qemu-e2k/docs/devel/tcg-ops.rst

.. _tcg-ops-ref:

*******************************
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.

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.

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
===========================

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 '$'.

.. code-block:: none

   add_i32 t0, t1, t2    /* (t0 <- t1 + t2) */


Assumptions
-----------

Basic blocks
^^^^^^^^^^^^

* Basic blocks end after branches (e.g. brcond_i32 instruction),
  goto_tb and exit_tb instructions.

* 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.

Floating point types
^^^^^^^^^^^^^^^^^^^^

* 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. 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.

  This 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`` 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_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
^^^^^^^^

* Use the instruction 'br' to jump to a label.

Code Optimizations
------------------

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

- Single instructions are simplified, e.g.

  .. code-block:: none

     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:

  .. code-block:: none

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

  only the last instruction is kept.


Instruction Reference
=====================

Function call
-------------

.. list-table::

   * - 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
------------

.. list-table::

   * - set_label $label

     - | Define label 'label' at the current program point.

   * - br $label

     - | Jump to label.

   * - brcond_i32/i64 *t0*, *t1*, *cond*, *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
----------

.. list-table::

   * - 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
-------

.. list-table::

   * - 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*

   * - andc_i32/i64 *t0*, *t1*, *t2*

     - | *t0* = *t1* & ~\ *t2*

   * - eqv_i32/i64 *t0*, *t1*, *t2*

     - | *t0* = ~(*t1* ^ *t2*), or equivalently, *t0* = *t1* ^ ~\ *t2*

   * - nand_i32/i64 *t0*, *t1*, *t2*

     - | *t0* = ~(*t1* & *t2*)

   * - nor_i32/i64 *t0*, *t1*, *t2*

     - | *t0* = ~(*t1* | *t2*)

   * - orc_i32/i64 *t0*, *t1*, *t2*

     - | *t0* = *t1* | ~\ *t2*

   * - clz_i32/i64 *t0*, *t1*, *t2*

     - | *t0* = *t1* ? clz(*t1*) : *t2*

   * - ctz_i32/i64 *t0*, *t1*, *t2*

     - | *t0* = *t1* ? ctz(*t1*) : *t2*

   * - ctpop_i32/i64 *t0*, *t1*

     - | *t0* = number of bits set in *t1*
       |
       | With *ctpop* short for "count population", matching
       | the function name used in ``include/qemu/host-utils.h``.


Shifts/Rotates
--------------

.. list-table::

   * - shl_i32/i64 *t0*, *t1*, *t2*

     - | *t0* = *t1* << *t2*
       | Unspecified behavior if *t2* < 0 or *t2* >= 32 (resp 64)

   * - shr_i32/i64 *t0*, *t1*, *t2*

     - | *t0* = *t1* >> *t2* (unsigned)
       | Unspecified behavior if *t2* < 0 or *t2* >= 32 (resp 64)

   * - sar_i32/i64 *t0*, *t1*, *t2*

     - | *t0* = *t1* >> *t2* (signed)
       | Unspecified behavior if *t2* < 0 or *t2* >= 32 (resp 64)

   * - rotl_i32/i64 *t0*, *t1*, *t2*

     - | Rotation of *t2* bits to the left
       | Unspecified behavior if *t2* < 0 or *t2* >= 32 (resp 64)

   * - rotr_i32/i64 *t0*, *t1*, *t2*

     - | Rotation of *t2* bits to the right.
       | Unspecified behavior if *t2* < 0 or *t2* >= 32 (resp 64)


Misc
----

.. list-table::

   * - 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/i64 *t0*, *t1*, *flags*

     - | 16 bit byte swap on the low bits of a 32/64 bit input.
       |
       | If *flags* & ``TCG_BSWAP_IZ``, then *t1* is known to be zero-extended from bit 15.
       | If *flags* & ``TCG_BSWAP_OZ``, then *t0* will be zero-extended from bit 15.
       | If *flags* & ``TCG_BSWAP_OS``, then *t0* will be sign-extended from bit 15.
       |
       | If neither ``TCG_BSWAP_OZ`` nor ``TCG_BSWAP_OS`` are set, then the bits of *t0* above bit 15 may contain any value.

   * - bswap32_i64 *t0*, *t1*, *flags*

     - | 32 bit byte swap on a 64-bit value.  The flags are the same as for bswap16,
         except they apply from bit 31 instead of bit 15.

   * - bswap32_i32 *t0*, *t1*, *flags*

       bswap64_i64 *t0*, *t1*, *flags*

     - | 32/64 bit byte swap. The flags are ignored, but still present
         for consistency with the other bswap opcodes.

   * - discard_i32/i64 *t0*

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

   * - deposit_i32/i64 *dest*, *t1*, *t2*, *pos*, *len*

     - | Deposit *t2* as a bitfield into *t1*, placing the result in *dest*.
       |
       | The bitfield is described by *pos*/*len*, which are immediate values:
       |
       |     *len* - the length of the bitfield
       |     *pos* - the position of the first bit, counting from the LSB
       |
       | For example, "deposit_i32 dest, t1, t2, 8, 4" indicates a 4-bit field
         at bit 8. This operation would be equivalent to
       |
       |     *dest* = (*t1* & ~0x0f00) | ((*t2* << 8) & 0x0f00)

   * - extract_i32/i64 *dest*, *t1*, *pos*, *len*

       sextract_i32/i64 *dest*, *t1*, *pos*, *len*

     - | Extract a bitfield from *t1*, placing the result in *dest*.
       |
       | The bitfield is described by *pos*/*len*, which are immediate values,
         as above for deposit.  For extract_*, the result will be extended
         to the left with zeros; for sextract_*, the result will be extended
         to the left with copies of the bitfield sign bit at *pos* + *len* - 1.
       |
       | For example, "sextract_i32 dest, t1, 8, 4" indicates a 4-bit field
         at bit 8. This operation would be equivalent to
       |
       |    *dest* = (*t1* << 20) >> 28
       |
       | (using an arithmetic right shift).

   * - extract2_i32/i64 *dest*, *t1*, *t2*, *pos*

     - | For N = {32,64}, extract an N-bit quantity from the concatenation
         of *t2*:*t1*, beginning at *pos*. The tcg_gen_extract2_{i32,i64} expander
         accepts 0 <= *pos* <= N as inputs. The backend code generator will
         not see either 0 or N as inputs for these opcodes.

   * - extrl_i64_i32 *t0*, *t1*

     - | For 64-bit hosts only, extract the low 32-bits of input *t1* and place it
         into 32-bit output *t0*.  Depending on the host, this may be a simple move,
         or may require additional canonicalization.

   * - extrh_i64_i32 *t0*, *t1*

     - | For 64-bit hosts only, extract the high 32-bits of input *t1* and place it
         into 32-bit output *t0*.  Depending on the host, this may be a simple shift,
         or may require additional canonicalization.


Conditional moves
-----------------

.. list-table::

   * - setcond_i32/i64 *dest*, *t1*, *t2*, *cond*

     - | *dest* = (*t1* *cond* *t2*)
       |
       | Set *dest* to 1 if (*t1* *cond* *t2*) is true, otherwise set to 0.

   * - movcond_i32/i64 *dest*, *c1*, *c2*, *v1*, *v2*, *cond*

     - | *dest* = (*c1* *cond* *c2* ? *v1* : *v2*)
       |
       | Set *dest* to *v1* if (*c1* *cond* *c2*) is true, otherwise set to *v2*.


Type conversions
----------------

.. list-table::

   * - 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
----------

.. list-table::

   * - 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.

All this opcodes assume that the pointed host memory doesn't correspond
to a global. In the latter case the behaviour is unpredictable.


Multiword arithmetic support
----------------------------

.. list-table::

   * - add2_i32/i64 *t0_low*, *t0_high*, *t1_low*, *t1_high*, *t2_low*, *t2_high*

       sub2_i32/i64 *t0_low*, *t0_high*, *t1_low*, *t1_high*, *t2_low*, *t2_high*

     - | Similar to add/sub, except that the double-word inputs *t1* and *t2* are
         formed from two single-word arguments, and the double-word output *t0*
         is returned in two single-word outputs.

   * - mulu2_i32/i64 *t0_low*, *t0_high*, *t1*, *t2*

     - | Similar to mul, except two unsigned inputs *t1* and *t2* yielding the full
         double-word product *t0*. The latter is returned in two single-word outputs.

   * - muls2_i32/i64 *t0_low*, *t0_high*, *t1*, *t2*

     - | Similar to mulu2, except the two inputs *t1* and *t2* are signed.

   * - mulsh_i32/i64 *t0*, *t1*, *t2*

       muluh_i32/i64 *t0*, *t1*, *t2*

     - | Provide the high part of a signed or unsigned multiply, respectively.
       |
       | If mulu2/muls2 are not provided by the backend, the tcg-op generator
         can obtain the same results by emitting a pair of opcodes, mul + muluh/mulsh.


Memory Barrier support
----------------------

.. list-table::

   * - mb *<$arg>*

     - | Generate a target memory barrier instruction to ensure memory ordering
         as being  enforced by a corresponding guest memory barrier instruction.
       |
       | The ordering enforced by the backend may be stricter than the ordering
         required by the guest. It cannot be weaker. This opcode takes a constant
         argument which is required to generate the appropriate barrier
         instruction. The backend should take care to emit the target barrier
         instruction only when necessary i.e., for SMP guests and when MTTCG is
         enabled.
       |
       | The guest translators should generate this opcode for all guest instructions
         which have ordering side effects.
       |
       | Please see :ref:`atomics-ref` for more information on memory barriers.


64-bit guest on 32-bit host support
-----------------------------------

The following opcodes are internal to TCG.  Thus they are to be implemented by
32-bit host code generators, but are not to be emitted by guest translators.
They are emitted as needed by inline functions within ``tcg-op.h``.

.. list-table::

   * - brcond2_i32 *t0_low*, *t0_high*, *t1_low*, *t1_high*, *cond*, *label*

     - | Similar to brcond, except that the 64-bit values *t0* and *t1*
         are formed from two 32-bit arguments.

   * - setcond2_i32 *dest*, *t1_low*, *t1_high*, *t2_low*, *t2_high*, *cond*

     - | Similar to setcond, except that the 64-bit values *t1* and *t2* are
         formed from two 32-bit arguments. The result is a 32-bit value.


QEMU specific operations
------------------------

.. list-table::

   * - exit_tb *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. Only indices 0 and 1 are valid and tcg_gen_goto_tb may be issued
         at most once with each slot index per TB.

   * - lookup_and_goto_ptr *tb_addr*

     - | Look up a TB address *tb_addr* and jump to it if valid. If not valid,
         jump to the TCG epilogue to go back to the exec loop.
       |
       | This operation is optional. If the TCG backend does not implement the
         goto_ptr opcode, emitting this op is equivalent to emitting exit_tb(0).

   * - qemu_ld_i32/i64 *t0*, *t1*, *flags*, *memidx*

       qemu_st_i32/i64 *t0*, *t1*, *flags*, *memidx*

       qemu_st8_i32 *t0*, *t1*, *flags*, *memidx*

     - | Load data at the guest address *t1* into *t0*, or store data in *t0* at guest
         address *t1*.  The _i32/_i64 size applies to the size of the input/output
         register *t0* only.  The address *t1* is always sized according to the guest,
         and the width of the memory operation is controlled by *flags*.
       |
       | Both *t0* and *t1* may be split into little-endian ordered pairs of registers
         if dealing with 64-bit quantities on a 32-bit host.
       |
       | The *memidx* selects the qemu tlb index to use (e.g. user or kernel access).
         The flags are the MemOp bits, selecting the sign, width, and endianness
         of the memory access.
       |
       | For a 32-bit host, qemu_ld/st_i64 is guaranteed to only be used with a
         64-bit memory access specified in *flags*.
       |
       | For i386, qemu_st8_i32 is exactly like qemu_st_i32, except the size of
         the memory operation is known to be 8-bit.  This allows the backend to
         provide a different set of register constraints.


Host vector operations
----------------------

All of the vector ops have two parameters, ``TCGOP_VECL`` & ``TCGOP_VECE``.
The former specifies the length of the vector in log2 64-bit units; the
latter specifies the length of the element (if applicable) in log2 8-bit units.
E.g. VECL = 1 -> 64 << 1 -> v128, and VECE = 2 -> 1 << 2 -> i32.

.. list-table::

   * - mov_vec *v0*, *v1*
       ld_vec *v0*, *t1*
       st_vec *v0*, *t1*

     - | Move, load and store.

   * - dup_vec *v0*, *r1*

     - | Duplicate the low N bits of *r1* into VECL/VECE copies across *v0*.

   * - dupi_vec *v0*, *c*

     - | Similarly, for a constant.
       | Smaller values will be replicated to host register size by the expanders.

   * - dup2_vec *v0*, *r1*, *r2*

     - | Duplicate *r2*:*r1* into VECL/64 copies across *v0*. This opcode is
         only present for 32-bit hosts.

   * - add_vec *v0*, *v1*, *v2*

     - | *v0* = *v1* + *v2*, in elements across the vector.

   * - sub_vec *v0*, *v1*, *v2*

     - | Similarly, *v0* = *v1* - *v2*.

   * - mul_vec *v0*, *v1*, *v2*

     - | Similarly, *v0* = *v1* * *v2*.

   * - neg_vec *v0*, *v1*

     - | Similarly, *v0* = -*v1*.

   * - abs_vec *v0*, *v1*

     - | Similarly, *v0* = *v1* < 0 ? -*v1* : *v1*, in elements across the vector.

   * - smin_vec *v0*, *v1*, *v2*

       umin_vec *v0*, *v1*, *v2*

     - | Similarly, *v0* = MIN(*v1*, *v2*), for signed and unsigned element types.

   * - smax_vec *v0*, *v1*, *v2*

       umax_vec *v0*, *v1*, *v2*

     - | Similarly, *v0* = MAX(*v1*, *v2*), for signed and unsigned element types.

   * - ssadd_vec *v0*, *v1*, *v2*

       sssub_vec *v0*, *v1*, *v2*

       usadd_vec *v0*, *v1*, *v2*

       ussub_vec *v0*, *v1*, *v2*

     - | Signed and unsigned saturating addition and subtraction.
       |
       | If the true result is not representable within the element type, the
         element is set to the minimum or maximum value for the type.

   * - and_vec *v0*, *v1*, *v2*

       or_vec *v0*, *v1*, *v2*

       xor_vec *v0*, *v1*, *v2*

       andc_vec *v0*, *v1*, *v2*

       orc_vec *v0*, *v1*, *v2*

       not_vec *v0*, *v1*

     - | Similarly, logical operations with and without complement.
       |
       | Note that VECE is unused.

   * - shli_vec *v0*, *v1*, *i2*

       shls_vec *v0*, *v1*, *s2*

     - | Shift all elements from v1 by a scalar *i2*/*s2*. I.e.

       .. code-block:: c

          for (i = 0; i < VECL/VECE; ++i) {
              v0[i] = v1[i] << s2;
          }

   * - shri_vec *v0*, *v1*, *i2*

       sari_vec *v0*, *v1*, *i2*

       rotli_vec *v0*, *v1*, *i2*

       shrs_vec *v0*, *v1*, *s2*

       sars_vec *v0*, *v1*, *s2*

     - | Similarly for logical and arithmetic right shift, and left rotate.

   * - shlv_vec *v0*, *v1*, *v2*

     - | Shift elements from *v1* by elements from *v2*. I.e.

       .. code-block:: c

          for (i = 0; i < VECL/VECE; ++i) {
              v0[i] = v1[i] << v2[i];
          }

   * - shrv_vec *v0*, *v1*, *v2*

       sarv_vec *v0*, *v1*, *v2*

       rotlv_vec *v0*, *v1*, *v2*

       rotrv_vec *v0*, *v1*, *v2*

     - | Similarly for logical and arithmetic right shift, and rotates.

   * - cmp_vec *v0*, *v1*, *v2*, *cond*

     - | Compare vectors by element, storing -1 for true and 0 for false.

   * - bitsel_vec *v0*, *v1*, *v2*, *v3*

     - | Bitwise select, *v0* = (*v2* & *v1*) | (*v3* & ~\ *v1*), across the entire vector.

   * - cmpsel_vec *v0*, *c1*, *c2*, *v3*, *v4*, *cond*

     - | Select elements based on comparison results:

       .. code-block:: c

          for (i = 0; i < n; ++i) {
              v0[i] = (c1[i] cond c2[i]) ? v3[i] : v4[i].
          }

**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).


Backend
=======

``tcg-target.h`` contains the target specific definitions. ``tcg-target.c.inc``
contains the target specific code; it is #included by ``tcg/tcg.c``, rather
than being a standalone C file.

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).

On a 64 bit target, the values are transferred between 32 and 64-bit
registers using the following ops:

- trunc_shr_i64_i32
- ext_i32_i64
- extu_i32_i64

They ensure that the values are correctly truncated or extended when
moved from a 32-bit to a 64-bit register or vice-versa. Note that the
trunc_shr_i64_i32 is an optional op. It is not necessary to implement
it if all the following conditions are met:

- 64-bit registers can hold 32-bit values
- 32-bit values in a 64-bit register do not need to stay zero or
  sign extended
- all 32-bit TCG ops ignore the high part of 64-bit registers

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.

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.

The same register may be used for both an input and an output, even when
they are not explicitly aliased.  If an op expands to multiple target
instructions then care must be taken to avoid clobbering input values.
GCC style "early clobber" outputs are supported, with '``&``'.

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 constraint '``i``' is defined generically to accept any constant.
The constraint '``r``' is not defined generically, but is consistently
used by each backend to indicate all registers.

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/sti instructions must accept signed 32 bit constant offsets.
This can be implemented by reserving a specific register in which to
compute the address if the offset is too big.

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

The sti instruction may fail if it cannot store the given constant.

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.


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.

- 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 guest
  instructions. There is little performance advantage in using TCG to
  implement guest instructions taking more than about twenty TCG
  instructions. Note that this rule of thumb is more applicable to
  helpers doing complex logic or arithmetic, where the C compiler has
  scope to do a good job of optimisation; it is less relevant where
  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.