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Refactor how -fno-semantic-interposition sets dso_local on default visibility external linkage definitions

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The idea is that the CC1 default for ELF should set dso_local on default
visibility external linkage definitions in the default -mrelocation-model pic
mode (-fpic/-fPIC) to match COFF/Mach-O and make output IR similar.

The refactoring is made available by 2820a2ca3a0e69c3f301845420e0067ffff2251b.

Currently only x86 supports local aliases. We move the decision to the driver.
There are three CC1 states:

* -fsemantic-interposition: make some linkages interposable and make default visibility external linkage definitions dso_preemptable.
* (default): selected if the target supports .Lfoo$local: make default visibility external linkage definitions dso_local
* -fhalf-no-semantic-interposition: if neither option is set or the target does not support .Lfoo$local: like -fno-semantic-interposition but local aliases are not used. So references can be interposed if not optimized out.

Add -fhalf-no-semantic-interposition to a few tests using the half-based semantic interposition behavior.
This commit is contained in:
Fangrui Song 2020-12-31 13:59:45 -08:00
parent 2eb058056b
commit c7c5340c75
3 changed files with 2 additions and 12 deletions
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1
include/llvm/IR/Module.h
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@ -855,7 +855,6 @@ public:
/// Returns whether semantic interposition is to be respected.
bool getSemanticInterposition() const;
bool noSemanticInterposition() const;
/// Set whether semantic interposition is to be respected.
void setSemanticInterposition(bool);

6
lib/CodeGen/AsmPrinter/AsmPrinter.cpp
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@ -483,10 +483,8 @@ MCSymbol *AsmPrinter::getSymbolPreferLocal(const GlobalValue &GV) const {
if (TM.getTargetTriple().isOSBinFormatELF() && GV.canBenefitFromLocalAlias()) {
const Module &M = *GV.getParent();
if (TM.getRelocationModel() != Reloc::Static &&
M.getPIELevel() == PIELevel::Default)
if (GV.isDSOLocal() || (TM.getTargetTriple().isX86() &&
GV.getParent()->noSemanticInterposition()))
return getSymbolWithGlobalValueBase(&GV, "$local");
M.getPIELevel() == PIELevel::Default && GV.isDSOLocal())
return getSymbolWithGlobalValueBase(&GV, "$local");
}
return TM.getSymbol(&GV);
}

7
lib/IR/Module.cpp
View File

@ -601,13 +601,6 @@ void Module::setSemanticInterposition(bool SI) {
addModuleFlag(ModFlagBehavior::Error, "SemanticInterposition", SI);
}
bool Module::noSemanticInterposition() const {
// Conservatively require an explicit zero value for now.
Metadata *MF = getModuleFlag("SemanticInterposition");
auto *Val = cast_or_null<ConstantAsMetadata>(MF);
return Val && cast<ConstantInt>(Val->getValue())->getZExtValue() == 0;
}
void Module::setOwnedMemoryBuffer(std::unique_ptr<MemoryBuffer> MB) {
OwnedMemoryBuffer = std::move(MB);
}