From fabc740f072539d4b409f2a840c8afd3f191e656 Mon Sep 17 00:00:00 2001 From: David Majnemer Date: Tue, 1 Jul 2014 22:37:08 +0000 Subject: [PATCH] CodeGen: Add some comments describing VFTable CG No functionality change, just some comments to describe what is going on. git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212142 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/CodeGen/MicrosoftCXXABI.cpp | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) diff --git a/lib/CodeGen/MicrosoftCXXABI.cpp b/lib/CodeGen/MicrosoftCXXABI.cpp index 5dc0752aef..c586e034d1 100644 --- a/lib/CodeGen/MicrosoftCXXABI.cpp +++ b/lib/CodeGen/MicrosoftCXXABI.cpp @@ -1159,21 +1159,36 @@ llvm::GlobalVariable *MicrosoftCXXABI::getAddrOfVTable(const CXXRecordDecl *RD, VTable = CGM.getModule().getNamedGlobal(VFTableName); if (!VTable) { + // Create a backing variable for the contents of VTable. The VTable may + // or may not include space for a pointer to RTTI data. llvm::GlobalValue *VFTable = VTable = new llvm::GlobalVariable( CGM.getModule(), VTableType, /*isConstant=*/true, VTableLinkage, /*Initializer=*/nullptr, VTableName); VTable->setUnnamedAddr(true); + + // Only insert a pointer into the VFTable for RTTI data if we are not + // importing it. We never reference the RTTI data directly so there is no + // need to make room for it. if (getContext().getLangOpts().RTTIData && !RD->hasAttr()) { llvm::Value *GEPIndices[] = {llvm::ConstantInt::get(CGM.IntTy, 0), llvm::ConstantInt::get(CGM.IntTy, 1)}; + // Create a GEP which points just after the first entry in the VFTable, + // this should be the location of the first virtual method. llvm::Constant *VTableGEP = llvm::ConstantExpr::getInBoundsGetElementPtr(VTable, GEPIndices); + // The symbol for the VFTable is an alias to the GEP. It is + // transparent, to other modules, what the nature of this symbol is; all + // that matters is that the alias be the address of the first virtual + // method. VFTable = llvm::GlobalAlias::create( cast(VTableGEP->getType())->getElementType(), /*AddressSpace=*/0, llvm::GlobalValue::ExternalLinkage, VFTableName.str(), VTableGEP, &CGM.getModule()); } else { + // We don't need a GlobalAlias to be a symbol for the VTable if we won't + // be referencing any RTTI data. The GlobalVariable will end up being + // an appropriate definition of the VFTable. VTable->setName(VFTableName.str()); } @@ -1199,6 +1214,11 @@ llvm::GlobalVariable *MicrosoftCXXABI::getAddrOfVTable(const CXXRecordDecl *RD, VFTableLinkage = llvm::GlobalValue::ExternalLinkage; llvm::Comdat *C = CGM.getModule().getOrInsertComdat(VFTable->getName()); + // We must indicate which VFTable is larger to support linking between + // translation units which do and do not have RTTI data. The largest + // VFTable contains the RTTI data; translation units which reference + // the smaller VFTable always reference it relative to the first + // virtual method. C->setSelectionKind(llvm::Comdat::Largest); VTable->setComdat(C); } else { -- 2.40.0