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Revision 0a6b7b78 tcg/README

     A TCG "function" corresponds to a QEMU Translated Block (TB).
     A TCG "temporary" is a variable only live in a given
     function. Temporaries are allocated explicitly in each function.
     A TCG "temporary" is a variable only live in a basic
     block. Temporaries are allocated explicitly in each function.
     A TCG "global" is a variable which is live in all the functions. 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 "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.
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 .1) Introduction
     TCG instructions operate on variables which are 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, 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.
     Each instruction has a fixed number of output variable operands, input
     variable operands and always constant operands.
-...
     The notable exception is the call instruction which has a variable
     number of outputs and inputs.
     In the textual form, output operands come first, followed by input
     operands, followed by constant operands. The output type is included
     in the instruction name. Constants are prefixed with a '$'.
     In the textual form, output operands usually come first, followed by
     input operands, followed by constant operands. The output type is
     included in the instruction name. Constants are prefixed with a '$'.
     add_i32 t0, t1, t2  (t0 <- t1 + t2)
     sub_i64 t2, t3, $4  (t2 <- t3 - 4)
 .2) Assumptions
     * Basic blocks
-...
     - Basic blocks start after the end of a previous basic block, at a
       set_label instruction or after a legacy dyngen operation.
     After the end of a basic block, temporaries at destroyed and globals
     are stored at their initial storage (register or memory place
     depending on their declarations).
     After the end of a basic block, the content of temporaries is
     destroyed, but local temporaries and globals are preserved.
     * Floating point types are not supported yet
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       is suppressed.
     - A liveness analysis is done at the basic block level. The
       information is used to suppress moves from a dead temporary to
       information is used to suppress moves from a dead variable to
       another one. It is also used to remove instructions which compute
       dead results. The later is especially useful for condition code
       optimization in QEMU.
-...
       only the last instruction is kept.
     - A macro system is supported (may get closer to function inlining
       some day). It is useful if the liveness analysis is likely to prove
       that some results of a computation are indeed not useful. With the
       macro system, the user can provide several alternative
       implementations which are used depending on the used results. It is
       especially useful for condition code optimization in QEMU.
       Here is an example:
       macro_2 t0, t1, $1
       mov_i32 t0, $0x1234
       The macro identified by the ID "$1" normally returns the values t0
       and t1. Suppose its implementation is:
       macro_start
       brcond_i32  t2, $0, $TCG_COND_EQ, $1
       mov_i32 t0, $2
       br $2
       set_label $1
       mov_i32 t0, $3
       set_label $2
       add_i32 t1, t3, t4
       macro_end
       If t0 is not used after the macro, the user can provide a simpler
       implementation:
       macro_start
       add_i32 t1, t2, t4
       macro_end
       TCG automatically chooses the right implementation depending on
       which macro outputs are used after it.
       Note that if TCG did more expensive optimizations, macros would be
       less useful. In the previous example a macro is useful because the
       liveness analysis is done on each basic block separately. Hence TCG
       cannot remove the code computing 't0' even if it is not used after
       the first macro implementation.
 .4) Instruction Reference
     ********* Function call
-...
     t0=t1^t2
     * not_i32/i64 t0, t1
     t0=~t1
     ********* Shifts
     * shl_i32/i64 t0, t1, t2
-...
     the generated code.
     The exception model is the same as the dyngen one.
 ) Recommended coding rules for best performance
     - Use globals to represent the parts of the QEMU CPU state which are
       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. The other uses are to ensure backward
       compatibility with dyngen during the porting a new target to TCG.
     - 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.
     - Don't hesitate to use helpers for complicated or seldom used target
       intructions. There is little performance advantage in using TCG to
       implement target instructions taking more than about twenty TCG
       instructions.
     - 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 target uses it to improve the condition codes optimisation.

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Revision 0a6b7b78 tcg/README