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COFF: Symbol resolution for common and comdat symbols defined in bitcode. 

In the case where either a bitcode file and a regular file or two bitcode
files export a common or comdat symbol with the same name, the linker needs
to pick one of them following COFF semantics. This patch implements a design
for resolving such symbols that pushes most of the work onto either LLD's
regular mechanism for resolving common or comdat symbols or the IR linker's
mechanism for doing the same.

We modify SymbolBody::compare to always prefer non-bitcode symbols, so that
during the initial phase of symbol resolution, the symbol table always contains
a regular symbol in any case where we need to choose between a regular and
a bitcode symbol. In SymbolTable::addCombinedLTOObject, we force export
any bitcode symbols that were initially pre-empted by a regular symbol,
and later use SymbolBody::compare to choose between the regular symbol in
the symbol table and the regular symbol from the combined LTO object file.

This design seems to be sound, so long as the resolution mechanism is defined
to be commutative and associative modulo arbitrary choices between symbols
(which seems to be the case for COFF).

Differential Revision: http://reviews.llvm.org/D10329

llvm-svn: 239563
This commit is contained in:
Peter Collingbourne 2015-06-11 21:49:54 +00:00
parent 767c45719f
commit 1b6fd1f5fd
7 changed files with 214 additions and 3 deletions
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4
lld/COFF/InputFiles.cpp
View File

@ -266,7 +266,9 @@ std::error_code BitcodeFile::parse() {
if (SymbolDef == LTO_SYMBOL_DEFINITION_UNDEFINED) {
SymbolBodies.push_back(new (Alloc) Undefined(SymName));
} else {
SymbolBodies.push_back(new (Alloc) DefinedBitcode(SymName));
bool Replaceable = (SymbolDef == LTO_SYMBOL_DEFINITION_TENTATIVE ||
(Attrs & LTO_SYMBOL_COMDAT));
SymbolBodies.push_back(new (Alloc) DefinedBitcode(SymName, Replaceable));
}
}

15
lld/COFF/SymbolTable.cpp
View File

@ -268,6 +268,14 @@ std::error_code SymbolTable::addCombinedLTOObject() {
return make_error_code(LLDError::BrokenFile);
}
Sym->Body = Body;
} else {
int comp = Sym->Body->compare(Body);
if (comp < 0)
Sym->Body = Body;
if (comp == 0) {
llvm::errs() << "LTO: unexpected duplicate symbol: " << Name << "\n";
return make_error_code(LLDError::BrokenFile);
}
}
// We may see new references to runtime library symbols such as __chkstk
@ -300,6 +308,13 @@ ErrorOr<ObjectFile *> SymbolTable::createLTOObject(LTOCodeGenerator *CG) {
if (auto *S = dyn_cast<DefinedBitcode>(Body->getReplacement()))
CG->addMustPreserveSymbol(S->getName());
// Likewise for bitcode symbols which we initially resolved to non-bitcode.
for (std::unique_ptr<BitcodeFile> &File : BitcodeFiles)
for (SymbolBody *Body : File->getSymbols())
if (isa<DefinedBitcode>(Body) &&
!isa<DefinedBitcode>(Body->getReplacement()))
CG->addMustPreserveSymbol(Body->getName());
// Likewise for other symbols that must be preserved.
for (StringRef Name : Config->GCRoots)
if (isa<DefinedBitcode>(Symtab[Name]->Body))

35
lld/COFF/Symbols.cpp
View File

@ -21,14 +21,31 @@ using llvm::sys::fs::file_magic;
namespace lld {
namespace coff {
// As an approximation, regular symbols win over bitcode symbols, but we
// definitely have a conflict if the regular symbol is not replaceable and
// neither is the bitcode symbol. We do not replicate the rest of the symbol
// resolution logic here; symbol resolution will be done accurately after
// lowering bitcode symbols to regular symbols in addCombinedLTOObject().
static int compareRegularBitcode(DefinedRegular *R, DefinedBitcode *B) {
if (!R->isCommon() && !R->isCOMDAT() && !B->isReplaceable())
return 0;
return 1;
}
// Returns 1, 0 or -1 if this symbol should take precedence over the
// Other in the symbol table, tie or lose, respectively.
int Defined::compare(SymbolBody *Other) {
if (!isa<Defined>(Other))
return 1;
auto *X = dyn_cast<DefinedRegular>(this);
if (!X)
return 0;
if (auto *B = dyn_cast<DefinedBitcode>(Other))
return compareRegularBitcode(X, B);
auto *Y = dyn_cast<DefinedRegular>(Other);
if (!X || !Y)
if (!Y)
return 0;
// Common symbols are weaker than other types of defined symbols.
@ -46,6 +63,22 @@ int Defined::compare(SymbolBody *Other) {
return 0;
}
int DefinedBitcode::compare(SymbolBody *Other) {
if (!isa<Defined>(Other))
return 1;
if (auto *R = dyn_cast<DefinedRegular>(Other))
return -compareRegularBitcode(R, this);
if (auto *B = dyn_cast<DefinedBitcode>(Other)) {
if (!isReplaceable() && !B->isReplaceable())
return 0;
// Non-replaceable symbols win.
return isReplaceable() ? -1 : 1;
}
return 0;
}
int Lazy::compare(SymbolBody *Other) {
if (isa<Defined>(Other))
return -1;

6
lld/COFF/Symbols.h
View File

@ -267,7 +267,8 @@ private:
class DefinedBitcode : public Defined {
public:
DefinedBitcode(StringRef N) : Defined(DefinedBitcodeKind), Name(N) {}
DefinedBitcode(StringRef N, bool R)
: Defined(DefinedBitcodeKind), Name(N), Replaceable(R) {}
static bool classof(const SymbolBody *S) {
return S->kind() == DefinedBitcodeKind;
@ -276,9 +277,12 @@ public:
StringRef getName() override { return Name; }
uint64_t getRVA() override { llvm_unreachable("bitcode reached writer"); }
uint64_t getFileOff() override { llvm_unreachable("bitcode reached writer"); }
int compare(SymbolBody *Other) override;
bool isReplaceable() const { return Replaceable; }
private:
StringRef Name;
bool Replaceable;
};
} // namespace coff

13
lld/test/COFF/Inputs/lto-comdat1.ll Normal file
View File

@ -0,0 +1,13 @@
target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc"
$comdat = comdat any
define void @f1() {
call void @comdat()
ret void
}
define linkonce_odr void @comdat() comdat {
ret void
}

13
lld/test/COFF/Inputs/lto-comdat2.ll Normal file
View File

@ -0,0 +1,13 @@
target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc"
$comdat = comdat any
define void @f2() {
call void @comdat()
ret void
}
define linkonce_odr void @comdat() comdat {
ret void
}

131
lld/test/COFF/lto-comdat.ll Normal file
View File

@ -0,0 +1,131 @@
; RUN: llvm-as -o %T/comdat-main.lto.obj %s
; RUN: llvm-as -o %T/comdat1.lto.obj %S/Inputs/lto-comdat1.ll
; RUN: llvm-as -o %T/comdat2.lto.obj %S/Inputs/lto-comdat2.ll
; RUN: rm -f %T/comdat.lto.lib
; RUN: llvm-ar cru %T/comdat.lto.lib %T/comdat1.lto.obj %T/comdat2.lto.obj
; RUN: llc -filetype=obj -o %T/comdat-main.obj %s
; RUN: llc -filetype=obj -o %T/comdat1.obj %S/Inputs/lto-comdat1.ll
; RUN: llc -filetype=obj -o %T/comdat2.obj %S/Inputs/lto-comdat2.ll
; RUN: rm -f %T/comdat.lib
; RUN: llvm-ar cru %T/comdat.lib %T/comdat1.obj %T/comdat2.obj
; RUN: lld -flavor link2 /out:%T/comdat-main.exe /entry:main /subsystem:console %T/comdat-main.lto.obj %T/comdat1.lto.obj %T/comdat2.lto.obj
; RUN: llvm-readobj -file-headers %T/comdat-main.exe | FileCheck -check-prefix=HEADERS-11 %s
; RUN: llvm-objdump -d %T/comdat-main.exe | FileCheck -check-prefix=TEXT-11 %s
; RUN: lld -flavor link2 /out:%T/comdat-main.exe /entry:main /subsystem:console %T/comdat-main.lto.obj %T/comdat.lto.lib
; RUN: llvm-readobj -file-headers %T/comdat-main.exe | FileCheck -check-prefix=HEADERS-11 %s
; RUN: llvm-objdump -d %T/comdat-main.exe | FileCheck -check-prefix=TEXT-11 %s
; RUN: lld -flavor link2 /out:%T/comdat-main.exe /entry:main /subsystem:console %T/comdat-main.obj %T/comdat1.lto.obj %T/comdat2.lto.obj
; RUN: llvm-readobj -file-headers %T/comdat-main.exe | FileCheck -check-prefix=HEADERS-01 %s
; RUN: llvm-objdump -d %T/comdat-main.exe | FileCheck -check-prefix=TEXT-01 %s
; RUN: lld -flavor link2 /out:%T/comdat-main.exe /entry:main /subsystem:console %T/comdat-main.obj %T/comdat.lto.lib
; RUN: llvm-readobj -file-headers %T/comdat-main.exe | FileCheck -check-prefix=HEADERS-01 %s
; RUN: llvm-objdump -d %T/comdat-main.exe | FileCheck -check-prefix=TEXT-01 %s
; RUN: lld -flavor link2 /out:%T/comdat-main.exe /entry:main /subsystem:console %T/comdat-main.lto.obj %T/comdat1.obj %T/comdat2.obj
; RUN: llvm-readobj -file-headers %T/comdat-main.exe | FileCheck -check-prefix=HEADERS-10 %s
; RUN: llvm-objdump -d %T/comdat-main.exe | FileCheck -check-prefix=TEXT-10 %s
; RUN: lld -flavor link2 /out:%T/comdat-main.exe /entry:main /subsystem:console %T/comdat-main.lto.obj %T/comdat.lib
; RUN: llvm-readobj -file-headers %T/comdat-main.exe | FileCheck -check-prefix=HEADERS-10 %s
; RUN: llvm-objdump -d %T/comdat-main.exe | FileCheck -check-prefix=TEXT-10 %s
; HEADERS-11: AddressOfEntryPoint: 0x1000
; TEXT-11: Disassembly of section .text:
; TEXT-11-NEXT: .text:
; TEXT-11-NEXT: xorl %eax, %eax
; TEXT-11-NEXT: retq
; HEADERS-01: AddressOfEntryPoint: 0x2000
; TEXT-01: Disassembly of section .text:
; TEXT-01-NEXT: .text:
; TEXT-01-NEXT: subq $40, %rsp
; TEXT-01-NEXT: callq 39
; TEXT-01-NEXT: callq 50
; TEXT-01-NEXT: callq 13
; TEXT-01-NEXT: xorl %eax, %eax
; TEXT-01-NEXT: addq $40, %rsp
; TEXT-01-NEXT: retq
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: retq
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: int3
; TEXT-01-NEXT: retq
; TEXT-01-NEXT: nopw %cs:(%rax,%rax)
; TEXT-01-NEXT: retq
; HEADERS-10: AddressOfEntryPoint: 0x2030
; TEXT-10: Disassembly of section .text:
; TEXT-10-NEXT: .text:
; TEXT-10-NEXT: subq $40, %rsp
; TEXT-10-NEXT: callq 7
; TEXT-10-NEXT: nop
; TEXT-10-NEXT: addq $40, %rsp
; TEXT-10-NEXT: retq
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: retq
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: subq $40, %rsp
; TEXT-10-NEXT: callq -25
; TEXT-10-NEXT: nop
; TEXT-10-NEXT: addq $40, %rsp
; TEXT-10-NEXT: retq
; TEXT-10-NEXT: int3
; TEXT-10-NEXT: subq $40, %rsp
; TEXT-10-NEXT: callq -57
; TEXT-10-NEXT: callq -30
; TEXT-10-NEXT: xorl %eax, %eax
; TEXT-10-NEXT: addq $40, %rsp
; TEXT-10-NEXT: retq
target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc"
$comdat = comdat any
define i32 @main() {
call void @f1()
call void @f2()
call void @comdat()
ret i32 0
}
define linkonce_odr void @comdat() comdat {
ret void
}
declare void @f1()
declare void @f2()