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[ELF] Define symbols "_end" and "end" if referenced.

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These symbols are expected to point to the end of the data segment.

Implements http://llvm.org/pr25528.

Differential Revision: http://reviews.llvm.org/D14833

llvm-svn: 253637
This commit is contained in:
Igor Kudrin
2015-11-20 02:32:35 +00:00
parent dcb5653516
commit b044af50f2
7 changed files with 159 additions and 5 deletions
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21
lld/ELF/Writer.cpp
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@@ -623,6 +623,23 @@ template <class ELFT> void Writer<ELFT>::createSections() {
if (!isOutputDynamic())
Symtab.addIgnoredSym("__tls_get_addr");
// If the "_end" symbol is referenced, it is expected to point to the address
// right after the data segment. Usually, this symbol points to the end
// of .bss section or to the end of .data section if .bss section is absent.
// The order of the sections can be affected by linker script,
// so it is hard to predict which section will be the last one.
// So, if this symbol is referenced, we just add the placeholder here
// and update its value later.
if (Symtab.find("_end"))
Symtab.addAbsoluteSym("_end", DefinedAbsolute<ELFT>::End);
// If there is an undefined symbol "end", we should initialize it
// with the same value as "_end". In any other case it should stay intact,
// because it is an allowable name for a user symbol.
if (SymbolBody *B = Symtab.find("end"))
if (B->isUndefined())
Symtab.addAbsoluteSym("end", DefinedAbsolute<ELFT>::End);
// Scan relocations. This must be done after every symbol is declared so that
// we can correctly decide if a dynamic relocation is needed.
for (const std::unique_ptr<ObjectFile<ELFT>> &F : Symtab.getObjectFiles()) {
@@ -879,6 +896,10 @@ template <class ELFT> void Writer<ELFT>::assignAddresses() {
SectionHeaderOff = RoundUpToAlignment(FileOff, ELFT::Is64Bits ? 8 : 4);
FileSize = SectionHeaderOff + getNumSections() * sizeof(Elf_Shdr);
// Update "_end" and "end" symbols so that they
// point to the end of the data segment.
DefinedAbsolute<ELFT>::End.st_value = VA;
// Update MIPS _gp absolute symbol so that it points to the static data.
if (Config->EMachine == EM_MIPS)
DefinedAbsolute<ELFT>::MipsGp.st_value = getMipsGpAddr<ELFT>();