Tiny Code Generator - Fabrice Bellard.

1) 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. 

2) Definitions

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.

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. 

3) Intermediate representation

3.1) 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.

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 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)

3.2) Assumptions

* Basic blocks

- Basic blocks end after branches (e.g. brcond_i32 instruction),
  goto_tb and exit_tb instructions.
- Basic blocks end before legacy dyngen operations.
- 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, the content of temporaries is
destroyed, but local temporaries and globals are preserved.

* Floating point types are not supported yet

* Pointers: 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.

* Helpers:

Using the tcg_gen_helper_x_y it is possible to call any function
taking i32, i64 or pointer types. Before calling an helper, all
globals are stored at their canonical location and it is assumed that
the function can modify them. In the future, function modifiers will
be allowed to tell that the helper does not read or write some globals.

On some TCG targets (e.g. x86), several calling conventions are
supported.

* Branches:

Use the instruction 'br' to jump to a label. Use 'jmp' to jump to an
explicit address. Conditional branches can only jump to labels.

3.3) Code Optimizations

When generating instructions, you can count on at least the following
optimizations:

- Single instructions are simplified, e.g.

   and_i32 t0, t0, $0xffffffff
    
  is suppressed.

- A liveness analysis is done at the basic block level. The
  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.

  In the following example:

  add_i32 t0, t1, t2
  add_i32 t0, t0, $1
  mov_i32 t0, $1

  only the last instruction is kept.

3.4) Instruction Reference

********* Function call

* call <ret> <params> ptr

call function 'ptr' (pointer type)

<ret> optional 32 bit or 64 bit return value
<params> optional 32 bit or 64 bit parameters

********* Jumps/Labels

* jmp t0

Absolute jump to address t0 (pointer type).

* set_label $label

Define label 'label' at the current program point.

* br $label

Jump to label.

* brcond_i32/i64 cond, t0, t1, label

Conditional jump if t0 cond t1 is true. cond can be:
    TCG_COND_EQ
    TCG_COND_NE
    TCG_COND_LT /* signed */
    TCG_COND_GE /* signed */
    TCG_COND_LE /* signed */
    TCG_COND_GT /* signed */
    TCG_COND_LTU /* unsigned */
    TCG_COND_GEU /* unsigned */
    TCG_COND_LEU /* unsigned */
    TCG_COND_GTU /* unsigned */

********* Arithmetic

* add_i32/i64 t0, t1, t2

t0=t1+t2

* sub_i32/i64 t0, t1, t2

t0=t1-t2

* neg_i32/i64 t0, t1

t0=-t1 (two's complement)

* mul_i32/i64 t0, t1, t2

t0=t1*t2

* div_i32/i64 t0, t1, t2

t0=t1/t2 (signed). Undefined behavior if division by zero or overflow.

* divu_i32/i64 t0, t1, t2

t0=t1/t2 (unsigned). Undefined behavior if division by zero.

* rem_i32/i64 t0, t1, t2

t0=t1%t2 (signed). Undefined behavior if division by zero or overflow.

* remu_i32/i64 t0, t1, t2

t0=t1%t2 (unsigned). Undefined behavior if division by zero.

********* Logical

* and_i32/i64 t0, t1, t2

t0=t1&t2

* or_i32/i64 t0, t1, t2

t0=t1|t2

* xor_i32/i64 t0, t1, t2

t0=t1^t2

* not_i32/i64 t0, t1

t0=~t1

********* Shifts

* shl_i32/i64 t0, t1, t2

t0=t1 << t2. Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)

* shr_i32/i64 t0, t1, t2

t0=t1 >> t2 (unsigned). Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)

* sar_i32/i64 t0, t1, t2

t0=t1 >> t2 (signed). Undefined behavior if t2 < 0 or t2 >= 32 (resp 64)

********* Misc

* mov_i32/i64 t0, t1

t0 = t1

Move t1 to t0 (both operands must have the same type).

* ext8s_i32/i64 t0, t1
ext8u_i32/i64 t0, t1
ext16s_i32/i64 t0, t1
ext16u_i32/i64 t0, t1
ext32s_i64 t0, t1
ext32u_i64 t0, t1

8, 16 or 32 bit sign/zero extension (both operands must have the same type)

* bswap16_i32 t0, t1

16 bit byte swap on a 32 bit value. The two high order bytes must be set
to zero.

* bswap_i32 t0, t1

32 bit byte swap

* bswap_i64 t0, t1

64 bit byte swap

* discard_i32/i64 t0

Indicate that the value of t0 won't be used later. It is useful to
force dead code elimination.

********* Type conversions

* ext_i32_i64 t0, t1
Convert t1 (32 bit) to t0 (64 bit) and does sign extension

* extu_i32_i64 t0, t1
Convert t1 (32 bit) to t0 (64 bit) and does zero extension

* trunc_i64_i32 t0, t1
Truncate t1 (64 bit) to t0 (32 bit)

* concat_i32_i64 t0, t1, t2
Construct t0 (64-bit) taking the low half from t1 (32 bit) and the high half
from t2 (32 bit).

* concat32_i64 t0, t1, t2
Construct t0 (64-bit) taking the low half from t1 (64 bit) and the high half
from t2 (64 bit).

********* Load/Store

* ld_i32/i64 t0, t1, offset
ld8s_i32/i64 t0, t1, offset
ld8u_i32/i64 t0, t1, offset
ld16s_i32/i64 t0, t1, offset
ld16u_i32/i64 t0, t1, offset
ld32s_i64 t0, t1, offset
ld32u_i64 t0, t1, offset

t0 = read(t1 + offset)
Load 8, 16, 32 or 64 bits with or without sign extension from host memory. 
offset must be a constant.

* st_i32/i64 t0, t1, offset
st8_i32/i64 t0, t1, offset
st16_i32/i64 t0, t1, offset
st32_i64 t0, t1, offset

write(t0, t1 + offset)
Write 8, 16, 32 or 64 bits to host memory.

********* QEMU specific operations

* tb_exit t0

Exit the current TB and return the value t0 (word type).

* goto_tb index

Exit the current TB and jump to the TB index 'index' (constant) if the
current TB was linked to this TB. Otherwise execute the next
instructions.

* qemu_ld_i32/i64 t0, t1, flags
qemu_ld8u_i32/i64 t0, t1, flags
qemu_ld8s_i32/i64 t0, t1, flags
qemu_ld16u_i32/i64 t0, t1, flags
qemu_ld16s_i32/i64 t0, t1, flags
qemu_ld32u_i64 t0, t1, flags
qemu_ld32s_i64 t0, t1, flags

Load data at the QEMU CPU address t1 into t0. t1 has the QEMU CPU
address type. 'flags' contains the QEMU memory index (selects user or
kernel access) for example.

* qemu_st_i32/i64 t0, t1, flags
qemu_st8_i32/i64 t0, t1, flags
qemu_st16_i32/i64 t0, t1, flags
qemu_st32_i64 t0, t1, flags

Store the data t0 at the QEMU CPU Address t1. t1 has the QEMU CPU
address type. 'flags' contains the QEMU memory index (selects user or
kernel access) for example.

Note 1: Some shortcuts are defined when the last operand is known to be
a constant (e.g. addi for add, movi for mov).

Note 2: When using TCG, the opcodes must never be generated directly
as some of them may not be available as "real" opcodes. Always use the
function tcg_gen_xxx(args).

4) Backend

tcg-target.h contains the target specific definitions. tcg-target.c
contains the target specific code.

4.1) Assumptions

The target word size (TCG_TARGET_REG_BITS) is expected to be 32 bit or
64 bit. It is expected that the pointer has the same size as the word.

On a 32 bit target, all 64 bit operations are converted to 32 bits. A
few specific operations must be implemented to allow it (see add2_i32,
sub2_i32, brcond2_i32).

Floating point operations are not supported in this version. A
previous incarnation of the code generator had full support of them,
but it is better to concentrate on integer operations first.

On a 64 bit target, no assumption is made in TCG about the storage of
the 32 bit values in 64 bit registers.

4.2) Constraints

GCC like constraints are used to define the constraints of every
instruction. Memory constraints are not supported in this
version. Aliases are specified in the input operands as for GCC.

A target can define specific register or constant constraints. If an
operation uses a constant input constraint which does not allow all
constants, it must also accept registers in order to have a fallback.

The movi_i32 and movi_i64 operations must accept any constants.

The mov_i32 and mov_i64 operations must accept any registers of the
same type.

The ld/st instructions must accept signed 32 bit constant offsets. It
can be implemented by reserving a specific register to compute the
address if the offset is too big.

The ld/st instructions must accept any destination (ld) or source (st)
register.

4.3) Function call assumptions

- The only supported types for parameters and return value are: 32 and
  64 bit integers and pointer.
- The stack grows downwards.
- The first N parameters are passed in registers.
- The next parameters are passed on the stack by storing them as words.
- Some registers are clobbered during the call. 
- The function can return 0 or 1 value in registers. On a 32 bit
  target, functions must be able to return 2 values in registers for
  64 bit return type.

5) Migration from dyngen to TCG

TCG is backward compatible with QEMU "dyngen" operations. It means
that TCG instructions can be freely mixed with dyngen operations. It
is expected that QEMU targets will be progressively fully converted to
TCG. Once a target is fully converted to TCG, it will be possible
to apply more optimizations because more registers will be free for
the generated code.

The exception model is the same as the dyngen one.

6) 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.
