tcg: convert tcg/README to rst

Convert tcg/README to rst and move it to docs/devel as a new "TCG Intermediate
Representation" page. There are a few minor changes to improve the aesthetic
of the final output which are as follows:

  - Rename the title from "Tiny Code Generator - Fabrice Bellard" to "TCG
    Intermediate Representation"

  - Remove the section numbering

  - Add the missing parameters to the ssadd_vec operations in the "Host
    vector operations" section

  - Change the path to the Atomic Operations document to use a proper
    reference

  - Replace tcg/README in tcg.rst with a proper reference to the new document

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20221130100434.64207-2-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 0 insertions, 784 deletions

diff --git a/tcg/README b/tcg/README
deleted file mode 100644
index bc15cc3..0000000
--- a/tcg/README
+++ /dev/null
@@ -1,784 +0,0 @@
-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
-
-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".
-
-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 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 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.
-
-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
-
-* 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
-
-* 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
-
-* 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
-
-* 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
-
-* 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
-
-* 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
-
-* 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.
-
-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
-
-* 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 later 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 can be obtained by emitting a pair of
-opcodes, mul+muluh/mulsh.
-
-********* Memory Barrier support
-
-* 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 docs/devel/atomics.rst 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".
-
-* 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
-
-* 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
-later 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.
-
-* 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:
-* umin_vec:
-
-  Similarly, v0 = MIN(v1, v2), for signed and unsigned element types.
-
-* smax_vec:
-* umax_vec:
-
-  Similarly, v0 = MAX(v1, v2), for signed and unsigned element types.
-
-* ssadd_vec:
-* sssub_vec:
-* usadd_vec:
-* ussub_vec:
-
-  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.
-
-    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.
-
-    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:
-  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).
-
-4) 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.
-
-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).
-
-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.
-
-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.
-
-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.
-
-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) 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.