Rewrite the sections which talked about 'local temporaries'.
Remove some assumptions which no longer hold.
Signed-off-by: Richard Henderson <richard.hender...@linaro.org>
---
docs/devel/tcg-ops.rst | 103 +++++++++++++++++++++--------------------
1 file changed, 54 insertions(+), 49 deletions(-)
diff --git a/docs/devel/tcg-ops.rst b/docs/devel/tcg-ops.rst
index 9adc0c9b6c..53b7b6c93b 100644
--- a/docs/devel/tcg-ops.rst
+++ b/docs/devel/tcg-ops.rst
@@ -29,21 +29,42 @@ 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 *basic block* is a single entry, multiple exit region which
+corresponds to a list of instructions terminated by a label, or
+any branch instruction.
-A TCG *temporary* is a variable only live in a basic block. Temporaries are
allocated explicitly in each function.
+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.
-A TCG *local temporary* is a variable only live in a function. Local
temporaries are allocated explicitly in each function.
+There is one TCG *fixed global* (``TEMP_FIXED``) variable, ``cpu_env``
+which is live in all translation blocks, and holds a pointer to
``CPUArchState``.
+This fixed global is held in a host cpu register at all times in all
+translation blocks.
-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 *global* (``TEMP_GLOBAL``) is a variable which is live in all
+translation blocks, and correspond to memory locations that are 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.
-A TCG *basic block* corresponds to a list of instructions terminated
-by a branch instruction.
+A TCG *constant* (``TEMP_CONST``) is a variable which is live throughout
+the entire translation block, and contains a constant value.
+These temporaries are allocated explicitly during translation and are
+hashed so that there is exactly one variable holding a given value.
+
+A TCG *translation block temporary* (``TEMP_TB``) is a variable which is
+live throughout the entire translation block, but dies on any exit.
+These temporaries are allocated explicitly during translation.
+
+A TCG *extended basic block temporary* (``TEMP_EBB``) is a variable which
+is live throughout an extended basic block, but dies on any exit.
+These temporaries are allocated explicitly during translation.
An operation with *undefined behavior* may result in a crash.
@@ -57,11 +78,11 @@ Intermediate representation
Introduction
------------
-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.
+TCG instructions operate on variables which are temporaries.
+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.
Each instruction has a fixed number of output variable operands, input
variable operands and always constant operands.
@@ -81,17 +102,19 @@ included in the instruction name. Constants are prefixed
with a '$'.
Assumptions
-----------
-Basic blocks
+Basic Blocks
^^^^^^^^^^^^
-* Basic blocks end after branches (e.g. brcond_i32 instruction),
- goto_tb and exit_tb instructions.
+* Basic blocks end after conditional branches (e.g. brcond_i32),
+ br, goto_tb, exit_tb, goto_ptr, set_label instructions,
+ and calls that are defined to not return (``TCG_CALL_NO_RETURN``).
-* Basic blocks start after the end of a previous basic block, or at a
- set_label instruction.
+* Basic blocks start after the end of a previous basic block,
+ or at a set_label instruction.
-After the end of a basic block, the content of temporaries is
-destroyed, but local temporaries and globals are preserved.
+* Extended basic blocks are a sequence of basic blocks that are
+ connected by the fall through of conditional branches. Thus they end
+ at br, goto_tb, exit_tb, goto_ptr, set_label, and noreturn calls.
Floating point types
^^^^^^^^^^^^^^^^^^^^
@@ -120,18 +143,15 @@ Helpers
either directly or via an exception. They will not be saved to their
canonical locations before calling the helper.
- - ``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.
+ - ``TCG_CALL_NO_WRITE_GLOBALS`` means that the helper does not modify
+ any globals, but may read them. Globals will be saved to their canonical
+ location before calling helpers, but won't 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_SIDE_EFFECTS`` means that the call to the function is
+ removed if the return value is not used.
Note that ``TCG_CALL_NO_READ_GLOBALS`` implies ``TCG_CALL_NO_WRITE_GLOBALS``.
- On some TCG targets (e.g. x86), several calling conventions are
- supported.
-
Branches
^^^^^^^^
@@ -908,20 +928,9 @@ 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 +941,6 @@ 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
