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diff --git a/qemu/tcg/README b/qemu/tcg/README deleted file mode 100644 index f4a8ac170..000000000 --- a/qemu/tcg/README +++ /dev/null @@ -1,570 +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 - -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 afterwise. -- 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 - -********* 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 - -16 bit byte swap on a 32/64 bit value. It assumes that the two/six high order -bytes are set to zero. - -* bswap32_i32/i64 t0, t1 - -32 bit byte swap on a 32/64 bit value. With a 64 bit value, it assumes that -the four high order bytes are set to zero. - -* bswap64_i64 t0, t1 - -64 bit byte swap - -* discard_i32/i64 t0 - -Indicate that the value of t0 won't be used later. It is useful to -force dead code elimination. - -* 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, pos=8, len=4 indicates a 4-bit field at bit 8. -This operation would be equivalent to - - dest = (t1 & ~0x0f00) | ((t2 << 8) & 0x0f00) - -* 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. - -********* 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. - -* qemu_ld_i32/i64 t0, t1, flags, memidx -* qemu_st_i32/i64 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 TCGMemOp 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. - -********* - -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.inc.c -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 transfered 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 not currently supported. - -A target can define specific register or constant constraints. If an -operation uses a constant input constraint which does not allow all -constants, it must also accept registers in order to have a fallback. - -The movi_i32 and movi_i64 operations must accept any constants. - -The mov_i32 and mov_i64 operations must accept any registers of the -same type. - -The ld/st instructions must accept signed 32 bit constant offsets. It -can be implemented by reserving a specific register to compute the -address if the offset is too big. - -The ld/st instructions must accept any destination (ld) or source (st) -register. - -4.3) Function call assumptions - -- The only supported types for parameters and return value are: 32 and - 64 bit integers and pointer. -- The stack grows downwards. -- The first N parameters are passed in registers. -- The next parameters are passed on the stack by storing them as words. -- Some registers are clobbered during the call. -- The function can return 0 or 1 value in registers. On a 32 bit - target, functions must be able to return 2 values in registers for - 64 bit return type. - -5) 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. |