tcg: Update docs/devel/tcg-ops.rst for temporary changes
Rewrite the sections which talked about 'local temporaries'. Remove some assumptions which no longer hold. Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
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Richard Henderson
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@ -7,67 +7,51 @@ TCG Intermediate Representation
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Introduction
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============
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TCG (Tiny Code Generator) began as a generic backend for a C
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compiler. It was simplified to be used in QEMU. It also has its roots
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in the QOP code generator written by Paul Brook.
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TCG (Tiny Code Generator) began as a generic backend for a C compiler.
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It was simplified to be used in QEMU. It also has its roots in the
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QOP code generator written by Paul Brook.
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Definitions
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===========
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TCG receives RISC-like *TCG ops* and performs some optimizations on them,
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including liveness analysis and trivial constant expression
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evaluation. TCG ops are then implemented in the host CPU back end,
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also known as the TCG target.
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The TCG *target* is the architecture for which we generate the
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code. It is of course not the same as the "target" of QEMU which is
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the emulated architecture. As TCG started as a generic C backend used
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for cross compiling, it is assumed that the TCG target is different
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from the host, although it is never the case for QEMU.
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The TCG *target* is the architecture for which we generate the code.
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It is of course not the same as the "target" of QEMU which is the
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emulated architecture. As TCG started as a generic C backend used
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for cross compiling, the assumption was that TCG target might be
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different from the host, although this is never the case for QEMU.
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In this document, we use *guest* to specify what architecture we are
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emulating; *target* always means the TCG target, the machine on which
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we are running QEMU.
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A TCG *function* corresponds to a QEMU Translated Block (TB).
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A TCG *temporary* is a variable only live in a basic block. Temporaries are allocated explicitly in each function.
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A TCG *local temporary* is a variable only live in a function. Local temporaries are allocated explicitly in each function.
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A TCG *global* is a variable which is live in all the functions
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(equivalent of a C global variable). They are defined before the
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functions defined. A TCG global can be a memory location (e.g. a QEMU
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CPU register), a fixed host register (e.g. the QEMU CPU state pointer)
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or a memory location which is stored in a register outside QEMU TBs
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(not implemented yet).
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A TCG *basic block* corresponds to a list of instructions terminated
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by a branch instruction.
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An operation with *undefined behavior* may result in a crash.
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An operation with *unspecified behavior* shall not crash. However,
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the result may be one of several possibilities so may be considered
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an *undefined result*.
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Intermediate representation
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===========================
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Basic Blocks
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============
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Introduction
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------------
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A TCG *basic block* is a single entry, multiple exit region which
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corresponds to a list of instructions terminated by a label, or
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any branch instruction.
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TCG instructions operate on variables which are temporaries, local
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temporaries or globals. TCG instructions and variables are strongly
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typed. Two types are supported: 32 bit integers and 64 bit
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integers. Pointers are defined as an alias to 32 bit or 64 bit
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integers depending on the TCG target word size.
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A TCG *extended basic block* is a single entry, multiple exit region
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which corresponds to a list of instructions terminated by a label or
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an unconditional branch. Specifically, an extended basic block is
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a sequence of basic blocks connected by the fall-through paths of
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zero or more conditional branch instructions.
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Each instruction has a fixed number of output variable operands, input
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variable operands and always constant operands.
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Operations
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==========
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The notable exception is the call instruction which has a variable
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number of outputs and inputs.
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TCG instructions or *ops* operate on TCG *variables*, both of which
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are strongly typed. Each instruction has a fixed number of output
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variable operands, input variable operands and constant operands.
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Vector instructions have a field specifying the element size within
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the vector. The notable exception is the call instruction which has
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a variable number of outputs and inputs.
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In the textual form, output operands usually come first, followed by
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input operands, followed by constant operands. The output type is
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@ -77,68 +61,127 @@ included in the instruction name. Constants are prefixed with a '$'.
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add_i32 t0, t1, t2 /* (t0 <- t1 + t2) */
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Variables
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=========
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Assumptions
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-----------
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* ``TEMP_FIXED``
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Basic blocks
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^^^^^^^^^^^^
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There is one TCG *fixed global* variable, ``cpu_env``, which is
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live in all translation blocks, and holds a pointer to ``CPUArchState``.
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This variable is held in a host cpu register at all times in all
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translation blocks.
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* Basic blocks end after branches (e.g. brcond_i32 instruction),
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goto_tb and exit_tb instructions.
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* ``TEMP_GLOBAL``
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* Basic blocks start after the end of a previous basic block, or at a
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set_label instruction.
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A TCG *global* is a variable which is live in all translation blocks,
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and corresponds to memory location that is within ``CPUArchState``.
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These may be specified as an offset from ``cpu_env``, in which case
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they are called *direct globals*, or may be specified as an offset
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from a direct global, in which case they are called *indirect globals*.
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Even indirect globals should still reference memory within
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``CPUArchState``. All TCG globals are defined during
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``TCGCPUOps.initialize``, before any translation blocks are generated.
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After the end of a basic block, the content of temporaries is
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destroyed, but local temporaries and globals are preserved.
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* ``TEMP_CONST``
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Floating point types
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^^^^^^^^^^^^^^^^^^^^
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A TCG *constant* is a variable which is live throughout the entire
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translation block, and contains a constant value. These variables
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are allocated on demand during translation and are hashed so that
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there is exactly one variable holding a given value.
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* Floating point types are not supported yet
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* ``TEMP_TB``
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Pointers
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^^^^^^^^
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A TCG *translation block temporary* is a variable which is live
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throughout the entire translation block, but dies on any exit.
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These temporaries are allocated explicitly during translation.
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* Depending on the TCG target, pointer size is 32 bit or 64
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bit. The type ``TCG_TYPE_PTR`` is an alias to ``TCG_TYPE_I32`` or
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``TCG_TYPE_I64``.
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* ``TEMP_EBB``
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A TCG *extended basic block temporary* is a variable which is live
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throughout an extended basic block, but dies on any exit.
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These temporaries are allocated explicitly during translation.
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Types
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=====
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* ``TCG_TYPE_I32``
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A 32-bit integer.
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* ``TCG_TYPE_I64``
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A 64-bit integer. For 32-bit hosts, such variables are split into a pair
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of variables with ``type=TCG_TYPE_I32`` and ``base_type=TCG_TYPE_I64``.
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The ``temp_subindex`` for each indicates where it falls within the
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host-endian representation.
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* ``TCG_TYPE_PTR``
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An alias for ``TCG_TYPE_I32`` or ``TCG_TYPE_I64``, depending on the size
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of a pointer for the host.
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* ``TCG_TYPE_REG``
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An alias for ``TCG_TYPE_I32`` or ``TCG_TYPE_I64``, depending on the size
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of the integer registers for the host. This may be larger
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than ``TCG_TYPE_PTR`` depending on the host ABI.
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* ``TCG_TYPE_I128``
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A 128-bit integer. For all hosts, such variables are split into a number
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of variables with ``type=TCG_TYPE_REG`` and ``base_type=TCG_TYPE_I128``.
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The ``temp_subindex`` for each indicates where it falls within the
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host-endian representation.
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* ``TCG_TYPE_V64``
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A 64-bit vector. This type is valid only if the TCG target
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sets ``TCG_TARGET_HAS_v64``.
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* ``TCG_TYPE_V128``
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A 128-bit vector. This type is valid only if the TCG target
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sets ``TCG_TARGET_HAS_v128``.
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* ``TCG_TYPE_V256``
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A 256-bit vector. This type is valid only if the TCG target
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sets ``TCG_TARGET_HAS_v256``.
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Helpers
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^^^^^^^
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=======
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* Using the tcg_gen_helper_x_y it is possible to call any function
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taking i32, i64 or pointer types. By default, before calling a helper,
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all globals are stored at their canonical location and it is assumed
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that the function can modify them. By default, the helper is allowed to
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modify the CPU state or raise an exception.
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Helpers are registered in a guest-specific ``helper.h``,
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which is processed to generate ``tcg_gen_helper_*`` functions.
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With these functions it is possible to call a function taking
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i32, i64, i128 or pointer types.
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This can be overridden using the following function modifiers:
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By default, before calling a helper, all globals are stored at their
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canonical location. By default, the helper is allowed to modify the
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CPU state (including the state represented by tcg globals)
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or may raise an exception. This default can be overridden using the
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following function modifiers:
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- ``TCG_CALL_NO_READ_GLOBALS`` means that the helper does not read globals,
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either directly or via an exception. They will not be saved to their
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canonical locations before calling the helper.
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* ``TCG_CALL_NO_WRITE_GLOBALS``
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- ``TCG_CALL_NO_WRITE_GLOBALS`` means that the helper does not modify any globals.
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They will only be saved to their canonical location before calling helpers,
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but they won't be reloaded afterwards.
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The helper does not modify any globals, but may read them.
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Globals will be saved to their canonical location before calling helpers,
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but need not be reloaded afterwards.
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- ``TCG_CALL_NO_SIDE_EFFECTS`` means that the call to the function is removed if
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the return value is not used.
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* ``TCG_CALL_NO_READ_GLOBALS``
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Note that ``TCG_CALL_NO_READ_GLOBALS`` implies ``TCG_CALL_NO_WRITE_GLOBALS``.
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The helper does not read globals, either directly or via an exception.
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They will not be saved to their canonical locations before calling
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the helper. This implies ``TCG_CALL_NO_WRITE_GLOBALS``.
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On some TCG targets (e.g. x86), several calling conventions are
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supported.
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* ``TCG_CALL_NO_SIDE_EFFECTS``
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Branches
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^^^^^^^^
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* Use the instruction 'br' to jump to a label.
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The call to the helper function may be removed if the return value is
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not used. This means that it may not modify any CPU state nor may it
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raise an exception.
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Code Optimizations
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------------------
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==================
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When generating instructions, you can count on at least the following
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optimizations:
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@ -908,20 +951,9 @@ Recommended coding rules for best performance
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often modified, e.g. the integer registers and the condition
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codes. TCG will be able to use host registers to store them.
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- Avoid globals stored in fixed registers. They must be used only to
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store the pointer to the CPU state and possibly to store a pointer
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to a register window.
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- Use temporaries. Use local temporaries only when really needed,
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e.g. when you need to use a value after a jump. Local temporaries
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introduce a performance hit in the current TCG implementation: their
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content is saved to memory at end of each basic block.
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- Free temporaries and local temporaries when they are no longer used
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(tcg_temp_free). Since tcg_const_x() also creates a temporary, you
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should free it after it is used. Freeing temporaries does not yield
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a better generated code, but it reduces the memory usage of TCG and
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the speed of the translation.
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- Free temporaries when they are no longer used (``tcg_temp_free``).
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Since ``tcg_const_x`` also creates a temporary, you should free it
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after it is used.
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- Don't hesitate to use helpers for complicated or seldom used guest
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instructions. There is little performance advantage in using TCG to
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@ -932,10 +964,6 @@ Recommended coding rules for best performance
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the instruction is mostly doing loads and stores, and in those cases
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inline TCG may still be faster for longer sequences.
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- The hard limit on the number of TCG instructions you can generate
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per guest instruction is set by ``MAX_OP_PER_INSTR`` in ``exec-all.h`` --
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you cannot exceed this without risking a buffer overrun.
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- Use the 'discard' instruction if you know that TCG won't be able to
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prove that a given global is "dead" at a given program point. The
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x86 guest uses it to improve the condition codes optimisation.
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