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Files:
clang/docs/ClangRepl.rst
clang/docs/autoprint.png
clang/docs/prettyprint.png
clang/docs/valuesynth.png
Index: clang/docs/ClangRepl.rst
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--- clang/docs/ClangRepl.rst
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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()``).
+
+.. image:: valuesynth.png
+ :align: center
+ :alt: valuesynth design
+
+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 'memory' to be allocated. Built-in types (bool, char, int,
+float, double, etc.) are simpler, since their memory allocation size is known.
+In case of objects, a pointer can be saved, since the size of the object is
+not known.
+
+For further improvement, the underlying Clang Type is also identified. For
+example, ``X(char, Char_S)``, where ``Char_S`` is the Clang type. Clang types are
+very efficient, which is important since these will be used in hotspots (high
+utilization areas of the program). The ``Value.h`` header file has a very low
+token count and was developed with strict constraints in mind, since it can
+affect the performance of the interpreter.
+
+This also enables the user to receive the computed 'type' back in their code
+and then transform the type into something else (e.g., transform 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, the CPPYY code makes use of this feature to enable running
+C++ within Python. It enables transporting values/information between C++
+and Python.
+
+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(âvoid setGlobal(int val);â);
+ Interp.ParseAndExecute(âint getGlobal();â);
+ Value V;
+ Interp.ParseAndExecute(âgetGlobal()â, &V);
+ std::cout << V.getAs<int>() << â\nâ; // 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**.
+
+.. image:: autoprint.png
+ :align: center
+ :alt: autoprint design
+
+
+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.
+
+.. image:: prettyprint.png
+ :align: center
+ :alt: prettyprint design
+
+
+**Note:** Following is a sample code snippet. Actual code may vary over time.
+
+.. code-block:: console
+
+ llvm::Error Interpreter::ParseAndExecute(llvm::StringRef Code, Value *V) {
+
+ auto PTU = Parse(Code);
+ if (!PTU)
+ return PTU.takeError();
+ if (PTU->TheModule)
+ if (llvm::Error Err = Execute(*PTU))
+ return Err;
+
+ if (LastValue.isValid()) {
+ if (!V) {
+ LastValue.dump();
+ LastValue.clear();
+ } else
+ *V = std::move(LastValue);
+ }
+ return llvm::Error::success();
+ }
+
+The ``dump()`` function (in ``value.cpp``) calls the ``print()`` function.
+
+Printing the Data and Type are handled in their respective functions:
+``ReplPrintDataImpl()`` and ``ReplPrintTypeImpl()``.
+
+Annotation Token (annot_repl_input_end)
+---------------------------------------
+
+This feature uses a new token (annot_repl_input_end) to consider printing the
+value of an expression if it doesn't end with a semicolon. When parsing an
+Expression Statement, if the last semicolon is missing, then the code will
+pretend that there one and set a marker there for later utilization, and
+continue parsing.
+
+A semicolon is normally required in C++, but this feature expands the C++
+syntax to handle cases where a missing semicolon is expected (i.e., when
+handling an expression statement). It also makes sure that an error is not
+generated for the missing semicolon in this specific case.
+
+This is accomplished by identifying the end position of the user input
+(expression statement). This helps store and return the expression statement
+effectively, so that it can be printed (displayed to the user automatically).
+
+**Note:** This logic is only available for C++ for now, since part of the
+implementation itself requires C++ features. Future versions may support more
+languages.
+
+.. code-block:: console
+
+ Token *CurTok = nullptr;
+ // If the semicolon is missing at the end of REPL input, consider if
+ // we want to do value printing. Note this is only enabled in C++ mode
+ // since part of the implementation requires C++ language features.
+ // Note we shouldn't eat the token since the callback needs it.
+ if (Tok.is(tok::annot_repl_input_end) && Actions.getLangOpts().CPlusPlus)
+ CurTok = &Tok;
+ else
+ // Otherwise, eat the semicolon.
+ ExpectAndConsumeSemi(diag::err_expected_semi_after_expr);
+
+ StmtResult R = handleExprStmt(Expr, StmtCtx);
+ if (CurTok && !R.isInvalid())
+ CurTok->setAnnotationValue(R.get());
+
+ return R;
+ }
+
+AST Transformation
+-------------------
+
+When Sema encounters the ``annot_repl_input_end`` token, it knows to transform
+the AST before the real CodeGen process. It will consume the token and set a
+'semi missing' bit in the respective decl.
+
+.. code-block:: console
+
+ if (Tok.is(tok::annot_repl_input_end) &&
+ Tok.getAnnotationValue() != nullptr) {
+ ConsumeAnnotationToken();
+ cast<TopLevelStmtDecl>(DeclsInGroup.back())->setSemiMissing();
+ }
+
+In the AST Consumer, traverse all the Top Level Decls, to look for expressions
+to synthesize. If the current Decl is the Top Level Statement
+Decl(``TopLevelStmtDecl``) and has a semicolon missing, then ask the interpreter
+to synthesize another expression (an internal function call) to replace this
+original expression.
+
+
+Detailed RFC and Discussion:
+----------------------------
+
+For more technical details, community discussion and links to patches related
+to these features,
+Please visit: `RFC on LLVM Discourse <https://discourse.llvm.org/t/rfc-handle-execution-results-in-clang-repl/68493>`_.
+
+Some logic presented in the RFC (e.g. ValueGetter()) may be outdated,
+compared to the final developed solution.
Related Reading
===============
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