AMDGPU/GlobalISel: StandaloneRegBankSelect

Assign register banks to virtual registers. Does not use generic
RegBankSelect. Defs and uses of G_ instructions have 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)

StandaloneRegBankSelect 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

@@ -7,13 +7,17 @@
 //===----------------------------------------------------------------------===//
 
 #include "AMDGPUGlobalISelUtils.h"
+#include "AMDGPURegisterBankInfo.h"
 #include "GCNSubtarget.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>
@@ -69,3 +73,37 @@ 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) {
+  return S32S64LaneMask.contains(Reg);
+}
+
+void IntrinsicLaneMaskAnalyzer::initLaneMaskIntrinsics(MachineFunction &MF) {
+  for (auto &MBB : MF) {
+    for (auto &MI : MBB) {
+      if (isa<GIntrinsic>(MI) &&
+          cast<GIntrinsic>(MI).getIntrinsicID() == 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 +9,8 @@
 #ifndef LLVM_LIB_TARGET_AMDGPU_AMDGPUGLOBALISELUTILS_H
 #define LLVM_LIB_TARGET_AMDGPU_AMDGPUGLOBALISELUTILS_H
 
+#include "llvm/ADT/DenseSet.h"
+#include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/Register.h"
 #include <utility>
 
@@ -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 {
+  DenseSet<Register> S32S64LaneMask;
+  MachineRegisterInfo &MRI;
+
+public:
+  IntrinsicLaneMaskAnalyzer(MachineFunction &MF);
+  bool isS32S64LaneMask(Register Reg);
+
+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/AMDGPUStandaloneRegBankSelect.cpp

@@ -16,7 +16,13 @@
 //===----------------------------------------------------------------------===//
 
 #include "AMDGPU.h"
+#include "AMDGPUGlobalISelUtils.h"
+#include "AMDGPURegisterBankInfo.h"
+#include "GCNSubtarget.h"
+#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
+#include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
 #include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineUniformityAnalysis.h"
 #include "llvm/InitializePasses.h"
 
 #define DEBUG_TYPE "amdgpu-standalone-regbankselect"
@@ -41,6 +47,7 @@ class AMDGPUStandaloneRegBankSelect : public MachineFunctionPass {
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<MachineUniformityAnalysisPass>();
     MachineFunctionPass::getAnalysisUsage(AU);
   }
 
@@ -68,9 +75,220 @@ FunctionPass *llvm::createAMDGPUStandaloneRegBankSelectPass() {
   return new AMDGPUStandaloneRegBankSelect();
 }
 
+class RegBankSelectHelper {
+  MachineFunction &MF;
+  MachineIRBuilder &B;
+  MachineRegisterInfo &MRI;
+  AMDGPU::IntrinsicLaneMaskAnalyzer &ILMA;
+  const MachineUniformityInfo &MUI;
+  const SIRegisterInfo &TRI;
+  const RegisterBank *SgprRB;
+  const RegisterBank *VgprRB;
+  const RegisterBank *VccRB;
+
+public:
+  RegBankSelectHelper(MachineFunction &MF, MachineIRBuilder &B,
+                      MachineRegisterInfo &MRI,
+                      AMDGPU::IntrinsicLaneMaskAnalyzer &ILMA,
+                      const MachineUniformityInfo &MUI,
+                      const SIRegisterInfo &TRI, const RegisterBankInfo &RBI)
+      : MF(MF), B(B), MRI(MRI), ILMA(ILMA), MUI(MUI), TRI(TRI),
+        SgprRB(&RBI.getRegBank(AMDGPU::SGPRRegBankID)),
+        VgprRB(&RBI.getRegBank(AMDGPU::VGPRRegBankID)),
+        VccRB(&RBI.getRegBank(AMDGPU::VCCRegBankID)) {}
+
+  bool shouldRegBankSelect(MachineInstr &MI) {
+    return MI.isPreISelOpcode() || MI.isCopy();
+  }
+
+  void setRBDef(MachineInstr &MI, MachineOperand &DefOP,
+                const RegisterBank *RB) {
+    Register Reg = DefOP.getReg();
+    // 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-rb-legalize artifact
+    // combiner and avoids need to special case some patterns.
+    if (MRI.getRegClassOrNull(Reg)) {
+      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.
+
+      // "Clear" reg classes from uses on generic instructions and put register
+      // banks instead.
+      for (auto &UseMI : MRI.use_instructions(Reg)) {
+        if (shouldRegBankSelect(UseMI)) {
+          for (MachineOperand &Op : UseMI.operands()) {
+            if (Op.isReg() && Op.getReg() == Reg)
+              Op.setReg(NewReg);
+          }
+        }
+      }
+
+    } else {
+      MRI.setRegBank(Reg, *RB);
+    }
+  }
+
+  void constrainRBUse(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);
+  }
+
+  std::optional<Register> tryGetVReg(MachineOperand &Op) {
+    if (!Op.isReg())
+      return std::nullopt;
+
+    Register Reg = Op.getReg();
+    if (!Reg.isVirtual())
+      return std::nullopt;
+
+    return Reg;
+  }
+
+  void assignBanksOnDefs() {
+    for (MachineBasicBlock &MBB : MF) {
+      for (MachineInstr &MI : MBB) {
+        if (!shouldRegBankSelect(MI))
+          continue;
+
+        for (MachineOperand &DefOP : MI.defs()) {
+          auto MaybeDefReg = tryGetVReg(DefOP);
+          if (!MaybeDefReg)
+            continue;
+          Register DefReg = *MaybeDefReg;
+
+          // Copies can have register class on def registers.
+          if (MI.isCopy() && MRI.getRegClassOrNull(DefReg)) {
+            continue;
+          }
+
+          if (MUI.isUniform(DefReg) || ILMA.isS32S64LaneMask(DefReg)) {
+            setRBDef(MI, DefOP, SgprRB);
+          } else {
+            if (MRI.getType(DefReg) == LLT::scalar(1))
+              setRBDef(MI, DefOP, VccRB);
+            else
+              setRBDef(MI, DefOP, VgprRB);
+          }
+        }
+      }
+    }
+  }
+
+  // Temporal divergence copy: COPY to vgpr with implicit use of $exec inside of
+  // the cycle
+  // 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
+  bool isTemporalDivergenceCopy(Register Reg) {
+    MachineInstr *MI = MRI.getVRegDef(Reg);
+    if (!MI->isCopy())
+      return false;
+
+    for (auto Op : MI->implicit_operands()) {
+      if (!Op.isReg())
+        continue;
+
+      if (Op.getReg() == TRI.getExec()) {
+        return true;
+      }
+    }
+
+    return false;
+  }
+
+  void constrainBanksOnUses() {
+    for (MachineBasicBlock &MBB : MF) {
+      for (MachineInstr &MI : MBB) {
+        if (!shouldRegBankSelect(MI))
+          continue;
+
+        // Copies can have register class on use registers.
+        if (MI.isCopy())
+          continue;
+
+        for (MachineOperand &UseOP : MI.uses()) {
+          auto MaybeUseReg = tryGetVReg(UseOP);
+          if (!MaybeUseReg)
+            continue;
+          Register UseReg = *MaybeUseReg;
+
+          // UseReg already has register bank.
+          if (MRI.getRegBankOrNull(UseReg))
+            continue;
+
+          if (!isTemporalDivergenceCopy(UseReg) &&
+              (MUI.isUniform(UseReg) || ILMA.isS32S64LaneMask(UseReg))) {
+            constrainRBUse(MI, UseOP, SgprRB);
+          } else {
+            if (MRI.getType(UseReg) == LLT::scalar(1))
+              constrainRBUse(MI, UseOP, VccRB);
+            else
+              constrainRBUse(MI, UseOP, VgprRB);
+          }
+        }
+      }
+    }
+  }
+};
+
 bool AMDGPUStandaloneRegBankSelect::runOnMachineFunction(MachineFunction &MF) {
   if (MF.getProperties().hasProperty(
           MachineFunctionProperties::Property::FailedISel))
     return false;
+
+  MachineUniformityInfo &MUI =
+      getAnalysis<MachineUniformityAnalysisPass>().getUniformityInfo();
+  AMDGPU::IntrinsicLaneMaskAnalyzer ILMA(MF);
+  MachineRegisterInfo &MRI = MF.getRegInfo();
+  const SIRegisterInfo &TRI =
+      *MF.getSubtarget<GCNSubtarget>().getRegisterInfo();
+  const RegisterBankInfo &RBI = *MF.getSubtarget().getRegBankInfo();
+
+  MachineIRBuilder B(MF);
+  RegBankSelectHelper RBSHelper(MF, B, MRI, ILMA, MUI, TRI, RBI);
+
+  // Assign register banks to ALL def registers on G_ instructions.
+  // Same for copies if they have no register bank or class on def.
+  RBSHelper.assignBanksOnDefs();
+
+  // At this point all virtual registers have register class or bank
+  // - Defs of G_ instructions have register banks.
+  // - Defs and uses of inst-selected instructions have register class.
+  // - Defs and uses of copies can have either register class or bank
+  // and most notably
+  // - Uses of G_ instructions can have either register class or bank
+
+  // Reassign uses of G_ instructions to only have register banks.
+  RBSHelper.constrainBanksOnUses();
+
+  // Defs and uses of G_ instructions have register banks exclusively.
   return true;
 }