Quartz 4

clang_link_backend

2 min read

clang llvm

Backend Link

We’ve parsed a complete top-level statement and now need to forward it to the backend for code generation. This section clarifies how Clang transitions from parsing to emitting IR via CodeGenModule::EmitTopLevelDecl:

Parsing Loop (clang::ParseAST)

for (bool AtEOF = P.ParseFirstTopLevelDecl(ADecl, ImportState);
     !AtEOF;
     AtEOF = P.ParseTopLevelDecl(ADecl, ImportState)) {
  if (ADecl && !Consumer->HandleTopLevelDecl(ADecl.get()))
    return;
}
  • ParseFirstTopLevelDecl / ParseTopLevelDecl: Retrieve the next DeclGroupRef from the parser
  • Consumer->HandleTopLevelDecl: Invokes the backend-specific handler with the parsed declaration

Backend Dispatch Chain

  1. BackendConsumer::HandleTopLevelDecl

    • Entry point from the frontend to the code-generation backend

  2. CodeGeneratorImpl::HandleTopLevelDecl

    • Iterates over each Decl in the DeclGroupRef, passing them on to the builder

  3. CodeGen::CodeGenModule::EmitTopLevelDecl

    • Final step to inspect and emit each individual Decl

CodeGenModule::EmitTopLevelDecl

  • Dispatch Role: Determines the nature of the Decl and delegates accordingly with a switch case
  • EmitGlobal(GlobalDecl): Converts FunctionDecl/VarDecl into LLVM IR objects
  • Contextual Processing: For containers like namespaces or records, it may recurse

CodeGenModule::EmitGlobal

  • If weak attribute don’t emit code by themselves
  • If alias return EmitAliasDefinition
  • If ifunc return EmitIFuncDefinition
  • If cpu_dispatch return emitCPUDispatchDefinition
  • Handle cuda
  • Handle OpenMP
  • If function decl
    • handle if attribute annotate("...")
    • if func dosent have a body emit the code (GetOrCreateLLVMFunction)
  • Else variable decl
    • if openMP
    • Ensure strong inline variable are emitted
  • Emit required global immediately if possible (EmitGlobalDefinition)
  • Track C++ global initializer order
  • Emit used declaration later
  • Defer required but non-eager decl
  • Register lazy deferred declaration

CodeGenModule::EmitGlobalDefinition

  • If function
    • Handle c++ specific
    • If multi version (target_clones("avx2","sse2") EmitMultiVersionFunctionDefinition
    • Else EmitGlobalFunctionDefinition
  • Else EmitGlobalVarDefinition

CodeGenModule::EmitGlobalFunctionDefinition

  • Get function info
  • Get function type
  • Get or create the func proto
  • If already emit return
  • Chooses external vs internal linkage based on static
  • Adds attributes (nounwind, uwtable, calling-conv, etc.)
  • Ensures C-name mangling but still internal linkage
  • Added comdat + weak_odr on the inline function
  • GenerateCode
  • Set attribute
  • If a Constructor or Destructor add it to the right table
  • If OpenMP add emit target function

CodeGenFunction::GenerateCode

  • Build function arg list
  • Handle if the function is a inline
  • Replace inline stub with definition
  • If no debug clear all info
  • Distinguish a function definition from a proto definition
  • Find the location
  • Handle function from template
  • Handle coroutines
  • Start function
  • Switch case for multiple special case
    • C++ constructor/destructor/method…
    • Else EmitFunctionBody
  • Inserting (as configured) a sanitizer check or trap and an unreachable before the closing brace
  • Finish function
  • Check if you can make the function a not throw

CodeGenFunction::EmitFunctionBody

To be explained

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