ELF: Simplify handling of *_start/*_end symbols.

[pcc]

Summary

We were previously using the (static) addSynthetic function to create
*_start/*_end symbols. This function was doing almost the same thing as
addOptionalSynthetic, except that it would also create the symbol in the
case where it is unreferenced. Because the symbol has hidden visibility,
creating it in that case would have no effect other than adding another
entry to the static symbol table. Remove addSynthetic and change callers to
use addOptionalSynthetic instead.

[ruiu]

My memory about it is not clear, but I think there was a reason to add addSynethic function. Eugene may know about that.

[Eugene.Zelenko]

Probably you meant @evgeny777?

[ruiu]

Ah, yes, sorry about that.

[evgeny777]

addSynthetic() was introduced to allow defining start/end symbols in linker scripts

Please take a look at
https://reviews.llvm.org/D23405

[ruiu]

LGTM.

Eugene, if you have any comment, please leave a comment as a post-commit review.

lld/trunk/ELF/Writer.cpp

addOptionalSynthetic(StringRef Name, OutputSectionBase<ELFT> *Sec,
   SymbolBody *S = Symtab<ELFT>::X->find(Name);
   if (!S)
     return nullptr;
   if (!S->isUndefined() && !S->isShared())
     return S->symbol();
   return Symtab<ELFT>::X->addSynthetic(Name, Sec, Val, StOther);
 }
 
-template <class ELFT>
-static void addSynthetic(StringRef Name, OutputSectionBase<ELFT> *Sec,
-                         typename ELFT::uint Val) {
-  SymbolBody *S = Symtab<ELFT>::X->find(Name);
-  if (!S || S->isUndefined() || S->isShared())
-    Symtab<ELFT>::X->addSynthetic(Name, Sec, Val, STV_HIDDEN);
-}
-
 // The beginning and the ending of .rel[a].plt section are marked
 // with __rel[a]_iplt_{start,end} symbols if it is a statically linked
 // executable. The runtime needs these symbols in order to resolve
 // all IRELATIVE relocs on startup. For dynamic executables, we don't
 // need these symbols, since IRELATIVE relocs are resolved through GOT
 // and PLT. For details, see http://www.airs.com/blog/archives/403.
 template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
   if (Out<ELFT>::DynSymTab || !Out<ELFT>::RelaPlt)
if (Out<ELFT>::Bss->getSize() > 0)
     Add(Out<ELFT>::Bss);
 }
 
 // The linker is expected to define SECNAME_start and SECNAME_end
 // symbols for a few sections. This function defines them.
 template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
   auto Define = [&](StringRef Start, StringRef End,
                     OutputSectionBase<ELFT> *OS) {
-    if (OS) {
-      addSynthetic(Start, OS, 0);
-      addSynthetic(End, OS, DefinedSynthetic<ELFT>::SectionEnd);
-    } else {
-      addOptionalSynthetic(Start, (OutputSectionBase<ELFT> *)nullptr, 0);
-      addOptionalSynthetic(End, (OutputSectionBase<ELFT> *)nullptr, 0);
-    }
+    // These symbols resolve to the image base if the section does not exist.
+    addOptionalSynthetic(Start, OS, 0);
+    addOptionalSynthetic(End, OS, OS ? DefinedSynthetic<ELFT>::SectionEnd : 0);
   };
 
   Define("__preinit_array_start", "__preinit_array_end",
          Out<ELFT>::PreinitArray);
   Define("__init_array_start", "__init_array_end", Out<ELFT>::InitArray);
   Define("__fini_array_start", "__fini_array_end", Out<ELFT>::FiniArray);
 }
 

lld/trunk/test/ELF/gc-sections.s

 # NOGC: Name: .fini
 # NOGC: Name: .debug_pubtypes
 # NOGC: Name: .comment
 # NOGC: Name: a
 # NOGC: Name: b
 # NOGC: Name: c
 # NOGC: Name: x
 # NOGC: Name: y
-# NOGC: Name: __preinit_array_start
-# NOGC: Name: __preinit_array_end
 # NOGC: Name: d
 
 # GC1:     Name: .eh_frame
 # GC1:     Name: .text
 # GC1:     Name: .ctors
 # GC1:     Name: .dtors
 # GC1:     Name: .init
 # GC1:     Name: .fini
 # GC1:     Name: .debug_pubtypes
 # GC1:     Name: .comment
 # GC1:     Name: a
 # GC1:     Name: b
 # GC1:     Name: c
 # GC1-NOT: Name: x
 # GC1-NOT: Name: y
-# GC1:     Name: __preinit_array_start
-# GC1:     Name: __preinit_array_end
 # GC1-NOT: Name: d
 
 # GC2:     Name: .eh_frame
 # GC2:     Name: .text
 # GC2:     Name: .ctors
 # GC2:     Name: .dtors
 # GC2:     Name: .init
 # GC2:     Name: .fini
 # GC2:     Name: .debug_pubtypes
 # GC2:     Name: .comment
 # GC2:     Name: a
 # GC2:     Name: b
 # GC2:     Name: c
 # GC2-NOT: Name: x
 # GC2-NOT: Name: y
-# GC2:     Name: __preinit_array_start
-# GC2:     Name: __preinit_array_end
 # GC2:     Name: d
 
 .globl _start, d
 .protected a, b, c, x, y
 _start:
   call a
 
 .section .text.a,"ax",@progbits