARHazardFixup(const MachineFunction &MF,
                                                  const GCNSubtarget &ST);
 
-GCNHazardRecognizer::GCNHazardRecognizer(const MachineFunction &MF) :
-  IsHazardRecognizerMode(false),
-  CurrCycleInstr(nullptr),
-  MF(MF),
-  ST(MF.getSubtarget<GCNSubtarget>()),
-  TII(*ST.getInstrInfo()),
-  TRI(TII.getRegisterInfo()),
-  ClauseUses(TRI.getNumRegUnits()),
-  ClauseDefs(TRI.getNumRegUnits()) {
+GCNHazardRecognizer::GCNHazardRecognizer(const MachineFunction &MF)
+    : IsHazardRecognizerMode(false), CurrCycleInstr(nullptr), MF(MF),
+      ST(MF.getSubtarget<GCNSubtarget>()), TII(*ST.getInstrInfo()),
+      TRI(TII.getRegisterInfo()), UseVALUReadHazardExhaustiveSearch(false),
+      ClauseUses(TRI.getNumRegUnits()), ClauseDefs(TRI.getNumRegUnits()) {
   MaxLookAhead = MF.getRegInfo().isPhysRegUsed(AMDGPU::AGPR0) ? 19 : 5;
   TSchedModel.init(&ST);
   RunLdsBranchVmemWARHazardFixup = shouldRunLdsBranchVmemWARHazardFixup(MF, ST);
@@ -1195,6 +1196,7 @@ void GCNHazardRecognizer::fixHazards(MachineInstr *MI) {
   fixWMMAHazards(MI);
   fixShift64HighRegBug(MI);
   fixVALUMaskWriteHazard(MI);
+  fixVALUReadSGPRHazard(MI);
   fixRequiredExportPriority(MI);
 }
 
@@ -3010,6 +3012,263 @@ bool GCNHazardRecognizer::fixVALUMaskWriteHazard(MachineInstr *MI) {
   return true;
 }
 
+static unsigned baseSGPRNumber(Register Reg, const SIRegisterInfo &TRI) {
+  unsigned RegN = TRI.getEncodingValue(Reg);
+  assert(RegN <= 127);
+  return (RegN >> 1) & 0x3f;
+}
+
+// For VALUReadSGPRHazard: pre-compute a bit vector of all SGPRs used by VALUs.
+void GCNHazardRecognizer::computeVALUHazardSGPRs(MachineFunction *MMF) {
+  assert(MMF == &MF);
+
+  // Assume non-empty vector means it has already been computed.
+  if (!VALUReadHazardSGPRs.empty())
+    return;
+
+  auto CallingConv = MF.getFunction().getCallingConv();
+  bool IsCallFree =
+      AMDGPU::isEntryFunctionCC(CallingConv) && !MF.getFrameInfo().hasCalls();
+
+  // Exhaustive search is only viable in non-caller/callee functions where
+  // VALUs will be exposed to the hazard recognizer.
+  UseVALUReadHazardExhaustiveSearch =
+      IsCallFree && MF.getTarget().getOptLevel() > CodeGenOptLevel::None &&
+      MF.getInstructionCount() <= MaxExhaustiveHazardSearch;
+
+  // Consider all SGPRs hazards if the shader uses function calls or is callee.
+  bool UseVALUUseCache =
+      IsCallFree && MF.getTarget().getOptLevel() > CodeGenOptLevel::None;
+  VALUReadHazardSGPRs.resize(64, !UseVALUUseCache);
+  if (!UseVALUUseCache)
+    return;
+
+  // Perform a post ordered reverse scan to find VALUs which read an SGPR
+  // before a SALU write to the same SGPR.  This provides a reduction in
+  // hazard insertion when all VALU access to an SGPR occurs after its last
+  // SALU write, when compared to a linear scan.
+  const unsigned SGPR_NULL = TRI.getEncodingValue(AMDGPU::SGPR_NULL_gfx11plus);
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+  BitVector SALUWriteSGPRs(64), ReadSGPRs(64);
+  MachineCycleInfo CI;
+  CI.compute(*MMF);
+
+  for (auto *MBB : post_order(&MF)) {
+    bool InCycle = CI.getCycle(MBB) != nullptr;
+    for (auto &MI : reverse(MBB->instrs())) {
+      bool IsVALU = SIInstrInfo::isVALU(MI);
+      bool IsSALU = SIInstrInfo::isSALU(MI);
+      if (!(IsVALU || IsSALU))
+        continue;
+
+      for (const MachineOperand &Op : MI.operands()) {
+        if (!Op.isReg())
+          continue;
+        Register Reg = Op.getReg();
+        // Only consider implicit operands of VCC.
+        if (Op.isImplicit() && !(Reg == AMDGPU::VCC_LO ||
+                                 Reg == AMDGPU::VCC_HI || Reg == AMDGPU::VCC))
+          continue;
+        if (!TRI.isSGPRReg(MRI, Reg))
+          continue;
+        if (TRI.getEncodingValue(Reg) >= SGPR_NULL)
+          continue;
+        unsigned RegN = baseSGPRNumber(Reg, TRI);
+        if (IsVALU && Op.isUse()) {
+          // Note: any access within a cycle must be considered a hazard.
+          if (InCycle || (ReadSGPRs[RegN] && SALUWriteSGPRs[RegN]))
+            VALUReadHazardSGPRs.set(RegN);
+          ReadSGPRs.set(RegN);
+        } else if (IsSALU) {
+          if (Op.isDef())
+            SALUWriteSGPRs.set(RegN);
+          else
+            ReadSGPRs.set(RegN);
+        }
+      }
+    }
+  }
+}
+
+bool GCNHazardRecognizer::fixVALUReadSGPRHazard(MachineInstr *MI) {
+  if (!ST.hasVALUReadSGPRHazard())
+    return false;
+
+  // The hazard sequence is fundamentally three instructions:
+  //   1. VALU reads SGPR
+  //   2. SALU writes SGPR
+  //   3. VALU/SALU reads SGPR
+  // Try to avoid searching for (1) because the expiry point of the hazard is
+  // indeterminate; however, the hazard between (2) and (3) can expire if the
+  // gap contains sufficient SALU instructions with no usage of SGPR from (1).
+  // Note: SGPRs must be considered as 64-bit pairs as hazard exists
+  // even if individual SGPRs are accessed.
+
+  bool MIIsSALU = SIInstrInfo::isSALU(*MI);
+  bool MIIsVALU = SIInstrInfo::isVALU(*MI);
+  if (!(MIIsSALU || MIIsVALU))
+    return false;
+
+  // Avoid expensive search when compile time is priority by
+  // mitigating every SALU which writes an SGPR.
+  if (MF.getTarget().getOptLevel() == CodeGenOptLevel::None) {
+    if (!SIInstrInfo::isSALU(*MI) || SIInstrInfo::isSOPP(*MI))
+      return false;
+
+    const MachineOperand *SDSTOp =
+        TII.getNamedOperand(*MI, AMDGPU::OpName::sdst);
+    if (!SDSTOp || !SDSTOp->isReg())
+      return false;
+
+    const Register HazardReg = SDSTOp->getReg();
+    if (HazardReg == AMDGPU::EXEC || HazardReg == AMDGPU::EXEC_LO ||
+        HazardReg == AMDGPU::EXEC_HI || HazardReg == AMDGPU::M0)
+      return false;
+
+    // Add s_wait_alu sa_sdst(0) after SALU write.
+    auto NextMI = std::next(MI->getIterator());
+    auto NewMI = BuildMI(*MI->getParent(), NextMI, MI->getDebugLoc(),
+                         TII.get(AMDGPU::S_WAITCNT_DEPCTR))
+                     .addImm(AMDGPU::DepCtr::encodeFieldSaSdst(0));
+
+    // SALU write may be s_getpc in a bundle.
+    updateGetPCBundle(NewMI);
+
+    return true;
+  }
+
+  // Pre-compute set of SGPR pairs read by VALUs.
+  // Note: pass mutable pointer to MachineFunction for CycleInfo.
+  computeVALUHazardSGPRs(MI->getMF());
+
+  // If no VALUs hazard SGPRs exist then nothing to do.
+  if (VALUReadHazardSGPRs.none())
+    return false;
+
+  // All SGPR writes before a call/return must be flushed as the callee/caller
+  // will not will not see the hazard chain, i.e. (2) to (3) described above.
+  const bool IsSetPC = (MI->getOpcode() == AMDGPU::S_SETPC_B64 ||
+                        MI->getOpcode() == AMDGPU::S_SETPC_B64_return ||
+                        MI->getOpcode() == AMDGPU::S_SWAPPC_B64 ||
+                        MI->getOpcode() == AMDGPU::S_CALL_B64);
+
+  // Collect all SGPR sources for MI which are read by a VALU.
+  const unsigned SGPR_NULL = TRI.getEncodingValue(AMDGPU::SGPR_NULL_gfx11plus);
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+  SmallSet<Register, 4> SGPRsUsed;
+
+  if (!IsSetPC) {
+    for (const MachineOperand &Op : MI->all_uses()) {
+      Register OpReg = Op.getReg();
+
+      // Only consider VCC implicit uses on VALUs.
+      // The only expected SALU implicit access is SCC which is no hazard.
+      if (MIIsSALU && Op.isImplicit())
+        continue;
+
+      if (!TRI.isSGPRReg(MRI, OpReg))
+        continue;
+
+      // Ignore special purposes registers such as NULL, EXEC, and M0.
+      if (TRI.getEncodingValue(OpReg) >= SGPR_NULL)
+        continue;
+
+      unsigned RegN = baseSGPRNumber(OpReg, TRI);
+      if (!VALUReadHazardSGPRs[RegN])
+        continue;
+
+      SGPRsUsed.insert(OpReg);
+    }
+
+    // No SGPRs -> nothing to do.
+    if (SGPRsUsed.empty())
+      return false;
+  }
+
+  // A hazard is any SALU which writes one of the SGPRs read by MI.
+  auto IsHazardFn = [this, IsSetPC, &SGPRsUsed](const MachineInstr &I) {
+    if (!SIInstrInfo::isSALU(I))
+      return false;
+    // Ensure SGPR flush before call/return by conservatively assuming every
+    // SALU writes an SGPR.
+    if (IsSetPC && I.getNumDefs() > 0)
+      return true;
+    // Check for any register writes.
+    return llvm::any_of(SGPRsUsed, [this, &I](Register Reg) {
+      return I.modifiesRegister(Reg, &TRI);
+    });
+  };
+
+  const int SALUExpiryCount = SIInstrInfo::isSALU(*MI) ? 10 : 11;
+  auto IsExpiredFn = [&](const MachineInstr &I, int Count) {
+    if (Count >= SALUExpiryCount)
+      return true;
+    // s_wait_alu sa_sdst(0) on path mitigates hazard.
+    if (I.getOpcode() == AMDGPU::S_WAITCNT_DEPCTR &&
+        AMDGPU::DepCtr::decodeFieldSaSdst(I.getOperand(0).getImm()) == 0)
+      return true;
+    return false;
+  };
+
+  auto WaitStatesFn = [this, &SGPRsUsed](const MachineInstr &I) {
+    // Only count true SALUs as wait states.
+    if (!SIInstrInfo::isSALU(I) || SIInstrInfo::isSOPP(I))
+      return 0;
+    // SALU must be unrelated to any hazard registers.
+    if (llvm::any_of(SGPRsUsed, [this, &I](Register Reg) {
+          return I.readsRegister(Reg, &TRI);
+        }))
+      return 0;
+    return 1;
+  };
+
+  // Check for the hazard.
+  DenseSet<const MachineBasicBlock *> Visited;
+  int WaitStates = ::getWaitStatesSince(IsHazardFn, MI->getParent(),
+                                        std::next(MI->getReverseIterator()), 0,
+                                        IsExpiredFn, Visited, WaitStatesFn);
+
+  if (WaitStates >= SALUExpiryCount)
+    return false;
+
+  // Validate hazard through an exhaustive search.
+  if (UseVALUReadHazardExhaustiveSearch) {
+    // A hazard is any VALU which reads one of the paired SGPRs read by MI.
+    // This is searching for (1) in the hazard description.
+    auto hazardPair = [this](Register Reg) {
+      if (Reg == AMDGPU::VCC || Reg == AMDGPU::VCC_LO || Reg == AMDGPU::VCC_HI)
+        return Register(AMDGPU::VCC);
+      // TODO: handle TTMP?
+      return Register(AMDGPU::SGPR0_SGPR1 + baseSGPRNumber(Reg, TRI));
+    };
+    auto SearchHazardFn = [this, hazardPair,
+                           &SGPRsUsed](const MachineInstr &I) {
+      if (!SIInstrInfo::isVALU(I))
+        return false;
+      // Check for any register reads.
+      return llvm::any_of(SGPRsUsed, [this, hazardPair, &I](Register Reg) {
+        return I.readsRegister(hazardPair(Reg), &TRI);
+      });
+    };
+    auto SearchExpiredFn = [&](const MachineInstr &I, int Count) {
+      return false;
+    };
+    if (::getWaitStatesSince(SearchHazardFn, MI, SearchExpiredFn) ==
+        std::numeric_limits<int>::max())
+      return false;
+  }
+
+  // Add s_wait_alu sa_sdst(0) before SALU read.
+  auto NewMI = BuildMI(*MI->getParent(), MI, MI->getDebugLoc(),
+                       TII.get(AMDGPU::S_WAITCNT_DEPCTR))
+                   .addImm(AMDGPU::DepCtr::encodeFieldSaSdst(0));
+
+  // SALU read may be after s_getpc in a bundle.
+  updateGetPCBundle(NewMI);
+
+  return true;
+}
+
 static bool ensureEntrySetPrio(MachineFunction *MF, int Priority,
                                const SIInstrInfo &TII) {
   MachineBasicBlock &EntryMBB = MF->front();

llvm/lib/Target/AMDGPU/GCNHazardRecognizer.h

@@ -48,6 +48,8 @@ class GCNHazardRecognizer final : public ScheduleHazardRecognizer {
   const SIRegisterInfo &TRI;
   TargetSchedModel TSchedModel;
   bool RunLdsBranchVmemWARHazardFixup;
+  BitVector VALUReadHazardSGPRs;
+  bool UseVALUReadHazardExhaustiveSearch;
 
   /// RegUnits of uses in the current soft memory clause.
   BitVector ClauseUses;
@@ -107,6 +109,8 @@ class GCNHazardRecognizer final : public ScheduleHazardRecognizer {
   bool fixWMMAHazards(MachineInstr *MI);
   bool fixShift64HighRegBug(MachineInstr *MI);
   bool fixVALUMaskWriteHazard(MachineInstr *MI);
+  void computeVALUHazardSGPRs(MachineFunction *MMF);
+  bool fixVALUReadSGPRHazard(MachineInstr *MI);
   bool fixRequiredExportPriority(MachineInstr *MI);
 
   int checkMAIHazards(MachineInstr *MI);

llvm/lib/Target/AMDGPU/GCNSubtarget.h

@@ -1247,6 +1247,8 @@ class GCNSubtarget final : public AMDGPUGenSubtargetInfo,
 
   bool hasVALUMaskWriteHazard() const { return getGeneration() == GFX11; }
 
+  bool hasVALUReadSGPRHazard() const { return getGeneration() == GFX12; }
+
   /// Return if operations acting on VGPR tuples require even alignment.
   bool needsAlignedVGPRs() const { return GFX90AInsts; }