Segfault caused by circular construction dependency between LowerModule and LowerTypes

[mahmood82]

I'm encountering a segmentation fault when calling a method on
LowerModule::getTargetLoweringInfo(). After debugging, it appears to be caused by a circular dependency between LowerModule and LowerTypes during construction.

The construction chain begins in cir::createLowerModule:

return std::make_unique<LowerModule>(std::move(langOpts),
                                     std::move(codeGenOpts), module,
                                     std::move(targetInfo), rewriter);

Inside LowerModule's constructor, the types member is constructed before the
target lowering info exists:

LowerModule::LowerModule(clang::LangOptions langOpts,
                         clang::CodeGenOptions codeGenOpts,
                         mlir::ModuleOp &module,
                         std::unique_ptr<clang::TargetInfo> target,
                         mlir::PatternRewriter &rewriter)
    : context(module, std::move(langOpts), std::move(codeGenOpts)),
      module(module), Target(std::move(target)), ABI(createCXXABI(*this)),
      types(*this), rewriter(rewriter) {
  context.initBuiltinTypes(*Target);
}

LowerTypes immediately queries LowerModule::getTargetLoweringInfo():

LowerTypes::LowerTypes(LowerModule &LM)
    : LM(LM), context(LM.getContext()), Target(LM.getTarget()),
      CXXABI(LM.getCXXABI()),
      TheABIInfo(LM.getTargetLoweringInfo().getABIInfo()),
      mlirContext(LM.getMLIRContext()), DL(LM.getModule()) {}

This triggers:

const TargetLoweringInfo &LowerModule::getTargetLoweringInfo() {
  if (!TheTargetCodeGenInfo)
    TheTargetCodeGenInfo = createTargetLoweringInfo(*this);
  return *TheTargetCodeGenInfo;
}

And in my configuration, createTargetLoweringInfo resolves to:

std::unique_ptr<TargetLoweringInfo>
createSPIRVTargetLoweringInfo(LowerModule &lowerModule) {
  return std::make_unique<SPIRVTargetLoweringInfo>(lowerModule.getTypes());
}

At this point, lowerModule.getTypes() returns the LowerTypes instance that is still under construction, and dereferencing fields inside it triggers a segmentation fault.

So effectively:

• LowerModule constructor ==> constructs types
• LowerTypes constructor ==> queries getTargetLoweringInfo
• createTargetLoweringInfo ==> asks again for LowerTypes
• ==> circular dependency ==> crash

This looks like a design-time initialization order violation.

[seven-mile]

Nice catch👍 Adjust the init order of fields in the ctor of LowerTypes may help.