qemu

README (23904B)

---

 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.