[AMDGPU] Speed up SIRegisterInfo::getReservedRegs (#79844)

reserveRegisterTuples is slow because it uses MCRegAliasIterator and
hence ends up reserving the same aliased registers many times. This
patch changes getReservedRegs not to use it for reserving SGPRs, VGPRs
and AGPRs. Instead it iterates through base register classes, which
should come closer to reserving each register once only.

Overall this speeds up the time to run check-llvm-codegen-amdgpu in my
Release build from 18.4 seconds to 16.9 seconds (all timings +/- 0.2).

Author: jayfoad

llvm/include/llvm/CodeGen/TargetRegisterInfo.h

@@ -119,6 +119,9 @@ class TargetRegisterClass {
   /// registers.
   bool isAllocatable() const { return MC->isAllocatable(); }
 
+  /// Return true if this register class has a defined BaseClassOrder.
+  bool isBaseClass() const { return MC->isBaseClass(); }
+
   /// Return true if the specified TargetRegisterClass
   /// is a proper sub-class of this TargetRegisterClass.
   bool hasSubClass(const TargetRegisterClass *RC) const {

llvm/include/llvm/MC/MCRegisterInfo.h

@@ -46,6 +46,7 @@ class MCRegisterClass {
   const uint16_t RegSizeInBits;
   const int8_t CopyCost;
   const bool Allocatable;
+  const bool BaseClass;
 
   /// getID() - Return the register class ID number.
   ///
@@ -97,6 +98,9 @@ class MCRegisterClass {
   /// isAllocatable - Return true if this register class may be used to create
   /// virtual registers.
   bool isAllocatable() const { return Allocatable; }
+
+  /// Return true if this register class has a defined BaseClassOrder.
+  bool isBaseClass() const { return BaseClass; }
 };
 
 /// MCRegisterDesc - This record contains information about a particular

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

@@ -622,9 +622,15 @@ BitVector SIRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
   //
   unsigned MaxNumSGPRs = ST.getMaxNumSGPRs(MF);
   unsigned TotalNumSGPRs = AMDGPU::SGPR_32RegClass.getNumRegs();
-  for (unsigned i = MaxNumSGPRs; i < TotalNumSGPRs; ++i) {
-    unsigned Reg = AMDGPU::SGPR_32RegClass.getRegister(i);
-    reserveRegisterTuples(Reserved, Reg);
+  for (const TargetRegisterClass *RC : regclasses()) {
+    if (RC->isBaseClass() && isSGPRClass(RC)) {
+      unsigned NumRegs = divideCeil(getRegSizeInBits(*RC), 32);
+      for (MCPhysReg Reg : *RC) {
+        unsigned Index = getHWRegIndex(Reg);
+        if (Index + NumRegs > MaxNumSGPRs && Index < TotalNumSGPRs)
+          Reserved.set(Reg);
+      }
+    }
   }
 
   Register ScratchRSrcReg = MFI->getScratchRSrcReg();
@@ -693,20 +699,29 @@ BitVector SIRegisterInfo::getReservedRegs(const MachineFunction &MF) const {
     }
   }
 
-  for (unsigned i = MaxNumVGPRs; i < TotalNumVGPRs; ++i) {
-    unsigned Reg = AMDGPU::VGPR_32RegClass.getRegister(i);
-    reserveRegisterTuples(Reserved, Reg);
+  for (const TargetRegisterClass *RC : regclasses()) {
+    if (RC->isBaseClass() && isVGPRClass(RC)) {
+      unsigned NumRegs = divideCeil(getRegSizeInBits(*RC), 32);
+      for (MCPhysReg Reg : *RC) {
+        unsigned Index = getHWRegIndex(Reg);
+        if (Index + NumRegs > MaxNumVGPRs)
+          Reserved.set(Reg);
+      }
+    }
   }
 
-  if (ST.hasMAIInsts()) {
-    for (unsigned i = MaxNumAGPRs; i < TotalNumVGPRs; ++i) {
-      unsigned Reg = AMDGPU::AGPR_32RegClass.getRegister(i);
-      reserveRegisterTuples(Reserved, Reg);
+  // Reserve all the AGPRs if there are no instructions to use it.
+  if (!ST.hasMAIInsts())
+    MaxNumAGPRs = 0;
+  for (const TargetRegisterClass *RC : regclasses()) {
+    if (RC->isBaseClass() && isAGPRClass(RC)) {
+      unsigned NumRegs = divideCeil(getRegSizeInBits(*RC), 32);
+      for (MCPhysReg Reg : *RC) {
+        unsigned Index = getHWRegIndex(Reg);
+        if (Index + NumRegs > MaxNumAGPRs)
+          Reserved.set(Reg);
+      }
     }
-  } else {
-    // Reserve all the AGPRs if there are no instructions to use it.
-    for (MCRegister Reg : AMDGPU::AGPR_32RegClass)
-      reserveRegisterTuples(Reserved, Reg);
   }
 
   // On GFX908, in order to guarantee copying between AGPRs, we need a scratch

llvm/unittests/CodeGen/MachineInstrTest.cpp

@@ -530,7 +530,8 @@ TEST(MachineInstrTest, SpliceOperands) {
   EXPECT_EQ(MI->getOperand(8).getImm(), MachineOperand::CreateImm(4).getImm());
 
   // test tied operands
-  MCRegisterClass MRC{0, 0, 0, 0, 0, 0, 0, 0, /*Allocatable=*/true};
+  MCRegisterClass MRC{
+      0, 0, 0, 0, 0, 0, 0, 0, /*Allocatable=*/true, /*BaseClass=*/true};
   TargetRegisterClass RC{&MRC, 0, 0, {}, 0, 0, 0, 0, 0, 0, 0};
   // MachineRegisterInfo will be very upset if these registers aren't
   // allocatable.

llvm/utils/TableGen/RegisterInfoEmitter.cpp

@@ -1069,7 +1069,8 @@ RegisterInfoEmitter::runMCDesc(raw_ostream &OS, CodeGenTarget &Target,
        << RegClassStrings.get(RC.getName()) << ", " << RC.getOrder().size()
        << ", " << RCBitsSize << ", " << RC.getQualifiedIdName() << ", "
        << RegSize << ", " << RC.CopyCost << ", "
-       << (RC.Allocatable ? "true" : "false") << " },\n";
+       << (RC.Allocatable ? "true" : "false") << ", "
+       << (RC.getBaseClassOrder() ? "true" : "false") << " },\n";
   }
 
   OS << "};\n\n";
@@ -1846,6 +1847,7 @@ void RegisterInfoEmitter::debugDump(raw_ostream &OS) {
     OS << "\tCoveredBySubRegs: " << RC.CoveredBySubRegs << '\n';
     OS << "\tAllocatable: " << RC.Allocatable << '\n';
     OS << "\tAllocationPriority: " << unsigned(RC.AllocationPriority) << '\n';
+    OS << "\tBaseClassOrder: " << RC.getBaseClassOrder() << '\n';
     OS << "\tRegs:";
     for (const CodeGenRegister *R : RC.getMembers()) {
       OS << " " << R->getName();