AMDGPU/GlobalISel: AMDGPURegBankSelect (llvm#112863)

Assign register banks to virtual registers. Does not use generic
RegBankSelect. After register bank selection all register operand of
G_ instructions have LLT and register banks exclusively. If they had
register class, reassign appropriate register bank.

Assign register banks using machine uniformity analysis:
Sgpr - uniform values and some lane masks
Vgpr - divergent, non S1, values
Vcc  - divergent S1 values(lane masks)

AMDGPURegBankSelect does not consider available instructions and, in
some cases, G_ instructions with some register bank assignment can't be
inst-selected. This is solved in RegBankLegalize.

Exceptions when uniformity analysis does not work:
S32/S64 lane masks:
- need to end up with sgpr register class after instruction selection
- In most cases Uniformity analysis declares them as uniform
  (forced by tablegen) resulting in sgpr S32/S64 reg bank
- When Uniformity analysis declares them as divergent (some phis),
  use intrinsic lane mask analyzer to still assign sgpr register bank
temporal divergence copy:
- COPY to vgpr with implicit use of $exec inside of the cycle
- this copy is declared as uniform by uniformity analysis
- make sure that assigned bank is vgpr
Note: uniformity analysis does not consider that registers with vgpr def
are divergent (you can have uniform value in vgpr).
- TODO: implicit use of $exec could be implemented as indicator
  that instruction is divergent

Author: petar-avramovic

llvm/lib/Target/AMDGPU/AMDGPUGlobalISelUtils.cpp

@@ -7,12 +7,16 @@
 //===----------------------------------------------------------------------===//
 
 #include "AMDGPUGlobalISelUtils.h"
+#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
 #include "llvm/CodeGen/GlobalISel/GISelKnownBits.h"
+#include "llvm/CodeGen/GlobalISel/GenericMachineInstrs.h"
 #include "llvm/CodeGen/GlobalISel/MIPatternMatch.h"
 #include "llvm/CodeGenTypes/LowLevelType.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/IR/IntrinsicsAMDGPU.h"
 
 using namespace llvm;
+using namespace AMDGPU;
 using namespace MIPatternMatch;
 
 std::pair<Register, unsigned>
@@ -68,3 +72,37 @@ AMDGPU::getBaseWithConstantOffset(MachineRegisterInfo &MRI, Register Reg,
 
   return std::pair(Reg, 0);
 }
+
+IntrinsicLaneMaskAnalyzer::IntrinsicLaneMaskAnalyzer(MachineFunction &MF)
+    : MRI(MF.getRegInfo()) {
+  initLaneMaskIntrinsics(MF);
+}
+
+bool IntrinsicLaneMaskAnalyzer::isS32S64LaneMask(Register Reg) const {
+  return S32S64LaneMask.contains(Reg);
+}
+
+void IntrinsicLaneMaskAnalyzer::initLaneMaskIntrinsics(MachineFunction &MF) {
+  for (auto &MBB : MF) {
+    for (auto &MI : MBB) {
+      GIntrinsic *GI = dyn_cast<GIntrinsic>(&MI);
+      if (GI && GI->is(Intrinsic::amdgcn_if_break)) {
+        S32S64LaneMask.insert(MI.getOperand(3).getReg());
+        findLCSSAPhi(MI.getOperand(0).getReg());
+      }
+
+      if (MI.getOpcode() == AMDGPU::SI_IF ||
+          MI.getOpcode() == AMDGPU::SI_ELSE) {
+        findLCSSAPhi(MI.getOperand(0).getReg());
+      }
+    }
+  }
+}
+
+void IntrinsicLaneMaskAnalyzer::findLCSSAPhi(Register Reg) {
+  S32S64LaneMask.insert(Reg);
+  for (const MachineInstr &LCSSAPhi : MRI.use_instructions(Reg)) {
+    if (LCSSAPhi.isPHI())
+      S32S64LaneMask.insert(LCSSAPhi.getOperand(0).getReg());
+  }
+}

llvm/lib/Target/AMDGPU/AMDGPUGlobalISelUtils.h

@@ -9,6 +9,7 @@
 #ifndef LLVM_LIB_TARGET_AMDGPU_AMDGPUGLOBALISELUTILS_H
 #define LLVM_LIB_TARGET_AMDGPU_AMDGPUGLOBALISELUTILS_H
 
+#include "llvm/ADT/DenseSet.h"
 #include "llvm/CodeGen/Register.h"
 #include <utility>
 
@@ -18,6 +19,7 @@ class MachineRegisterInfo;
 class GCNSubtarget;
 class GISelKnownBits;
 class LLT;
+class MachineFunction;
 
 namespace AMDGPU {
 
@@ -26,6 +28,26 @@ std::pair<Register, unsigned>
 getBaseWithConstantOffset(MachineRegisterInfo &MRI, Register Reg,
                           GISelKnownBits *KnownBits = nullptr,
                           bool CheckNUW = false);
+
+// Currently finds S32/S64 lane masks that can be declared as divergent by
+// uniformity analysis (all are phis at the moment).
+// These are defined as i32/i64 in some IR intrinsics (not as i1).
+// Tablegen forces(via telling that lane mask IR intrinsics are uniform) most of
+// S32/S64 lane masks to be uniform, as this results in them ending up with sgpr
+// reg class after instruction-select, don't search for all of them.
+class IntrinsicLaneMaskAnalyzer {
+  SmallDenseSet<Register, 8> S32S64LaneMask;
+  MachineRegisterInfo &MRI;
+
+public:
+  IntrinsicLaneMaskAnalyzer(MachineFunction &MF);
+  bool isS32S64LaneMask(Register Reg) const;
+
+private:
+  void initLaneMaskIntrinsics(MachineFunction &MF);
+  // This will not be needed when we turn off LCSSA for global-isel.
+  void findLCSSAPhi(Register Reg);
+};
 }
 }
 

llvm/lib/Target/AMDGPU/AMDGPURegBankSelect.cpp

@@ -16,12 +16,18 @@
 //===----------------------------------------------------------------------===//
 
 #include "AMDGPU.h"
-#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "AMDGPUGlobalISelUtils.h"
+#include "GCNSubtarget.h"
+#include "llvm/CodeGen/GlobalISel/CSEInfo.h"
+#include "llvm/CodeGen/GlobalISel/CSEMIRBuilder.h"
+#include "llvm/CodeGen/MachineUniformityAnalysis.h"
+#include "llvm/CodeGen/TargetPassConfig.h"
 #include "llvm/InitializePasses.h"
 
 #define DEBUG_TYPE "amdgpu-regbankselect"
 
 using namespace llvm;
+using namespace AMDGPU;
 
 namespace {
 
@@ -40,6 +46,9 @@ class AMDGPURegBankSelect : public MachineFunctionPass {
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<TargetPassConfig>();
+    AU.addRequired<GISelCSEAnalysisWrapperPass>();
+    AU.addRequired<MachineUniformityAnalysisPass>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 
@@ -55,6 +64,9 @@ class AMDGPURegBankSelect : public MachineFunctionPass {
 
 INITIALIZE_PASS_BEGIN(AMDGPURegBankSelect, DEBUG_TYPE,
                       "AMDGPU Register Bank Select", false, false)
+INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
+INITIALIZE_PASS_DEPENDENCY(GISelCSEAnalysisWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(MachineUniformityAnalysisPass)
 INITIALIZE_PASS_END(AMDGPURegBankSelect, DEBUG_TYPE,
                     "AMDGPU Register Bank Select", false, false)
 
@@ -66,9 +78,201 @@ FunctionPass *llvm::createAMDGPURegBankSelectPass() {
   return new AMDGPURegBankSelect();
 }
 
+class RegBankSelectHelper {
+  MachineIRBuilder &B;
+  MachineRegisterInfo &MRI;
+  AMDGPU::IntrinsicLaneMaskAnalyzer &ILMA;
+  const MachineUniformityInfo &MUI;
+  const RegisterBank *SgprRB;
+  const RegisterBank *VgprRB;
+  const RegisterBank *VccRB;
+
+public:
+  RegBankSelectHelper(MachineIRBuilder &B,
+                      AMDGPU::IntrinsicLaneMaskAnalyzer &ILMA,
+                      const MachineUniformityInfo &MUI,
+                      const RegisterBankInfo &RBI)
+      : B(B), MRI(*B.getMRI()), ILMA(ILMA), MUI(MUI),
+        SgprRB(&RBI.getRegBank(AMDGPU::SGPRRegBankID)),
+        VgprRB(&RBI.getRegBank(AMDGPU::VGPRRegBankID)),
+        VccRB(&RBI.getRegBank(AMDGPU::VCCRegBankID)) {}
+
+  const RegisterBank *getRegBankToAssign(Register Reg) {
+    if (MUI.isUniform(Reg) || ILMA.isS32S64LaneMask(Reg))
+      return SgprRB;
+    if (MRI.getType(Reg) == LLT::scalar(1))
+      return VccRB;
+    return VgprRB;
+  }
+
+  // %rc:RegClass(s32) = G_ ...
+  // ...
+  // %a = G_ ..., %rc
+  // ->
+  // %rb:RegBank(s32) = G_ ...
+  // %rc:RegClass(s32) = COPY %rb
+  // ...
+  // %a = G_ ..., %rb
+  void reAssignRegBankOnDef(MachineInstr &MI, MachineOperand &DefOP,
+                            const RegisterBank *RB) {
+    // Register that already has Register class got it during pre-inst selection
+    // of another instruction. Maybe cross bank copy was required so we insert a
+    // copy that can be removed later. This simplifies post regbanklegalize
+    // combiner and avoids need to special case some patterns.
+    Register Reg = DefOP.getReg();
+    LLT Ty = MRI.getType(Reg);
+    Register NewReg = MRI.createVirtualRegister({RB, Ty});
+    DefOP.setReg(NewReg);
+
+    auto &MBB = *MI.getParent();
+    B.setInsertPt(MBB, MBB.SkipPHIsAndLabels(std::next(MI.getIterator())));
+    B.buildCopy(Reg, NewReg);
+
+    // The problem was discovered for uniform S1 that was used as both
+    // lane mask(vcc) and regular sgpr S1.
+    // - lane-mask(vcc) use was by si_if, this use is divergent and requires
+    //   non-trivial sgpr-S1-to-vcc copy. But pre-inst-selection of si_if sets
+    //   sreg_64_xexec(S1) on def of uniform S1 making it lane-mask.
+    // - the regular sgpr S1(uniform) instruction is now broken since
+    //   it uses sreg_64_xexec(S1) which is divergent.
+
+    // Replace virtual registers with register class on generic instructions
+    // uses with virtual registers with register bank.
+    for (auto &UseMI : make_early_inc_range(MRI.use_instructions(Reg))) {
+      if (UseMI.isPreISelOpcode()) {
+        for (MachineOperand &Op : UseMI.operands()) {
+          if (Op.isReg() && Op.getReg() == Reg)
+            Op.setReg(NewReg);
+        }
+      }
+    }
+  }
+
+  // %a = G_ ..., %rc
+  // ->
+  // %rb:RegBank(s32) = COPY %rc
+  // %a = G_ ..., %rb
+  void constrainRegBankUse(MachineInstr &MI, MachineOperand &UseOP,
+                           const RegisterBank *RB) {
+    Register Reg = UseOP.getReg();
+
+    LLT Ty = MRI.getType(Reg);
+    Register NewReg = MRI.createVirtualRegister({RB, Ty});
+    UseOP.setReg(NewReg);
+
+    if (MI.isPHI()) {
+      auto DefMI = MRI.getVRegDef(Reg)->getIterator();
+      MachineBasicBlock *DefMBB = DefMI->getParent();
+      B.setInsertPt(*DefMBB, DefMBB->SkipPHIsAndLabels(std::next(DefMI)));
+    } else {
+      B.setInstr(MI);
+    }
+
+    B.buildCopy(NewReg, Reg);
+  }
+};
+
+static Register getVReg(MachineOperand &Op) {
+  if (!Op.isReg())
+    return {};
+
+  // Operands of COPY and G_SI_CALL can be physical registers.
+  Register Reg = Op.getReg();
+  if (!Reg.isVirtual())
+    return {};
+
+  return Reg;
+}
+
 bool AMDGPURegBankSelect::runOnMachineFunction(MachineFunction &MF) {
   if (MF.getProperties().hasProperty(
           MachineFunctionProperties::Property::FailedISel))
     return false;
+
+  // Setup the instruction builder with CSE.
+  const TargetPassConfig &TPC = getAnalysis<TargetPassConfig>();
+  GISelCSEAnalysisWrapper &Wrapper =
+      getAnalysis<GISelCSEAnalysisWrapperPass>().getCSEWrapper();
+  GISelCSEInfo &CSEInfo = Wrapper.get(TPC.getCSEConfig());
+  GISelObserverWrapper Observer;
+  Observer.addObserver(&CSEInfo);
+
+  CSEMIRBuilder B(MF);
+  B.setCSEInfo(&CSEInfo);
+  B.setChangeObserver(Observer);
+
+  RAIIDelegateInstaller DelegateInstaller(MF, &Observer);
+  RAIIMFObserverInstaller MFObserverInstaller(MF, Observer);
+
+  IntrinsicLaneMaskAnalyzer ILMA(MF);
+  MachineUniformityInfo &MUI =
+      getAnalysis<MachineUniformityAnalysisPass>().getUniformityInfo();
+  MachineRegisterInfo &MRI = *B.getMRI();
+  const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
+  RegBankSelectHelper RBSHelper(B, ILMA, MUI, *ST.getRegBankInfo());
+  // Virtual registers at this point don't have register banks.
+  // Virtual registers in def and use operands of already inst-selected
+  // instruction have register class.
+
+  for (MachineBasicBlock &MBB : MF) {
+    for (MachineInstr &MI : MBB) {
+      // Vregs in def and use operands of COPY can have either register class
+      // or bank. If there is neither on vreg in def operand, assign bank.
+      if (MI.isCopy()) {
+        Register DefReg = getVReg(MI.getOperand(0));
+        if (!DefReg.isValid() || MRI.getRegClassOrNull(DefReg))
+          continue;
+
+        assert(!MRI.getRegBankOrNull(DefReg));
+        MRI.setRegBank(DefReg, *RBSHelper.getRegBankToAssign(DefReg));
+        continue;
+      }
+
+      if (!MI.isPreISelOpcode())
+        continue;
+
+      // Vregs in def and use operands of G_ instructions need to have register
+      // banks assigned. Before this loop possible case are
+      // - (1) vreg without register class or bank in def or use operand
+      // - (2) vreg with register class in def operand
+      // - (3) vreg, defined by G_ instruction, in use operand
+      // - (4) vreg, defined by pre-inst-selected instruction, in use operand
+
+      // First three cases are handled in loop through all def operands of G_
+      // instructions. For case (1) simply setRegBank. Cases (2) and (3) are
+      // handled by reAssignRegBankOnDef.
+      for (MachineOperand &DefOP : MI.defs()) {
+        Register DefReg = getVReg(DefOP);
+        if (!DefReg.isValid())
+          continue;
+
+        const RegisterBank *RB = RBSHelper.getRegBankToAssign(DefReg);
+        if (MRI.getRegClassOrNull(DefReg))
+          RBSHelper.reAssignRegBankOnDef(MI, DefOP, RB);
+        else {
+          assert(!MRI.getRegBankOrNull(DefReg));
+          MRI.setRegBank(DefReg, *RB);
+        }
+      }
+
+      // Register bank select doesn't modify pre-inst-selected instructions.
+      // For case (4) need to insert a copy, handled by constrainRegBankUse.
+      for (MachineOperand &UseOP : MI.uses()) {
+        Register UseReg = getVReg(UseOP);
+        if (!UseReg.isValid())
+          continue;
+
+        // Skip case (3).
+        if (!MRI.getRegClassOrNull(UseReg) ||
+            MRI.getVRegDef(UseReg)->isPreISelOpcode())
+          continue;
+
+        // Use with register class defined by pre-inst-selected instruction.
+        const RegisterBank *RB = RBSHelper.getRegBankToAssign(UseReg);
+        RBSHelper.constrainRegBankUse(MI, UseOP, RB);
+      }
+    }
+  }
+
   return true;
 }