[PowerPC] Add linker opt for PC Relative GOT indirect accesses

A linker optimization is available on PowerPC for GOT indirect PCRelative loads.

The idea is that we can mark a usual GOT indirect load:

pld 3, vec@got@pcrel(0), 1
lwa 3, 4(3)

With a relocation to say that if we don't need to go through the GOT we can let
the linker further optimize this and replace a load with a nop.

  pld 3, vec@got@pcrel(0), 1
.Lpcrel1:
.reloc .Lpcrel1-8,R_PPC64_PCREL_OPT,.-(.Lpcrel1-8)
  lwa 3, 4(3)

This patch adds the logic that allows the compiler to add the R_PPC64_PCREL_OPT.

Reviewers: nemanjai, lei, hfinkel, sfertile, efriedma, tstellar, grosbach

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D79864

Author: stefanp-synopsys

llvm/include/llvm/MC/MCExpr.h

@@ -302,6 +302,7 @@ class MCSymbolRefExpr : public MCExpr {
     VK_PPC_TLSLD,           // symbol@tlsld
     VK_PPC_LOCAL,           // symbol@local
     VK_PPC_NOTOC,           // symbol@notoc
+    VK_PPC_PCREL_OPT,       // .reloc expr, R_PPC64_PCREL_OPT, expr
 
     VK_COFF_IMGREL32, // symbol@imgrel (image-relative)
 

llvm/lib/MC/MCExpr.cpp

@@ -324,6 +324,7 @@ StringRef MCSymbolRefExpr::getVariantKindName(VariantKind Kind) {
   case VK_PPC_TLSLD: return "tlsld";
   case VK_PPC_LOCAL: return "local";
   case VK_PPC_NOTOC: return "notoc";
+  case VK_PPC_PCREL_OPT: return "<<invalid>>";
   case VK_COFF_IMGREL32: return "IMGREL";
   case VK_Hexagon_LO16: return "LO16";
   case VK_Hexagon_HI16: return "HI16";

llvm/lib/Target/PowerPC/MCTargetDesc/PPCELFStreamer.cpp

@@ -20,6 +20,7 @@
 
 
 #include "PPCELFStreamer.h"
+#include "PPCFixupKinds.h"
 #include "PPCInstrInfo.h"
 #include "PPCMCCodeEmitter.h"
 #include "llvm/BinaryFormat/ELF.h"
@@ -89,12 +90,33 @@ void PPCELFStreamer::emitInstruction(const MCInst &Inst,
   PPCMCCodeEmitter *Emitter =
       static_cast<PPCMCCodeEmitter*>(getAssembler().getEmitterPtr());
 
+  // If the instruction is a part of the GOT to PC-Rel link time optimization
+  // instruction pair, return a value, otherwise return None. A true returned
+  // value means the instruction is the PLDpc and a false value means it is
+  // the user instruction.
+  Optional<bool> IsPartOfGOTToPCRelPair = isPartOfGOTToPCRelPair(Inst, STI);
+
+  // User of the GOT-indirect address.
+  // For example, the load that will get the relocation as follows:
+  // .reloc .Lpcrel1-8,R_PPC64_PCREL_OPT,.-(.Lpcrel1-8)
+  //  lwa 3, 4(3)
+  if (IsPartOfGOTToPCRelPair.hasValue() && !IsPartOfGOTToPCRelPair.getValue())
+    emitGOTToPCRelReloc(Inst);
+
   // Special handling is only for prefixed instructions.
   if (!Emitter->isPrefixedInstruction(Inst)) {
     MCELFStreamer::emitInstruction(Inst, STI);
     return;
   }
   emitPrefixedInstruction(Inst, STI);
+
+  // Producer of the GOT-indirect address.
+  // For example, the prefixed load from the got that will get the label as
+  // follows:
+  //  pld 3, vec@got@pcrel(0), 1
+  // .Lpcrel1:
+  if (IsPartOfGOTToPCRelPair.hasValue() && IsPartOfGOTToPCRelPair.getValue())
+    emitGOTToPCRelLabel(Inst);
 }
 
 void PPCELFStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
@@ -103,6 +125,102 @@ void PPCELFStreamer::emitLabel(MCSymbol *Symbol, SMLoc Loc) {
   MCELFStreamer::emitLabel(Symbol);
 }
 
+// This linker time GOT PC Relative optimization relocation will look like this:
+//   pld <reg> symbol@got@pcrel
+// <Label###>:
+//   .reloc Label###-8,R_PPC64_PCREL_OPT,.-(Label###-8)
+//   load <loadedreg>, 0(<reg>)
+// The reason we place the label after the PLDpc instruction is that there
+// may be an alignment nop before it since prefixed instructions must not
+// cross a 64-byte boundary (please see
+// PPCELFStreamer::emitPrefixedInstruction()). When referring to the
+// label, we subtract the width of a prefixed instruction (8 bytes) to ensure
+// we refer to the PLDpc.
+void PPCELFStreamer::emitGOTToPCRelReloc(const MCInst &Inst) {
+  // Get the last operand which contains the symbol.
+  const MCOperand &Operand = Inst.getOperand(Inst.getNumOperands() - 1);
+  assert(Operand.isExpr() && "Expecting an MCExpr.");
+  // Cast the last operand to MCSymbolRefExpr to get the symbol.
+  const MCExpr *Expr = Operand.getExpr();
+  const MCSymbolRefExpr *SymExpr = static_cast<const MCSymbolRefExpr *>(Expr);
+  assert(SymExpr->getKind() == MCSymbolRefExpr::VK_PPC_PCREL_OPT &&
+         "Expecting a symbol of type VK_PPC_PCREL_OPT");
+  MCSymbol *LabelSym =
+      getContext().getOrCreateSymbol(SymExpr->getSymbol().getName());
+  const MCExpr *LabelExpr = MCSymbolRefExpr::create(LabelSym, getContext());
+  const MCExpr *Eight = MCConstantExpr::create(8, getContext());
+  // SubExpr is just Label###-8
+  const MCExpr *SubExpr =
+      MCBinaryExpr::createSub(LabelExpr, Eight, getContext());
+  MCSymbol *CurrentLocation = getContext().createTempSymbol();
+  const MCExpr *CurrentLocationExpr =
+      MCSymbolRefExpr::create(CurrentLocation, getContext());
+  // SubExpr2 is .-(Label###-8)
+  const MCExpr *SubExpr2 =
+      MCBinaryExpr::createSub(CurrentLocationExpr, SubExpr, getContext());
+
+  MCDataFragment *DF = static_cast<MCDataFragment *>(LabelSym->getFragment());
+  assert(DF && "Expecting a valid data fragment.");
+  MCFixupKind FixupKind = static_cast<MCFixupKind>(FirstLiteralRelocationKind +
+                                                   ELF::R_PPC64_PCREL_OPT);
+  DF->getFixups().push_back(
+      MCFixup::create(LabelSym->getOffset() - 8, SubExpr2,
+                      FixupKind, Inst.getLoc()));
+  emitLabel(CurrentLocation, Inst.getLoc());
+}
+
+// Emit the label that immediately follows the PLDpc for a link time GOT PC Rel
+// optimization.
+void PPCELFStreamer::emitGOTToPCRelLabel(const MCInst &Inst) {
+  // Get the last operand which contains the symbol.
+  const MCOperand &Operand = Inst.getOperand(Inst.getNumOperands() - 1);
+  assert(Operand.isExpr() && "Expecting an MCExpr.");
+  // Cast the last operand to MCSymbolRefExpr to get the symbol.
+  const MCExpr *Expr = Operand.getExpr();
+  const MCSymbolRefExpr *SymExpr = static_cast<const MCSymbolRefExpr *>(Expr);
+  assert(SymExpr->getKind() == MCSymbolRefExpr::VK_PPC_PCREL_OPT &&
+         "Expecting a symbol of type VK_PPC_PCREL_OPT");
+  MCSymbol *LabelSym =
+      getContext().getOrCreateSymbol(SymExpr->getSymbol().getName());
+  emitLabel(LabelSym, Inst.getLoc());
+}
+
+// This funciton checks if the parameter Inst is part of the setup for a link
+// time GOT PC Relative optimization. For example in this situation:
+// <MCInst PLDpc <MCOperand Reg:282> <MCOperand Expr:(glob_double@got@pcrel)>
+//   <MCOperand Imm:0> <MCOperand Expr:(.Lpcrel@<<invalid>>)>>
+// <MCInst SOME_LOAD <MCOperand Reg:22> <MCOperand Imm:0> <MCOperand Reg:282>
+//   <MCOperand Expr:(.Lpcrel@<<invalid>>)>>
+// The above is a pair of such instructions and this function will not return
+// None for either one of them. In both cases we are looking for the last
+// operand <MCOperand Expr:(.Lpcrel@<<invalid>>)> which needs to be an MCExpr
+// and has the flag MCSymbolRefExpr::VK_PPC_PCREL_OPT. After that we just look
+// at the opcode and in the case of PLDpc we will return true. For the load
+// (or store) this function will return false indicating it has found the second
+// instruciton in the pair.
+Optional<bool> llvm::isPartOfGOTToPCRelPair(const MCInst &Inst,
+                                            const MCSubtargetInfo &STI) {
+  // Need at least two operands.
+  if (Inst.getNumOperands() < 2)
+    return None;
+
+  unsigned LastOp = Inst.getNumOperands() - 1;
+  // The last operand needs to be an MCExpr and it needs to have a variant kind
+  // of VK_PPC_PCREL_OPT. If it does not satisfy these conditions it is not a
+  // link time GOT PC Rel opt instruction and we can ignore it and return None.
+  const MCOperand &Operand = Inst.getOperand(LastOp);
+  if (!Operand.isExpr())
+    return None;
+
+  // Check for the variant kind VK_PPC_PCREL_OPT in this expression.
+  const MCExpr *Expr = Operand.getExpr();
+  const MCSymbolRefExpr *SymExpr = static_cast<const MCSymbolRefExpr *>(Expr);
+  if (!SymExpr || SymExpr->getKind() != MCSymbolRefExpr::VK_PPC_PCREL_OPT)
+    return None;
+
+  return (Inst.getOpcode() == PPC::PLDpc);
+}
+
 MCELFStreamer *llvm::createPPCELFStreamer(
     MCContext &Context, std::unique_ptr<MCAsmBackend> MAB,
     std::unique_ptr<MCObjectWriter> OW,

llvm/lib/Target/PowerPC/MCTargetDesc/PPCELFStreamer.h

@@ -43,8 +43,15 @@ class PPCELFStreamer : public MCELFStreamer {
   void emitLabel(MCSymbol *Symbol, SMLoc Loc = SMLoc()) override;
 private:
   void emitPrefixedInstruction(const MCInst &Inst, const MCSubtargetInfo &STI);
+  void emitGOTToPCRelReloc(const MCInst &Inst);
+  void emitGOTToPCRelLabel(const MCInst &Inst);
 };
 
+// Check if the instruction Inst is part of a pair of instructions that make up
+// a link time GOT PC Rel optimization.
+Optional<bool> isPartOfGOTToPCRelPair(const MCInst &Inst,
+                                      const MCSubtargetInfo &STI);
+
 MCELFStreamer *createPPCELFStreamer(MCContext &Context,
                                     std::unique_ptr<MCAsmBackend> MAB,
                                     std::unique_ptr<MCObjectWriter> OW,

llvm/lib/Target/PowerPC/MCTargetDesc/PPCInstPrinter.cpp

@@ -92,6 +92,36 @@ void PPCInstPrinter::printInst(const MCInst *MI, uint64_t Address,
     return;
   }
 
+  // Check if the last operand is an expression with the variant kind
+  // VK_PPC_PCREL_OPT. If this is the case then this is a linker optimization
+  // relocation and the .reloc directive needs to be added.
+  unsigned LastOp = MI->getNumOperands() - 1;
+  if (MI->getNumOperands() > 1) {
+    const MCOperand &Operand = MI->getOperand(LastOp);
+    if (Operand.isExpr()) {
+      const MCExpr *Expr = Operand.getExpr();
+      const MCSymbolRefExpr *SymExpr =
+          static_cast<const MCSymbolRefExpr *>(Expr);
+
+      if (SymExpr && SymExpr->getKind() == MCSymbolRefExpr::VK_PPC_PCREL_OPT) {
+        const MCSymbol &Symbol = SymExpr->getSymbol();
+        if (MI->getOpcode() == PPC::PLDpc) {
+          printInstruction(MI, Address, O);
+          O << "\n";
+          Symbol.print(O, &MAI);
+          O << ":";
+          return;
+        } else {
+          O << "\t.reloc ";
+          Symbol.print(O, &MAI);
+          O << "-8,R_PPC64_PCREL_OPT,.-(";
+          Symbol.print(O, &MAI);
+          O << "-8)\n";
+        }
+      }
+    }
+  }
+
   // Check for slwi/srwi mnemonics.
   if (MI->getOpcode() == PPC::RLWINM) {
     unsigned char SH = MI->getOperand(2).getImm();

llvm/lib/Target/PowerPC/PPC.h

@@ -107,6 +107,10 @@ namespace llvm {
     /// produce the relocation @got@pcrel. Fixup is VK_PPC_GOT_PCREL.
     MO_GOT_FLAG = 8,
 
+    // MO_PCREL_OPT_FLAG - If this bit is set the operand is part of a
+    // PC Relative linker optimization.
+    MO_PCREL_OPT_FLAG = 16,
+
     /// The next are not flags but distinct values.
     MO_ACCESS_MASK = 0xf00,
 

llvm/lib/Target/PowerPC/PPCInstrInfo.cpp

@@ -2283,7 +2283,8 @@ PPCInstrInfo::getSerializableBitmaskMachineOperandTargetFlags() const {
       {MO_PLT, "ppc-plt"},
       {MO_PIC_FLAG, "ppc-pic"},
       {MO_PCREL_FLAG, "ppc-pcrel"},
-      {MO_GOT_FLAG, "ppc-got"}};
+      {MO_GOT_FLAG, "ppc-got"},
+      {MO_PCREL_OPT_FLAG, "ppc-opt-pcrel"}};
   return makeArrayRef(TargetFlags);
 }
 

llvm/lib/Target/PowerPC/PPCMCInstLower.cpp

@@ -100,6 +100,8 @@ static MCOperand GetSymbolRef(const MachineOperand &MO, const MCSymbol *Symbol,
         MIOpcode == PPC::BL8_NOTOC) {
       RefKind = MCSymbolRefExpr::VK_PPC_NOTOC;
     }
+    if (MO.getTargetFlags() == PPCII::MO_PCREL_OPT_FLAG)
+      RefKind = MCSymbolRefExpr::VK_PPC_PCREL_OPT;
   }
 
   const MCExpr *Expr = MCSymbolRefExpr::create(Symbol, RefKind, Ctx);