Handle symbol assignments before the first section switch

[• rafael]

Summary

This is an alternative to https://reviews.llvm.org/D29391 which is a lot simpler IMHO.

[ruiu]

Does this always create a dummy section at the beginning of a file?

ELF/LinkerScript.cpp

779

mention -> mentioned

[ruiu]

Ah, Aether is just a placeholder object that is going to be output as a real section in the output. That's a clever way of handling the symbols defined before the first section. LGTM.

[grimar]

Thanks !

[grimar]

r294252

ELF/LinkerScript.cpp

template <class ELFT> void LinkerScript<ELFT>::placeOrphanSections() {
   }
 }
 
 template <class ELFT>
 void LinkerScript<ELFT>::assignAddresses(std::vector<PhdrEntry> &Phdrs) {
   // Assign addresses as instructed by linker script SECTIONS sub-commands.
   Dot = 0;
 
+  // A symbol can be assigned before any section is mention in the linker
+  // script. In an DSO, the symbol values are addresses, so the only important
+  // section values are:
+  // * SHN_UNDEF
+  // * SHN_ABS
+  // * Any value meaning a regular section.
+  // To handle that, create a dummy aether section that fills the void before
+  // the linker scripts switches to another section. It has an index of one
+  // which will map to whatever the first actual section is.
+  auto *Aether = make<OutputSectionBase>("", 0, SHF_ALLOC);
+  Aether->SectionIndex = 1;
+  switchTo(Aether);
+
   for (const std::unique_ptr<BaseCommand> &Base : Opt.Commands) {
     if (auto *Cmd = dyn_cast<SymbolAssignment>(Base.get())) {
       if (Cmd->Name == ".") {
         Dot = Cmd->Expression(Dot);
       } else if (Cmd->Sym) {
         assignSectionSymbol<ELFT>(Cmd, Dot);
       }
       continue;
template <class ELFT> bool LinkerScript<ELFT>::isAbsolute(StringRef S) {
   return DR && !DR->Section;
 }
 
 // Gets section symbol belongs to. Symbol "." doesn't belong to any
 // specific section but isn't absolute at the same time, so we try
 // to find suitable section for it as well.
 template <class ELFT>
 const OutputSectionBase *LinkerScript<ELFT>::getSymbolSection(StringRef S) {
-  SymbolBody *Sym = Symtab<ELFT>::X->find(S);
-  if (!Sym) {
-    if (OutputSections->empty())
-      return nullptr;
-    return CurOutSec ? CurOutSec : (*OutputSections)[0];
-  }
-
+  if (SymbolBody *Sym = Symtab<ELFT>::X->find(S))
     return SymbolTableSection<ELFT>::getOutputSection(Sym);
+  return CurOutSec;
 }
 
 // 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) {

test/ELF/linkerscript/non-absolute2.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t1.o
+# RUN: echo "SECTIONS { A = . + 0x1; . += 0x1000; }" > %t.script
+# RUN: ld.lld -shared %t1.o --script %t.script -o %t
+# RUN: llvm-objdump -section-headers -t %t | FileCheck %s
+
+# CHECK:       Sections:
+# CHECK-NEXT:   Idx Name          Size      Address
+# CHECK-NEXT:    0               00000000 0000000000000000
+# CHECK-NEXT:    1 .text         00000000 0000000000001000
+
+# CHECK: 0000000000000001         .text            00000000 A