[clang] Add Documentation for Execution Results Handling in Clang-Repl (PR #65650)
Vassil Vassilev via cfe-commits
cfe-commits at lists.llvm.org
Fri Oct 13 04:16:07 PDT 2023
================
@@ -213,6 +213,411 @@ concept helps support advanced use cases such as template instantiations on dema
automatic language interoperability. It also helps static languages such as C/C++ become
apt for data science.
+Execution Results Handling in Clang-Repl
+========================================
+
+Execution Results Handling features discussed below help extend the Clang-Repl
+functionality by creating an interface between the execution results of a
+program and the compiled program.
+
+1. **Capture Execution Results**: This feature helps capture the execution results
+of a program and bring them back to the compiled program.
+
+2. **Dump Captured Execution Results**: This feature helps create a temporary dump
+for Value Printing/Automatic Printf, that is, to display the value and type of
+the captured data.
+
+
+1. Capture Execution Results
+============================
+
+In many cases, it is useful to bring back the program execution result to the
+compiled program. This result can be stored in an object of type **Value**.
+
+How Execution Results are captured (Value Synthesis):
+-----------------------------------------------------
+
+The synthesizer chooses which expression to synthesize, and then it replaces
+the original expression with the synthesized expression. Depending on the
+expression type, it may choose to save an object (``LastValue``) of type 'value'
+while allocating memory to it (``SetValueWithAlloc()``), or not (
+``SetValueNoAlloc()``).
+
+.. graphviz::
+ :name: valuesynthesis
+ :caption: Value Synthesis
+ :alt: Shows how an object of type 'Value' is synthesized
+ :align: center
+
+ digraph "valuesynthesis" {
+ rankdir="LR";
+ graph [fontname="Verdana", fontsize="12"];
+ node [fontname="Verdana", fontsize="12"];
+ edge [fontname="Sans", fontsize="9"];
+
+ start [label=" Create an Object \n 'Last Value' \n of type 'Value' ", shape="note", fontcolor=white, fillcolor="#3333ff", style=filled];
+ assign [label=" Assign the result \n to the 'LastValue' \n (based on respective \n Memory Allocation \n scenario) ", shape="box"]
+ print [label=" Pretty Print \n the Value Object ", shape="Msquare", fillcolor="yellow", style=filled];
+ start -> assign;
+ assign -> print;
+
+ subgraph SynthesizeExpression {
+ synth [label=" SynthesizeExpr() ", shape="note", fontcolor=white, fillcolor="#3333ff", style=filled];
+ mem [label=" New Memory \n Allocation? ", shape="diamond"];
+ withaloc [label=" SetValueWithAlloc() ", shape="box"];
+ noaloc [label=" SetValueNoAlloc() ", shape="box"];
+ right [label=" 1. RValue Structure \n (a temporary value)", shape="box"];
+ left2 [label=" 2. LValue Structure \n (a variable with \n an address)", shape="box"];
+ left3 [label=" 3. Built-In Type \n (int, float, etc.)", shape="box"];
+ output [label=" move to 'Assign' step ", shape="box"];
+
+ synth -> mem;
+ mem -> withaloc [label="Yes"];
+ mem -> noaloc [label="No"];
+ withaloc -> right;
+ noaloc -> left2;
+ noaloc -> left3;
+ right -> output;
+ left2 -> output;
+ left3 -> output;
+ }
+ output -> assign
+ }
+
+Where is the captured result stored?
+------------------------------------
+
+``LastValue`` holds the last result of the value printing. It is a class member
+because it can be accessed even after subsequent inputs.
+
+**Note:** If no value printing happens, then it is in an invalid state.
+
+Improving Efficiency and User Experience
+----------------------------------------
+
+The Value object is essentially used to create a mapping between an expression
+'type' and the allocated 'memory'. Built-in types (bool, char, int,
+float, double, etc.) are copyable. Their memory allocation size is known
+and the Value object can introduce a small-buffer optimization.
+In case of objects, the ``Value`` class provides reference-counted memory
+management.
+
+The implementation maps the type as written and the Clang Type to be able to use
+the preprocessor to synthesize the relevant cast operations. For example,
+``X(char, Char_S)``, where ``char`` is the type from the language's type system
+and ``Char_S`` is the Clang builtin type which represents it. This mapping helps
+to import execution results from the interpreter in a compiled program and vice
+versa. The ``Value.h`` header file can be included at runtime and this is why it
+has a very low token count and was developed with strict constraints in mind.
+
+This also enables the user to receive the computed 'type' back in their code
+and then transform the type into something else (e.g., re-cast a double into
+a float). Normally, the compiler can handle these conversions transparently,
+but in interpreter mode, the compiler cannot see all the 'from' and 'to' types,
+so it cannot implicitly do the conversions. So this logic enables providing
+these conversions on request.
+
+On-request conversions can help improve the user experience, by allowing
+conversion to a desired 'to' type, when the 'from' type is unknown or unclear.
+
+Significance of this Feature
+----------------------------
+
+The 'Value' object enables wrapping a memory region that comes from the
+JIT, and bringing it back to the compiled code (and vice versa).
+This is a very useful functionality when:
+
+- connecting an interpreter to the compiled code, or
+- connecting an interpreter in another language.
+
+For example, this feature helps transport values across boundaries. A notable
+example is the cppyy project code makes use of this feature to enable running C++
+within Python. It enables transporting values/information between C++
+and Python.
+
+Note: `cppyy <https://github.com/wlav/cppyy/>`_ is an automatic, run-time,
+Python-to-C++ bindings generator, for calling C++ from Python and Python from C++.
+It uses LLVM along with a C++ interpreter (e.g., Cling) to enable features like
+run-time instantiation of C++ templates, cross-inheritance, callbacks,
+auto-casting, transparent use of smart pointers, etc.
+
+In a nutshell, this feature enables a new way of developing code, paving the
+way for language interoperability and easier interactive programming.
+
+Implementation Details
+======================
+
+Interpreter as a REPL vs. as a Library
+--------------------------------------
+
+1 - If we're using the interpreter in interactive (REPL) mode, it will dump
+the value (i.e., value printing).
+
+.. code-block:: console
+
+ if (LastValue.isValid()) {
+ if (!V) {
+ LastValue.dump();
+ LastValue.clear();
+ } else
+ *V = std::move(LastValue);
+ }
+
+
+2 - If we're using the interpreter as a library, then it will pass the value
+to the user.
+
+Incremental AST Consumer
+------------------------
+
+The ``IncrementalASTConsumer`` class wraps the original code generator
+``ASTConsumer`` and it performs a hook, to traverse all the top-level decls, to
+look for expressions to synthesize, based on the ``isSemiMissing()`` condition.
+
+If this condition is found to be true, then ``Interp.SynthesizeExpr()`` will be
+invoked.
+
+**Note:** Following is a sample code snippet. Actual code may vary over time.
+
+.. code-block:: console
+
+ for (Decl *D : DGR)
+ if (auto *TSD = llvm::dyn_cast<TopLevelStmtDecl>(D);
+ TSD && TSD->isSemiMissing())
+ TSD->setStmt(Interp.SynthesizeExpr(cast<Expr>(TSD->getStmt())));
+
+ return Consumer->HandleTopLevelDecl(DGR);
+
+The synthesizer will then choose the relevant expression, based on its type.
+
+Communication between Compiled Code and Interpreted Code
+--------------------------------------------------------
+
+In Clang-Repl there is **interpreted code**, and this feature adds a 'value'
+runtime that can talk to the **compiled code**.
+
+Following is an example where the compiled code interacts with the interpreter
+code. The execution results of an expression are stored in the object 'V' of
+type Value. This value is then printed, effectively helping the interpreter
+use a value from the compiled code.
+
+.. code-block:: console
+
+ int Global = 42;
+ void setGlobal(int val) { Global = val; }
+ int getGlobal() { return Global; }
+ Interp.ParseAndExecute(“void setGlobal(int val);”);
+ Interp.ParseAndExecute(“int getGlobal();”);
+ Value V;
+ Interp.ParseAndExecute(“getGlobal()”, &V);
+ std::cout << V.getAs<int>() << “\n”; // Prints 42
+
+
+**Note:** Above is an example of interoperability between the compiled code and
+the interpreted code. Interoperability between languages (e.g., C++ and Python)
+works similarly.
+
+
+2. Dump Captured Execution Results
+==================================
+
+This feature helps create a temporary dump to display the value and type
+(pretty print) of the desired data. This is a good way to interact with the
+interpreter during interactive programming.
+
+How value printing is simplified (Automatic Printf)
+---------------------------------------------------
+
+The ``Automatic Printf`` feature makes it easy to display variable values during
+program execution. Using the ``printf`` function repeatedly is not required.
+This is achieved using an extension in the ``libclangInterpreter`` library.
+
+To automatically print the value of an expression, simply write the expression
+in the global scope **without a semicolon**.
+
+.. graphviz::
+ :name: automaticprintf
+ :caption: Automatic PrintF
+ :alt: Shows how Automatic PrintF can be used
+ :align: center
+
+ digraph "AutomaticPrintF" {
+ size="6,4";
+ rankdir="LR";
+ graph [fontname="Verdana", fontsize="12"];
+ node [fontname="Verdana", fontsize="12"];
+ edge [fontname="Sans", fontsize="9"];
+
+ manual [label=" Manual PrintF ", shape="box"];
+ int1 [label=" int ( &) 42 ", shape="box"]
+ auto [label=" Automatic PrintF ", shape="box"];
+ int2 [label=" int ( &) 42 ", shape="box"]
+
+ auto -> int2 [label="int x = 42; \n x"];
+ manual -> int1 [label="int x = 42; \n printf("(int &) %d \\n", x);"];
+ }
+
+
+Significance of this feature
+----------------------------
+
+Inspired by a similar implementation in `Cling <https://github.com/root-project/cling>`_,
+this feature added to upstream Clang repo has essentially extended the syntax of
+C++, so that it can be more helpful for people that are writing code for data
+science applications.
+
+This is useful, for example, when you want to experiment with a set of values
+against a set of functions, and you'd like to know the results right away.
+This is similar to how Python works (hence its popularity in data science
+research), but the superior performance of C++, along with this flexibility
+makes it a more attractive option.
+
+Implementation Details
+======================
+
+Parsing mechanism:
+------------------
+
+The Interpreter in Clang-Repl (``Interpreter.cpp``) includes the function
+``ParseAndExecute()`` that can accept a 'Value' parameter to capture the result.
+But if the value parameter is made optional and it is omitted (i.e., that the
+user does not want to utilize it elsewhere), then the last value can be
+validated and pushed into the ``dump()`` function.
+
+.. graphviz::
----------------
vgvassilev wrote:
We could use ascii however we want something better than that making documentation more welcoming to the user.
https://github.com/llvm/llvm-project/pull/65650
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