[ELF] - Linkerscript: implement DEFINED() command.

[grimar]

Summary

DEFINED(symbol)
Return 1 if symbol is in the linker global symbol table and is defined before
the statement using DEFINED in the script, otherwise return 0.

Can be used to define default values for symbols. Found it in the wild here:
https://searchcode.com/file/103745382/board/bf561-ezkit/u-boot.lds.S

Patch implements it.

lld/trunk/ELF/LinkerScript.h

protected:
   ~LinkerScriptBase() = default;
 
 public:
   virtual uint64_t getOutputSectionAddress(StringRef Name) = 0;
   virtual uint64_t getOutputSectionSize(StringRef Name) = 0;
   virtual uint64_t getOutputSectionAlign(StringRef Name) = 0;
   virtual uint64_t getHeaderSize() = 0;
   virtual uint64_t getSymbolValue(StringRef S) = 0;
+  virtual bool isDefined(StringRef S) = 0;
 };
 
 // ScriptConfiguration holds linker script parse results.
 struct ScriptConfiguration {
   // Used to assign addresses to sections.
   std::vector<std::unique_ptr<BaseCommand>> Commands;
 
   // Used to assign sections to headers.
public:
   void assignOffsets(OutputSectionCommand *Cmd);
   void assignAddresses();
   bool hasPhdrsCommands();
   uint64_t getOutputSectionAddress(StringRef Name) override;
   uint64_t getOutputSectionSize(StringRef Name) override;
   uint64_t getOutputSectionAlign(StringRef Name) override;
   uint64_t getHeaderSize() override;
   uint64_t getSymbolValue(StringRef S) override;
+  bool isDefined(StringRef S) override;
 
   std::vector<OutputSectionBase<ELFT> *> *OutputSections;
 
   int getSectionIndex(StringRef Name);
 
 private:
   void computeInputSections(InputSectionDescription *);
 

lld/trunk/ELF/LinkerScript.cpp

 
 template <class ELFT> uint64_t LinkerScript<ELFT>::getSymbolValue(StringRef S) {
   if (SymbolBody *B = Symtab<ELFT>::X->find(S))
     return B->getVA<ELFT>();
   error("symbol not found: " + S);
   return 0;
 }
 
+template <class ELFT> bool LinkerScript<ELFT>::isDefined(StringRef S) {
+  return Symtab<ELFT>::X->find(S) != nullptr;
+}
+
 // Returns indices of ELF headers containing specific section, identified
 // by Name. Each index is a zero based number of ELF header listed within
 // PHDRS {} script block.
 template <class ELFT>
 std::vector<size_t> LinkerScript<ELFT>::getPhdrIndices(StringRef SectionName) {
   for (const std::unique_ptr<BaseCommand> &Base : Opt.Commands) {
     auto *Cmd = dyn_cast<OutputSectionCommand>(Base.get());
     if (!Cmd || Cmd->Name != SectionName)
if (Tok == "ALIGN") {
     return [=](uint64_t Dot) { return alignTo(Dot, E(Dot)); };
   }
   if (Tok == "CONSTANT") {
     expect("(");
     StringRef Tok = next();
     expect(")");
     return [=](uint64_t Dot) { return getConstant(Tok); };
   }
+  if (Tok == "DEFINED") {
+    expect("(");
+    StringRef Tok = next();
+    expect(")");
+    return [=](uint64_t Dot) {
+      return ScriptBase->isDefined(Tok) ? 1 : 0;
+    };
+  }
   if (Tok == "SEGMENT_START") {
     expect("(");
     next();
     expect(",");
     Expr E = readExpr();
     expect(")");
     return [=](uint64_t Dot) { return E(Dot); };
   }

lld/trunk/test/ELF/linkerscript/define.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+
+# RUN: echo "SECTIONS                                \
+# RUN: {                                             \
+# RUN:  . = DEFINED(defined) ? 0x11000 : .;          \
+# RUN:  .foo : { *(.foo*) }                          \
+# RUN:  . = DEFINED(notdefined) ? 0x12000 : 0x13000; \
+# RUN:  .bar : { *(.bar*) }                          \
+# RUN: }" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-objdump -section-headers %t1 | FileCheck %s
+
+# CHECK: 1 .foo  00000008 0000000000011000 DATA
+# CHECK: 2 .bar  00000008 0000000000013000 DATA
+# CHECK: 3 .text 00000000 0000000000013008 TEXT DATA
+
+.global defined
+defined = 0
+
+.section .foo,"a"
+.quad 1
+
+.section .bar,"a"
+.quad 1