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tcg/README
119
tcg/README
@ -16,14 +16,18 @@ from the host, although it is never the case for 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 given
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function. Temporaries are allocated explicitly in each function.
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A TCG "temporary" is a variable only live in a basic
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block. Temporaries are allocated explicitly in each function.
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A TCG "global" is a variable which is live in all the functions. They
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are defined before the functions defined. A TCG global can be a memory
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location (e.g. a QEMU CPU register), a fixed host register (e.g. the
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QEMU CPU state pointer) or a memory location which is stored in a
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register outside QEMU TBs (not implemented yet).
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A TCG "local temporary" is a variable only live in a function. Local
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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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@ -32,11 +36,11 @@ by a branch instruction.
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3.1) Introduction
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TCG instructions operate on variables which are temporaries or
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globals. TCG instructions and variables are strongly typed. Two types
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are supported: 32 bit integers and 64 bit integers. Pointers are
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defined as an alias to 32 bit or 64 bit integers depending on the TCG
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target word size.
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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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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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@ -44,14 +48,12 @@ variable operands and always constant operands.
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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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In the textual form, output operands come first, followed by input
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operands, followed by constant operands. The output type is included
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in the instruction name. Constants are prefixed with a '$'.
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In the textual form, output operands usually come first, followed by
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input operands, followed by constant operands. The output type is
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included in the instruction name. Constants are prefixed with a '$'.
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add_i32 t0, t1, t2 (t0 <- t1 + t2)
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sub_i64 t2, t3, $4 (t2 <- t3 - 4)
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3.2) Assumptions
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* Basic blocks
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@ -62,9 +64,8 @@ sub_i64 t2, t3, $4 (t2 <- t3 - 4)
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- Basic blocks start after the end of a previous basic block, at a
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set_label instruction or after a legacy dyngen operation.
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After the end of a basic block, temporaries at destroyed and globals
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are stored at their initial storage (register or memory place
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depending on their declarations).
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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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* Floating point types are not supported yet
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@ -100,7 +101,7 @@ optimizations:
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is suppressed.
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- A liveness analysis is done at the basic block level. The
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information is used to suppress moves from a dead temporary to
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information is used to suppress moves from a dead variable to
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another one. It is also used to remove instructions which compute
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dead results. The later is especially useful for condition code
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optimization in QEMU.
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@ -113,47 +114,6 @@ optimizations:
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only the last instruction is kept.
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- A macro system is supported (may get closer to function inlining
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some day). It is useful if the liveness analysis is likely to prove
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that some results of a computation are indeed not useful. With the
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macro system, the user can provide several alternative
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implementations which are used depending on the used results. It is
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especially useful for condition code optimization in QEMU.
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Here is an example:
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macro_2 t0, t1, $1
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mov_i32 t0, $0x1234
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The macro identified by the ID "$1" normally returns the values t0
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and t1. Suppose its implementation is:
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macro_start
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brcond_i32 t2, $0, $TCG_COND_EQ, $1
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mov_i32 t0, $2
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br $2
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set_label $1
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mov_i32 t0, $3
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set_label $2
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add_i32 t1, t3, t4
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macro_end
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If t0 is not used after the macro, the user can provide a simpler
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implementation:
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macro_start
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add_i32 t1, t2, t4
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macro_end
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TCG automatically chooses the right implementation depending on
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which macro outputs are used after it.
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Note that if TCG did more expensive optimizations, macros would be
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less useful. In the previous example a macro is useful because the
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liveness analysis is done on each basic block separately. Hence TCG
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cannot remove the code computing 't0' even if it is not used after
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the first macro implementation.
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3.4) Instruction Reference
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********* Function call
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@ -241,6 +201,10 @@ t0=t1|t2
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t0=t1^t2
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* not_i32/i64 t0, t1
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t0=~t1
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********* Shifts
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* shl_i32/i64 t0, t1, t2
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@ -428,3 +392,34 @@ to apply more optimizations because more registers will be free for
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the generated code.
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The exception model is the same as the dyngen one.
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6) Recommended coding rules for best performance
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- Use globals to represent the parts of the QEMU CPU state which are
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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. The other uses are to ensure backward
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compatibility with dyngen during the porting a new target to TCG.
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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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- Don't hesitate to use helpers for complicated or seldom used target
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intructions. There is little performance advantage in using TCG to
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implement target instructions taking more than about twenty TCG
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instructions.
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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 target uses it to improve the condition codes optimisation.
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31
tcg/TODO
31
tcg/TODO
@ -1,32 +1,15 @@
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- test macro system
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- Add new instructions such as: andnot, ror, rol, setcond, clz, ctz,
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popcnt.
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- test conditional jumps
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- See if it is worth exporting mul2, mulu2, div2, divu2.
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- test mul, div, ext8s, ext16s, bswap
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- generate a global TB prologue and epilogue to save/restore registers
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to/from the CPU state and to reserve a stack frame to optimize
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helper calls. Modify cpu-exec.c so that it does not use global
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register variables (except maybe for 'env').
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- fully convert the x86 target. The minimal amount of work includes:
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- add cc_src, cc_dst and cc_op as globals
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- disable its eflags optimization (the liveness analysis should
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suffice)
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- move complicated operations to helpers (in particular FPU, SSE, MMX).
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- optimize the x86 target:
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- move some or all the registers as globals
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- use the TB prologue and epilogue to have QEMU target registers in
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pre assigned host registers.
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- Support of globals saved in fixed registers between TBs.
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Ideas:
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- Move the slow part of the qemu_ld/st ops after the end of the TB.
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- Experiment: change instruction storage to simplify macro handling
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and to handle dynamic allocation and see if the translation speed is
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OK.
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- change exception syntax to get closer to QOP system (exception
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- Change exception syntax to get closer to QOP system (exception
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parameters given with a specific instruction).
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- Add float and vector support.
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