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1====================2Constant Interpreter3====================4 5.. contents::6 :local:7 8Introduction9============10 11The constexpr interpreter aims to replace the existing tree evaluator in12clang, improving performance on constructs which are executed inefficiently13by the evaluator. The interpreter is activated using the following flags:14 15* ``-fexperimental-new-constant-interpreter`` enables the interpreter,16 emitting an error if an unsupported feature is encountered17 18Bytecode Compilation19====================20 21Bytecode compilation is handled in ``Compiler.h`` for statements22and for expressions. The compiler has two different23backends: one to generate bytecode for functions (``ByteCodeEmitter``) and24one to directly evaluate expressions as they are compiled, without25generating bytecode (``EvalEmitter``). All functions are compiled to26bytecode, while toplevel expressions used in constant contexts are directly27evaluated since the bytecode would never be reused. This mechanism aims to28pave the way towards replacing the evaluator, improving its performance on29functions and loops, while being just as fast on single-use toplevel30expressions.31 32The interpreter relies on stack-based, strongly-typed opcodes. The glue33logic between the code generator, along with the enumeration and34description of opcodes, can be found in ``Opcodes.td``. The opcodes are35implemented as generic template methods in ``Interp.h`` and instantiated36with the relevant primitive types by the interpreter loop or by the37evaluating emitter.38 39Primitive Types40---------------41 42* ``PT_{U|S}int{8|16|32|64}``43 44 Signed or unsigned integers of a specific bit width, implemented using45 the ```Integral``` type.46 47* ``PT_IntAP{S}``48 49 Signed or unsigned integers of an arbitrary, but fixed width used to50 implement integral types which are required by the target, but are not51 supported by the host. Under the hood, they rely on ``APInt``. The52 ``Integral`` specialisation for these types is required by opcodes to53 share an implementation with fixed integrals.54 55* ``PT_Bool``56 57 Representation for boolean types, essentially a 1-bit unsigned58 ``Integral``.59 60* ``PT_Float``61 62 Arbitrary, but fixed precision floating point numbers. Could be63 specialised in the future similarly to integers in order to improve64 floating point performance.65 66* ``PT_Ptr``67 68 Pointer type, defined in ``"Pointer.h"``. The most common type of69 pointer is a "BlockPointer", which points to an ``interp::Block``.70 But other pointer types exist, such as typeid pointers or71 integral pointers.72 73* ``PT_FnPtr``74 75 Function pointer type, can also be a null function pointer. Defined76 in ``"FunctionPointer.h"``.77 78* ``PT_MemberPtr``79 80 Member pointer type, can also be a null member pointer. Defined81 in ``"MemberPointer.h"``82 83Composite types84---------------85 86The interpreter distinguishes two kinds of composite types: arrays and87records (structs and classes). Unions are represented as records, except88at most a single field can be marked as active. The contents of inactive89fields are kept until they are reactivated and overwritten.90Complex numbers (``_Complex``) and vectors91(``__attribute((vector_size(16)))``) are treated as arrays.92 93 94Bytecode Execution95==================96 97Bytecode is executed using a stack-based interpreter. The execution98context consists of an ``InterpStack``, along with a chain of99``InterpFrame`` objects storing the call frames. Frames are built by100call instructions and destroyed by return instructions. They perform101one allocation to reserve space for all locals in a single block.102These objects store all the required information to emit stack traces103whenever evaluation fails.104 105Memory Organisation106===================107 108Memory management in the interpreter relies on 3 data structures: ``Block``109objects which store the data and associated inline metadata, ``Pointer``110objects which refer to or into blocks, and ``Descriptor`` structures which111describe blocks and subobjects nested inside blocks.112 113Blocks114------115 116Blocks contain data interleaved with metadata. They are allocated either117statically in the code generator (globals, static members, dummy parameter118values etc.) or dynamically in the interpreter, when creating the frame119containing the local variables of a function. Blocks are associated with a120descriptor that characterises the entire allocation, along with a few121additional attributes:122 123* ``IsStatic`` indicates whether the block has static duration in the124 interpreter, i.e. it is not a local in a frame.125 126* ``DeclID`` identifies each global declaration (it is set to an invalid127 and irrelevant value for locals) in order to prevent illegal writes and128 reads involving globals and temporaries with static storage duration.129 130Static blocks are never deallocated, but local ones might be deallocated131even when there are live pointers to them. Pointers are only valid as132long as the blocks they point to are valid, so a block with pointers to133it whose lifetime ends is kept alive until all pointers to it go out of134scope. Since the frame is destroyed on function exit, such blocks are135turned into a ``DeadBlock`` and copied to storage managed by the136interpreter itself, not the frame. Reads and writes to these blocks are137illegal and cause an appropriate diagnostic to be emitted. When the last138pointer goes out of scope, dead blocks are also deallocated.139 140The lifetime of blocks is managed through 3 methods stored in the141descriptor of the block:142 143* **CtorFn**: initializes the metadata which is stored in the block,144 alongside actual data. Invokes the default constructors of objects145 which are not trivial (``Pointer``, ``RealFP``, etc.)146 147* **DtorFn**: invokes the destructors of non-trivial objects.148 149* **MoveFn**: moves a block to dead storage.150 151Non-static blocks track all the pointers into them through an intrusive152doubly-linked list, required to adjust and invalidate all pointers when153transforming a block into a dead block. If the lifetime of an object ends,154all pointers to it are invalidated, emitting the appropriate diagnostics when155dereferenced.156 157The interpreter distinguishes 3 different kinds of blocks:158 159* **Primitives**160 161 A block containing a single primitive with no additional metadata.162 163* **Arrays of primitives**164 165 An array of primitives contains a pointer to an ``InitMap`` storage as its166 first field: the initialisation map is a bit map indicating all elements of167 the array which were initialised. If the pointer is null, no elements were168 initialised, while a value of ``(InitMap*)-1`` indicates that the object was169 fully initialised. When all fields are initialised, the map is deallocated170 and replaced with that token.171 172 Array elements are stored sequentially, without padding, after the pointer173 to the map.174 175* **Arrays of composites and records**176 177 Each element in an array of composites is preceded by an ``InlineDescriptor``178 which stores the attributes specific to the field and not the whole179 allocation site. Descriptors and elements are stored sequentially in the180 block.181 Records are laid out identically to arrays of composites: each field and base182 class is preceded by an inline descriptor. The ``InlineDescriptor``183 has the following fields:184 185 * **Offset**: byte offset into the array or record, used to step back to the186 parent array or record.187 * **IsConst**: flag indicating if the field is const-qualified.188 * **IsInitialized**: flag indicating whether the field or element was189 initialized. For non-primitive fields, this is only relevant to determine190 the dynamic type of objects during construction.191 * **IsBase**: flag indicating whether the record is a base class. In that192 case, the offset can be used to identify the derived class.193 * **IsActive**: indicates if the field is the active field of a union.194 * **IsMutable**: indicates if the field is marked as mutable.195 196Inline descriptors are filled in by the `CtorFn` of blocks, which leaves storage197in an uninitialised, but valid state.198 199Descriptors200-----------201 202Descriptors are generated at bytecode compilation time and contain information203required to determine if a particular memory access is allowed in constexpr.204They also carry all the information required to emit a diagnostic involving205a memory access, such as the declaration which originates the block.206Currently there is a single kind of descriptor encoding information for all207block types.208 209Pointers210--------211 212Pointers, implemented in ``Pointer.h`` are represented as a tagged union.213 214 * **BlockPointer**: used to reference memory allocated and managed by the215 interpreter, being the only pointer kind which allows dereferencing in the216 interpreter217 * **TypeIDPointer**: tracks information for the opaque type returned by218 ``typeid``219 * **IntegralPointer**: a pointer formed from an integer,220 think ``(int*)123``.221 222Besides the previously mentioned union, a number of other pointer-like types223have their own type:224 225 * **FunctionPointer** tracks functions.226 * **MemberPointer** tracks C++ object members227 228BlockPointer229~~~~~~~~~~~~230 231Block pointers track a ``Pointee``, the block to which they point, along232with a ``Base`` and an ``Offset``. The base identifies the innermost field,233while the offset points to an array element relative to the base (including234one-past-end pointers). The offset identifies the array element or field235which is referenced, while the base points to the outer object or array which236contains the field. These two fields allow all pointers to be uniquely237identified, disambiguated and characterised.238 239As an example, consider the following structure:240 241.. code-block:: c242 243 struct A {244 struct B {245 int x;246 int y;247 } b;248 struct C {249 int a;250 int b;251 } c[2];252 int z;253 };254 constexpr A a;255 256On the target, ``&a`` and ``&a.b.x`` are equal. So are ``&a.c[0]`` and257``&a.c[0].a``. In the interpreter, all these pointers must be258distinguished since the are all allowed to address distinct range of259memory.260 261In the interpreter, the object would require 240 bytes of storage and262would have its field interleaved with metadata. The pointers which can263be derived to the object are illustrated in the following diagram:264 265::266 267 0 16 32 40 56 64 80 96 112 120 136 144 160 176 184 200 208 224 240268 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+269 + B | D | D | x | D | y | D | D | D | a | D | b | D | D | a | D | b | D | z |270 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+271 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^272 | | | | | | | &a.c[0].b | | &a.c[1].b |273 a |&a.b.x &a.y &a.c |&a.c[0].a |&a.c[1].a |274 &a.b &a.c[0] &a.c[1] &a.z275 276The ``Base`` offset of all pointers points to the start of a field or277an array and is preceded by an inline descriptor (unless ``Base`` is278zero, pointing to the root). All the relevant attributes can be read279from either the inline descriptor or the descriptor of the block.280 281 282Array elements are identified by the ``Offset`` field of pointers,283pointing to past the inline descriptors for composites and before284the actual data in the case of primitive arrays. The ``Offset``285points to the offset where primitives can be read from. As an example,286``a.c + 1`` would have the same base as ``a.c`` since it is an element287of ``a.c``, but its offset would point to ``&a.c[1]``. The288array-to-pointer decay operation adjusts a pointer to an array (where289the offset is equal to the base) to a pointer to the first element.290 291TypeInfoPointer292~~~~~~~~~~~~~~~293 294``TypeInfoPointer`` tracks two types: the type assigned to295``std::type_info`` and the type which was passed to ``typeinfo``.296It is part of the tagged union in ``Pointer``.297