# 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

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, the assumption was that TCG target might be different from the host, although this 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.

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.

## Basic Blocks

A TCG basic block is a single entry, multiple exit region which corresponds to a list of instructions terminated by a label, or any branch instruction.

A TCG extended basic block is a single entry, multiple exit region which corresponds to a list of instructions terminated by a label or an unconditional branch. Specifically, an extended basic block is a sequence of basic blocks connected by the fall-through paths of zero or more conditional branch instructions.

## Operations

TCG instructions or ops operate on TCG variables, both of which are strongly typed. Each instruction has a fixed number of output variable operands, input variable operands and constant operands. Vector instructions have a field specifying the element size within the vector. 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) */

## Variables

- `TEMP_FIXED`

  There is one TCG fixed global variable, `cpu_env`, which is live in all translation blocks, and holds a pointer to `CPUArchState`. This variable is held in a host cpu register at all times in all translation blocks.

- `TEMP_GLOBAL`

  A TCG global is a variable which is live in all translation blocks, and corresponds to memory location that is within `CPUArchState`. These may be specified as an offset from `cpu_env`, in which case they are called direct globals, or may be specified as an offset from a direct global, in which case they are called indirect globals. Even indirect globals should still reference memory within `CPUArchState`. All TCG globals are defined during `TCGCPUOps.initialize`, before any translation blocks are generated.

- `TEMP_CONST`

  A TCG constant is a variable which is live throughout the entire translation block, and contains a constant value. These variables are allocated on demand during translation and are hashed so that there is exactly one variable holding a given value.

- `TEMP_TB`

  A TCG translation block temporary is a variable which is live throughout the entire translation block, but dies on any exit. These temporaries are allocated explicitly during translation.

- `TEMP_EBB`

  A TCG extended basic block temporary is a variable which is live throughout an extended basic block, but dies on any exit. These temporaries are allocated explicitly during translation.

## Types

- `TCG_TYPE_I32`

  A 32-bit integer.

- `TCG_TYPE_I64`

  A 64-bit integer. For 32-bit hosts, such variables are split into a pair of variables with `type=TCG_TYPE_I32` and `base_type=TCG_TYPE_I64`. The `temp_subindex` for each indicates where it falls within the host-endian representation.

- `TCG_TYPE_PTR`

  An alias for `TCG_TYPE_I32` or `TCG_TYPE_I64`, depending on the size of a pointer for the host.

- `TCG_TYPE_REG`

  An alias for `TCG_TYPE_I32` or `TCG_TYPE_I64`, depending on the size of the integer registers for the host. This may be larger than `TCG_TYPE_PTR` depending on the host ABI.

- `TCG_TYPE_I128`

  A 128-bit integer. For all hosts, such variables are split into a number of variables with `type=TCG_TYPE_REG` and `base_type=TCG_TYPE_I128`. The `temp_subindex` for each indicates where it falls within the host-endian representation.

- `TCG_TYPE_V64`

  A 64-bit vector. This type is valid only if the TCG target sets `TCG_TARGET_HAS_v64`.

- `TCG_TYPE_V128`

  A 128-bit vector. This type is valid only if the TCG target sets `TCG_TARGET_HAS_v128`.

- `TCG_TYPE_V256`

  A 256-bit vector. This type is valid only if the TCG target sets `TCG_TARGET_HAS_v256`.

## Helpers

Helpers are registered in a guest-specific `helper.h`, which is processed to generate `tcg_gen_helper_*` functions. With these functions it is possible to call a function taking i32, i64, i128 or pointer types.

By default, before calling a helper, all globals are stored at their canonical location. By default, the helper is allowed to modify the CPU state (including the state represented by tcg globals) or may raise an exception. This default can be overridden using the following function modifiers:

- `TCG_CALL_NO_WRITE_GLOBALS`

  The helper does not modify any globals, but may read them. Globals will be saved to their canonical location before calling helpers, but need not be reloaded afterwards.

- `TCG_CALL_NO_READ_GLOBALS`

  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. This implies `TCG_CALL_NO_WRITE_GLOBALS`.

- `TCG_CALL_NO_SIDE_EFFECTS`

  The call to the helper function may be removed if the return value is not used. This means that it may not modify any CPU state nor may it raise an exception.

## 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.

## 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 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 */` `TCG_COND_TSTEQ /* t1 & t2 == 0 */` `TCG_COND_TSTNE /* t1 & t2 != 0 */`

### Arithmetic

add t0, t1, t2 | t0 = t1 + t2
sub t0, t1, t2 | t0 = t1 - t2
neg t0, t1 | t0 = -t1 (two’s complement)
mul t0, t1, t2 | t0 = t1 * t2
divs t0, t1, t2 | t0 = t1 / t2 (signed) Undefined behavior if division by zero or overflow.
divu t0, t1, t2 | t0 = t1 / t2 (unsigned) Undefined behavior if division by zero.
rems t0, t1, t2 | t0 = t1 % t2 (signed) Undefined behavior if division by zero or overflow.
remu t0, t1, t2 | t0 = t1 % t2 (unsigned) Undefined behavior if division by zero.
divs2 q, r, nl, nh, d | q = nh:nl / d (signed) r = nh:nl % d Undefined behaviour if division by zero, or the double-word numerator divided by the single-word divisor does not fit within the single-word quotient. The code generator will pass nh as a simple sign-extension of nl, so the only overflow should be INT_MIN / -1.
divu2 q, r, nl, nh, d | q = nh:nl / d (unsigned) r = nh:nl % d Undefined behaviour if division by zero, or the double-word numerator divided by the single-word divisor does not fit within the single-word quotient. The code generator will pass 0 to nh to make a simple zero-extension of nl, so overflow should never occur.

### Logical

and t0, t1, t2 | t0 = t1 & t2
or t0, t1, t2 | t0 = t1 | t2
xor t0, t1, t2 | t0 = t1 ^ t2
not t0, t1 | t0 = ~t1
andc t0, t1, t2 | t0 = t1 & ~t2
eqv t0, t1, t2 | t0 = ~(t1 ^ t2), or equivalently, t0 = t1 ^ ~t2
nand t0, t1, t2 | t0 = ~(t1 & t2)
nor t0, t1, t2 | t0 = ~(t1 | t2)
orc t0, t1, t2 | t0 = t1 | ~t2
clz t0, t1, t2 | t0 = t1 ? clz(t1) : t2
ctz t0, t1, t2 | t0 = t1 ? ctz(t1) : t2
ctpop t0, t1 | t0 = number of bits set in t1 The name ctpop is short for “count population”, and matches the function name used in `include/qemu/host-utils.h`.

### Shifts/Rotates

shl t0, t1, t2 | t0 = t1 << t2 Unspecified behavior for negative or out-of-range shifts.
shr t0, t1, t2 | t0 = t1 >> t2 (unsigned) Unspecified behavior for negative or out-of-range shifts.
sar t0, t1, t2 | t0 = t1 >> t2 (signed) Unspecified behavior for negative or out-of-range shifts.
rotl t0, t1, t2 | Rotation of t2 bits to the left Unspecified behavior for negative or out-of-range shifts.
rotr t0, t1, t2 | Rotation of t2 bits to the right. Unspecified behavior for negative or out-of-range shifts.

### Misc

mov t0, t1 | t0 = t1 Move t1 to t0.
bswap16 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 t0, t1, flags | 32 bit byte swap. The flags are the same as for bswap16, except they apply from bit 31 instead of bit 15. On TCG_TYPE_I32, the flags should be zero.
bswap64 t0, t1, flags | 64 bit byte swap. The flags are ignored, but still present for consistency with the other bswap opcodes. For future compatibility, the flags should be zero.
discard_i32/i64 t0 | Indicate that the value of t0 won’t be used later. It is useful to force dead code elimination.
deposit 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 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) on TCG_TYPE_I32.
extract dest, t1, pos, len sextract 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 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) on TCG_TYPE_I32.
extract2 dest, t1, t2, pos | For TCG_TYPE_I{N}, 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 dest, t1, t2, cond | dest = (t1 cond t2) Set dest to 1 if (t1 cond t2) is true, otherwise set to 0.
negsetcond dest, t1, t2, cond | dest = -(t1 cond t2) Set dest to -1 if (t1 cond t2) is true, otherwise set to 0.
movcond 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

addco t0, t1, t2 | Compute t0 = t1 + t2 and in addition output to the carry bit provided by the host architecture.
addci t0, t1, t2 | Compute t0 = t1 + t2 + C, where C is the input carry bit provided by the host architecture. The output carry bit need not be computed.
addcio t0, t1, t2 | Compute t0 = t1 + t2 + C, where C is the input carry bit provided by the host architecture, and also compute the output carry bit.
addc1o t0, t1, t2 | Compute t0 = t1 + t2 + 1, and in addition output to the carry bit provided by the host architecture. This is akin to addcio with a fixed carry-in value of 1. This is intended to be used by the optimization pass, intermediate to complete folding of the addition chain. In some cases complete folding is not possible and this opcode will remain until output. If this happens, the code generator will use `tcg_out_set_carry` and then the output routine for addcio.
subbo t0, t1, t2 | Compute t0 = t1 - t2 and in addition output to the borrow bit provided by the host architecture. Depending on the host architecture, the carry bit may or may not be identical to the borrow bit. Thus the addc* and subb* opcodes must not be mixed.
subbi t0, t1, t2 | Compute t0 = t1 - t2 - B, where B is the input borrow bit provided by the host architecture. The output borrow bit need not be computed.
subbio t0, t1, t2 | Compute t0 = t1 - t2 - B, where B is the input borrow bit provided by the host architecture, and also compute the output borrow bit.
subb1o t0, t1, t2 | Compute t0 = t1 - t2 - 1, and in addition output to the borrow bit provided by the host architecture. This is akin to subbio with a fixed borrow-in value of 1. This is intended to be used by the optimization pass, intermediate to complete folding of the subtraction chain. In some cases complete folding is not possible and this opcode will remain until output. If this happens, the code generator will use `tcg_out_set_borrow` and then the output routine for subbio.
mulu2 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 t0_low, t0_high, t1, t2 | Similar to mulu2, except the two inputs t1 and t2 are signed.
mulsh t0, t1, t2 muluh 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

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 Atomic operations in QEMU for more information on memory barriers.

### 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/i128 t0, t1, flags, memidx qemu_st_i32/i64/i128 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/_i128 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, or 128-bit quantities on a 64-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 qemu_ld/st_i128, these are only supported for a 64-bit host.

### Host vector operations

All of the vector ops have two parameters, `TCGOP_TYPE` & `TCGOP_VECE`. The former specifies the length of the vector as a TCGType; the latter specifies the length of the element (if applicable) in log2 8-bit units.

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 TYPE/VECE copies across v0.
dupi_vec v0, c | Similarly, for a constant. Smaller values will be replicated to host register size by the expanders.
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 nand_vec v0, v1, v2 or_vec v0, v1, v2 nor_vec v0, v1, v2 xor_vec v0, v1, v2 eqv_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 < TYPE/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 rotls_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 < TYPE/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).

## 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 64 bit. It is expected that the pointer has the same size as the word.

Values are transferred between 32 and 64-bit registers using the following ops:

- extrl_i64_i32

- extrh_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 extrl_i64_i32 and extrh_i64_i32 are optional ops. It is not necessary to implement them 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. If `TCG_REG_ZERO` is defined by the backend, the constraint ‘`z`’ is defined generically to map constant 0 to the hardware zero register.

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.

- 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.

- 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.