Reland r298901 with modifications (reverted in r298932)

Dont emit Mapping symbols for sections that contain only data.

Summary:
Dont emit mapping symbols for sections that contain only data.

Reviewers: rengolin, weimingz, kparzysz, t.p.northover, peter.smith

Reviewed By: t.p.northover

Patched by Shankar Easwaran <shankare@codeaurora.org>

Subscribers: alekseyshl, t.p.northover, llvm-commits

Differential Revision: https://reviews.llvm.org/D30724

llvm-svn: 299392

1 files changed, 71 insertions, 15 deletions

diff --git a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
index f7bdda491cb..774a0b3771b 100644
--- a/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
+++ b/llvm/lib/Target/ARM/MCTargetDesc/ARMELFStreamer.cpp
@@ -464,13 +464,14 @@ public:
   void emitUnwindRaw(int64_t Offset, const SmallVectorImpl<uint8_t> &Opcodes);
 
   void ChangeSection(MCSection *Section, const MCExpr *Subsection) override {
-    // We have to keep track of the mapping symbol state of any sections we
-    // use. Each one should start off as EMS_None, which is provided as the
-    // default constructor by DenseMap::lookup.
-    LastMappingSymbols[getPreviousSection().first] = LastEMS;
-    LastEMS = LastMappingSymbols.lookup(Section);
-
+    LastMappingSymbols[getPreviousSection().first] = std::move(LastEMSInfo);
     MCELFStreamer::ChangeSection(Section, Subsection);
+    auto LastMappingSymbol = LastMappingSymbols.find(Section);
+    if (LastMappingSymbol != LastMappingSymbols.end()) {
+      LastEMSInfo = std::move(LastMappingSymbol->second);
+      return;
+    }
+    LastEMSInfo.reset(new ElfMappingSymbolInfo(SMLoc(), nullptr, 0));
   }
 
   /// This function is the one used to emit instruction data into the ELF
@@ -532,15 +533,25 @@ public:
     MCELFStreamer::EmitBytes(Data);
   }
 
+  void FlushPendingMappingSymbol() {
+    if (!LastEMSInfo->hasInfo())
+      return;
+    ElfMappingSymbolInfo *EMS = LastEMSInfo.get();
+    EmitMappingSymbol("$d", EMS->Loc, EMS->F, EMS->Offset);
+    EMS->resetInfo();
+  }
+
   /// This is one of the functions used to emit data into an ELF section, so the
   /// ARM streamer overrides it to add the appropriate mapping symbol ($d) if
   /// necessary.
   void EmitValueImpl(const MCExpr *Value, unsigned Size, SMLoc Loc) override {
-    if (const MCSymbolRefExpr *SRE = dyn_cast_or_null<MCSymbolRefExpr>(Value))
+    if (const MCSymbolRefExpr *SRE = dyn_cast_or_null<MCSymbolRefExpr>(Value)) {
       if (SRE->getKind() == MCSymbolRefExpr::VK_ARM_SBREL && !(Size == 4)) {
         getContext().reportError(Loc, "relocated expression must be 32-bit");
         return;
       }
+      getOrCreateDataFragment();
+    }
 
     EmitDataMappingSymbol();
     MCELFStreamer::EmitValueImpl(Value, Size, Loc);
@@ -573,22 +584,54 @@ private:
     EMS_Data
   };
 
+  struct ElfMappingSymbolInfo {
+    explicit ElfMappingSymbolInfo(SMLoc Loc, MCFragment *F, uint64_t O)
+        : Loc(Loc), F(F), Offset(O), State(EMS_None) {}
+    void resetInfo() {
+      F = nullptr;
+      Offset = 0;
+    }
+    bool hasInfo() { return F != nullptr; }
+    SMLoc Loc;
+    MCFragment *F;
+    uint64_t Offset;
+    ElfMappingSymbol State;
+  };
+
   void EmitDataMappingSymbol() {
-    if (LastEMS == EMS_Data) return;
+    if (LastEMSInfo->State == EMS_Data)
+      return;
+    else if (LastEMSInfo->State == EMS_None) {
+      // This is a tentative symbol, it won't really be emitted until it's
+      // actually needed.
+      ElfMappingSymbolInfo *EMS = LastEMSInfo.get();
+      auto *DF = dyn_cast_or_null<MCDataFragment>(getCurrentFragment());
+      if (!DF)
+        return;
+      EMS->Loc = SMLoc();
+      EMS->F = getCurrentFragment();
+      EMS->Offset = DF->getContents().size();
+      LastEMSInfo->State = EMS_Data;
+      return;
+    }
     EmitMappingSymbol("$d");
-    LastEMS = EMS_Data;
+    LastEMSInfo->State = EMS_Data;
   }
 
   void EmitThumbMappingSymbol() {
-    if (LastEMS == EMS_Thumb) return;
+    if (LastEMSInfo->State == EMS_Thumb)
+      return;
+    FlushPendingMappingSymbol();
     EmitMappingSymbol("$t");
-    LastEMS = EMS_Thumb;
+    LastEMSInfo->State = EMS_Thumb;
   }
 
   void EmitARMMappingSymbol() {
-    if (LastEMS == EMS_ARM) return;
+    if (LastEMSInfo->State == EMS_ARM)
+      return;
+    FlushPendingMappingSymbol();
     EmitMappingSymbol("$a");
-    LastEMS = EMS_ARM;
+    LastEMSInfo->State = EMS_ARM;
   }
 
   void EmitMappingSymbol(StringRef Name) {
@@ -601,6 +644,17 @@ private:
     Symbol->setExternal(false);
   }
 
+  void EmitMappingSymbol(StringRef Name, SMLoc Loc, MCFragment *F,
+                         uint64_t Offset) {
+    auto *Symbol = cast<MCSymbolELF>(getContext().getOrCreateSymbol(
+        Name + "." + Twine(MappingSymbolCounter++)));
+    EmitLabel(Symbol, Loc, F);
+    Symbol->setType(ELF::STT_NOTYPE);
+    Symbol->setBinding(ELF::STB_LOCAL);
+    Symbol->setExternal(false);
+    Symbol->setOffset(Offset);
+  }
+
   void EmitThumbFunc(MCSymbol *Func) override {
     getAssembler().setIsThumbFunc(Func);
     EmitSymbolAttribute(Func, MCSA_ELF_TypeFunction);
@@ -626,8 +680,10 @@ private:
   bool IsThumb;
   int64_t MappingSymbolCounter = 0;
 
-  DenseMap<const MCSection *, ElfMappingSymbol> LastMappingSymbols;
-  ElfMappingSymbol LastEMS = EMS_None;
+  DenseMap<const MCSection *, std::unique_ptr<ElfMappingSymbolInfo>>
+      LastMappingSymbols;
+
+  std::unique_ptr<ElfMappingSymbolInfo> LastEMSInfo;
 
   // ARM Exception Handling Frame Information
   MCSymbol *ExTab;