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ELF: Compute used bit for --as-needed during symbol resolution. 

We can now use this to decide whether to emit a verneed during the final
pass over the symbols. We were previously wrongly creating a verneed entry
in the case where all references to a DSO's symbols were weak.

In a future change we may also want to use the used bit to control whether
shared symbols are preemptible and appear in the dynsym. This seems a little
tricky to do at the moment because isNeeded() is templated.

The only other functional change here is that we emit a DT_NEEDED for DSOs
whose symbols are all preempted by objects that appear later in the link. But
that doesn't seem too important to me.

Differential Revision: http://reviews.llvm.org/D21171

llvm-svn: 272282
This commit is contained in:
Peter Collingbourne 2016-06-09 18:01:35 +00:00
parent 00b385e3a5
commit ca8c994818
3 changed files with 26 additions and 10 deletions
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14
lld/ELF/SymbolTable.cpp
View File

@ -238,9 +238,12 @@ Symbol *SymbolTable<ELFT>::addUndefined(StringRef Name, uint8_t Binding,
cast<Undefined>(S->body())->File = File;
return S;
}
if (Binding != STB_WEAK &&
(S->body()->isShared() || S->body()->isLazy()))
S->Binding = Binding;
if (Binding != STB_WEAK) {
if (S->body()->isShared() || S->body()->isLazy())
S->Binding = Binding;
if (auto *SS = dyn_cast<SharedSymbol<ELFT>>(S->body()))
SS->File->IsUsed = true;
}
if (auto *L = dyn_cast<Lazy>(S->body())) {
// An undefined weak will not fetch archive members, but we have to remember
// its type. See also comment in addLazyArchive.
@ -393,8 +396,11 @@ void SymbolTable<ELFT>::addShared(SharedFile<ELFT> *F, StringRef Name,
// Make sure we preempt DSO symbols with default visibility.
if (Sym.getVisibility() == STV_DEFAULT)
S->ExportDynamic = true;
if (WasInserted || isa<Undefined>(S->body()))
if (WasInserted || isa<Undefined>(S->body())) {
replaceBody<SharedSymbol<ELFT>>(S, F, Name, Sym, Verdef);
if (!S->isWeak())
F->IsUsed = true;
}
}
template <class ELFT>

8
lld/ELF/Writer.cpp
View File

@ -813,11 +813,6 @@ template <class ELFT> void Writer<ELFT>::createSections() {
for (Symbol *S : Symtab.getSymbols()) {
SymbolBody *Body = S->body();
// Set "used" bit for --as-needed.
if (S->IsUsedInRegularObj && !S->isWeak())
if (auto *SS = dyn_cast<SharedSymbol<ELFT>>(Body))
SS->File->IsUsed = true;
// We only report undefined symbols in regular objects. This means that we
// will accept an undefined reference in bitcode if it can be optimized out.
if (S->IsUsedInRegularObj && Body->isUndefined() && !S->isWeak())
@ -834,7 +829,8 @@ template <class ELFT> void Writer<ELFT>::createSections() {
if (isOutputDynamic() && S->includeInDynsym()) {
Out<ELFT>::DynSymTab->addSymbol(Body);
if (auto *SS = dyn_cast<SharedSymbol<ELFT>>(Body))
Out<ELFT>::VerNeed->addSymbol(SS);
if (SS->File->isNeeded())
Out<ELFT>::VerNeed->addSymbol(SS);
}
}

14
lld/test/ELF/verneed-as-needed-weak.s Normal file
View File

@ -0,0 +1,14 @@
# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
# RUN: ld.lld %t.o --as-needed %S/Inputs/verneed1.so -o %t
# RUN: llvm-readobj -V %t | FileCheck %s
# CHECK: SHT_GNU_verneed {
# CHECK-NEXT: }
.weak f1
.globl _start
_start:
.data
.long f1