Rewrite the sections which talked about 'local temporaries'.
Remove some assumptions which no longer hold.
Reviewed-by: Daniel Henrique Barboza <dbarb...@ventanamicro.com>
Signed-off-by: Richard Henderson <richard.hender...@linaro.org>
---
docs/devel/tcg-ops.rst | 230 +++++++++++++++++++++++------------------
1 file changed, 129 insertions(+), 101 deletions(-)
diff --git a/docs/devel/tcg-ops.rst b/docs/devel/tcg-ops.rst
index 9adc0c9b6c..561c416574 100644
--- a/docs/devel/tcg-ops.rst
+++ b/docs/devel/tcg-ops.rst
@@ -7,67 +7,51 @@ TCG Intermediate Representation
Introduction
============
-TCG (Tiny Code Generator) began as a generic backend for a C
-compiler. It was simplified to be used in QEMU. It also has its roots
-in the QOP code generator written by Paul Brook.
+TCG (Tiny Code Generator) began as a generic backend for a C compiler.
+It was simplified to be used in QEMU. It also has its roots in the
+QOP code generator written by Paul Brook.
Definitions
===========
-TCG receives RISC-like *TCG ops* and performs some optimizations on them,
-including liveness analysis and trivial constant expression
-evaluation. TCG ops are then implemented in the host CPU back end,
-also known as the TCG target.
-
-The TCG *target* is the architecture for which we generate the
-code. It is of course not the same as the "target" of QEMU which is
-the emulated architecture. As TCG started as a generic C backend used
-for cross compiling, it is assumed that the TCG target is different
-from the host, although it is never the case for QEMU.
+The TCG *target* is the architecture for which we generate the code.
+It is of course not the same as the "target" of QEMU which is the
+emulated architecture. As TCG started as a generic C backend used
+for cross compiling, the assumption was that TCG target might be
+different from the host, although this is never the case for QEMU.
In this document, we use *guest* to specify what architecture we are
emulating; *target* always means the TCG target, the machine on which
we are running QEMU.
-A TCG *function* corresponds to a QEMU Translated Block (TB).
-
-A TCG *temporary* is a variable only live in a basic block. Temporaries are
allocated explicitly in each function.
-
-A TCG *local temporary* is a variable only live in a function. Local
temporaries are allocated explicitly in each function.
-
-A TCG *global* is a variable which is live in all the functions
-(equivalent of a C global variable). They are defined before the
-functions defined. A TCG global can be a memory location (e.g. a QEMU
-CPU register), a fixed host register (e.g. the QEMU CPU state pointer)
-or a memory location which is stored in a register outside QEMU TBs
-(not implemented yet).
-
-A TCG *basic block* corresponds to a list of instructions terminated
-by a branch instruction.
-
An operation with *undefined behavior* may result in a crash.
An operation with *unspecified behavior* shall not crash. However,
the result may be one of several possibilities so may be considered
an *undefined result*.
-Intermediate representation
-===========================
+Basic Blocks
+============
-Introduction
-------------
+A TCG *basic block* is a single entry, multiple exit region which
+corresponds to a list of instructions terminated by a label, or
+any branch instruction.
-TCG instructions operate on variables which are temporaries, local
-temporaries or globals. TCG instructions and variables are strongly
-typed. Two types are supported: 32 bit integers and 64 bit
-integers. Pointers are defined as an alias to 32 bit or 64 bit
-integers depending on the TCG target word size.
+A TCG *extended basic block* is a single entry, multiple exit region
+which corresponds to a list of instructions terminated by a label or
+an unconditional branch. Specifically, an extended basic block is
+a sequence of basic blocks connected by the fall-through paths of
+zero or more conditional branch instructions.
-Each instruction has a fixed number of output variable operands, input
-variable operands and always constant operands.
+Operations
+==========
-The notable exception is the call instruction which has a variable
-number of outputs and inputs.
+TCG instructions or *ops* operate on TCG *variables*, both of which
+are strongly typed. Each instruction has a fixed number of output
+variable operands, input variable operands and constant operands.
+Vector instructions have a field specifying the element size within
+the vector. The notable exception is the call instruction which has
+a variable number of outputs and inputs.
In the textual form, output operands usually come first, followed by
input operands, followed by constant operands. The output type is
@@ -77,68 +61,127 @@ included in the instruction name. Constants are prefixed
with a '$'.
add_i32 t0, t1, t2 /* (t0 <- t1 + t2) */
+Variables
+=========
-Assumptions
------------
+* ``TEMP_FIXED``
-Basic blocks
-^^^^^^^^^^^^
+ There is one TCG *fixed global* variable, ``cpu_env``, which is
+ live in all translation blocks, and holds a pointer to ``CPUArchState``.
+ This variable is held in a host cpu register at all times in all
+ translation blocks.
-* Basic blocks end after branches (e.g. brcond_i32 instruction),
- goto_tb and exit_tb instructions.
+* ``TEMP_GLOBAL``
-* Basic blocks start after the end of a previous basic block, or at a
- set_label instruction.
+ A TCG *global* is a variable which is live in all translation blocks,
+ and corresponds to memory location that is within ``CPUArchState``.
+ These may be specified as an offset from ``cpu_env``, in which case
+ they are called *direct globals*, or may be specified as an offset
+ from a direct global, in which case they are called *indirect globals*.
+ Even indirect globals should still reference memory within
+ ``CPUArchState``. All TCG globals are defined during
+ ``TCGCPUOps.initialize``, before any translation blocks are generated.
-After the end of a basic block, the content of temporaries is
-destroyed, but local temporaries and globals are preserved.
+* ``TEMP_CONST``
-Floating point types
-^^^^^^^^^^^^^^^^^^^^
+ A TCG *constant* is a variable which is live throughout the entire
+ translation block, and contains a constant value. These variables
+ are allocated on demand during translation and are hashed so that
+ there is exactly one variable holding a given value.
-* Floating point types are not supported yet
+* ``TEMP_TB``
-Pointers
-^^^^^^^^
+ A TCG *translation block temporary* is a variable which is live
+ throughout the entire translation block, but dies on any exit.
+ These temporaries are allocated explicitly during translation.
-* Depending on the TCG target, pointer size is 32 bit or 64
- bit. The type ``TCG_TYPE_PTR`` is an alias to ``TCG_TYPE_I32`` or
- ``TCG_TYPE_I64``.
+* ``TEMP_EBB``
+
+ A TCG *extended basic block temporary* is a variable which is live
+ throughout an extended basic block, but dies on any exit.
+ These temporaries are allocated explicitly during translation.
+
+Types
+=====
+
+* ``TCG_TYPE_I32``
+
+ A 32-bit integer.
+
+* ``TCG_TYPE_I64``
+
+ A 64-bit integer. For 32-bit hosts, such variables are split into a pair
+ of variables with ``type=TCG_TYPE_I32`` and ``base_type=TCG_TYPE_I64``.
+ The ``temp_subindex`` for each indicates where it falls within the
+ host-endian representation.
+
+* ``TCG_TYPE_PTR``
+
+ An alias for ``TCG_TYPE_I32`` or ``TCG_TYPE_I64``, depending on the size
+ of a pointer for the host.
+
+* ``TCG_TYPE_REG``
+
+ An alias for ``TCG_TYPE_I32`` or ``TCG_TYPE_I64``, depending on the size
+ of the integer registers for the host. This may be larger
+ than ``TCG_TYPE_PTR`` depending on the host ABI.
+
+* ``TCG_TYPE_I128``
+
+ A 128-bit integer. For all hosts, such variables are split into a number
+ of variables with ``type=TCG_TYPE_REG`` and ``base_type=TCG_TYPE_I128``.
+ The ``temp_subindex`` for each indicates where it falls within the
+ host-endian representation.
+
+* ``TCG_TYPE_V64``
+
+ A 64-bit vector. This type is valid only if the TCG target
+ sets ``TCG_TARGET_HAS_v64``.
+
+* ``TCG_TYPE_V128``
+
+ A 128-bit vector. This type is valid only if the TCG target
+ sets ``TCG_TARGET_HAS_v128``.
+
+* ``TCG_TYPE_V256``
+
+ A 256-bit vector. This type is valid only if the TCG target
+ sets ``TCG_TARGET_HAS_v256``.
Helpers
-^^^^^^^
+=======
-* Using the tcg_gen_helper_x_y it is possible to call any function
- taking i32, i64 or pointer types. By default, before calling a helper,
- all globals are stored at their canonical location and it is assumed
- that the function can modify them. By default, the helper is allowed to
- modify the CPU state or raise an exception.
+Helpers are registered in a guest-specific ``helper.h``,
+which is processed to generate ``tcg_gen_helper_*`` functions.
+With these functions it is possible to call a function taking
+i32, i64, i128 or pointer types.
- This can be overridden using the following function modifiers:
+By default, before calling a helper, all globals are stored at their
+canonical location. By default, the helper is allowed to modify the
+CPU state (including the state represented by tcg globals)
+or may raise an exception. This default can be overridden using the
+following function modifiers:
- - ``TCG_CALL_NO_READ_GLOBALS`` means that the helper does not read globals,
- either directly or via an exception. They will not be saved to their
- canonical locations before calling the helper.
+* ``TCG_CALL_NO_WRITE_GLOBALS``
- - ``TCG_CALL_NO_WRITE_GLOBALS`` means that the helper does not modify any
globals.
- They will only be saved to their canonical location before calling helpers,
- but they won't be reloaded afterwards.
+ The helper does not modify any globals, but may read them.
+ Globals will be saved to their canonical location before calling helpers,
+ but need not be reloaded afterwards.
- - ``TCG_CALL_NO_SIDE_EFFECTS`` means that the call to the function is
removed if
- the return value is not used.
+* ``TCG_CALL_NO_READ_GLOBALS``
- Note that ``TCG_CALL_NO_READ_GLOBALS`` implies ``TCG_CALL_NO_WRITE_GLOBALS``.
+ The helper does not read globals, either directly or via an exception.
+ They will not be saved to their canonical locations before calling
+ the helper. This implies ``TCG_CALL_NO_WRITE_GLOBALS``.
- On some TCG targets (e.g. x86), several calling conventions are
- supported.
+* ``TCG_CALL_NO_SIDE_EFFECTS``
-Branches
-^^^^^^^^
-
-* Use the instruction 'br' to jump to a label.
+ The call to the helper function may be removed if the return value is
+ not used. This means that it may not modify any CPU state nor may it
+ raise an exception.
Code Optimizations
-------------------
+==================
When generating instructions, you can count on at least the following
optimizations:
@@ -908,20 +951,9 @@ Recommended coding rules for best performance
often modified, e.g. the integer registers and the condition
codes. TCG will be able to use host registers to store them.
-- Avoid globals stored in fixed registers. They must be used only to
- store the pointer to the CPU state and possibly to store a pointer
- to a register window.
-
-- Use temporaries. Use local temporaries only when really needed,
- e.g. when you need to use a value after a jump. Local temporaries
- introduce a performance hit in the current TCG implementation: their
- content is saved to memory at end of each basic block.
-
-- Free temporaries and local temporaries when they are no longer used
- (tcg_temp_free). Since tcg_const_x() also creates a temporary, you
- should free it after it is used. Freeing temporaries does not yield
- a better generated code, but it reduces the memory usage of TCG and
- the speed of the translation.
+- Free temporaries when they are no longer used (``tcg_temp_free``).
+ Since ``tcg_const_x`` also creates a temporary, you should free it
+ after it is used.
- Don't hesitate to use helpers for complicated or seldom used guest
instructions. There is little performance advantage in using TCG to
@@ -932,10 +964,6 @@ Recommended coding rules for best performance
the instruction is mostly doing loads and stores, and in those cases
inline TCG may still be faster for longer sequences.
-- The hard limit on the number of TCG instructions you can generate
- per guest instruction is set by ``MAX_OP_PER_INSTR`` in ``exec-all.h`` --
- you cannot exceed this without risking a buffer overrun.
-
- Use the 'discard' instruction if you know that TCG won't be able to
prove that a given global is "dead" at a given program point. The
x86 guest uses it to improve the condition codes optimisation.
--
2.34.1
