Page History

...

4. Cross-compiled functions (TODO) 

I do not recommend plowing through the LLVM tutorial. It has some useful info, but is mostly useless for our purposes.

...

• Never use IRBuilder::CreateAlloca() directly. Instead, use LlvmCodegen::CreateEntryBlockAlloca(). Creating alloca's in the middle of functions will yield strange results (e.g. bad optimizations, blowing the stack).
• When you create a new cross-compiled file (i.e. a *-ir.cc file), you need to add it to codegen/impala-ir.cc to include it in the IR module.
• After generating an IR function, you should call OptimizeFunctionWithExprs() or FinalizeFunction() on it, as well as AddFunctionToJit() (TODO: expand on this)

Code Overview

The planner divides a query into fragments, the BE runs multiple fragment instances. Each instance is represented by FragmentInstance in fragment-instance-state.cc. The Open() method checks if code generation should be done, and if so, triggers the codegen work by calling the CodeGen() method on on the set of exec nodes (operators), which will call CodeGen() on expressions (for those nodes that have expressions.)

Consider an expression in the SELECT clause. Impala is row-based, with each row represented by TupleRow (tuple.h). TupleRow::MaterializeExprs() iterates over the expressions to be used to materialize a row, which will include either a slot reference or a scalar expr (ScalarExpr). For each, GetValue() is called, which is either a code generated function or an interpreted function. This function itself can be code generated as the MaterializeExprs function.

Useful APIs and references

...