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GNU Info File | 1995-06-16 | 50KB | 879 lines

This is Info file gcc.info, produced by Makeinfo-1.55 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public License," "Funding for Free Software," and "Protect Your Freedom--Fight `Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English. File: gcc.info, Node: Constants, Next: Regs and Memory, Prev: Machine Modes, Up: RTL Constant Expression Types ========================= The simplest RTL expressions are those that represent constant values. `(const_int I)' This type of expression represents the integer value I. I is customarily accessed with the macro `INTVAL' as in `INTVAL (EXP)', which is equivalent to `XWINT (EXP, 0)'. There is only one expression object for the integer value zero; it is the value of the variable `const0_rtx'. Likewise, the only expression for integer value one is found in `const1_rtx', the only expression for integer value two is found in `const2_rtx', and the only expression for integer value negative one is found in `constm1_rtx'. Any attempt to create an expression of code `const_int' and value zero, one, two or negative one will return `const0_rtx', `const1_rtx', `const2_rtx' or `constm1_rtx' as appropriate. Similarly, there is only one object for the integer whose value is `STORE_FLAG_VALUE'. It is found in `const_true_rtx'. If `STORE_FLAG_VALUE' is one, `const_true_rtx' and `const1_rtx' will point to the same object. If `STORE_FLAG_VALUE' is -1, `const_true_rtx' and `constm1_rtx' will point to the same object. `(const_double:M ADDR I0 I1 ...)' Represents either a floating-point constant of mode M or an integer constant too large to fit into `HOST_BITS_PER_WIDE_INT' bits but small enough to fit within twice that number of bits (GNU CC does not provide a mechanism to represent even larger constants). In the latter case, M will be `VOIDmode'. ADDR is used to contain the `mem' expression that corresponds to the location in memory that at which the constant can be found. If it has not been allocated a memory location, but is on the chain of all `const_double' expressions in this compilation (maintained using an undisplayed field), ADDR contains `const0_rtx'. If it is not on the chain, ADDR contains `cc0_rtx'. ADDR is customarily accessed with the macro `CONST_DOUBLE_MEM' and the chain field via `CONST_DOUBLE_CHAIN'. If M is `VOIDmode', the bits of the value are stored in I0 and I1. I0 is customarily accessed with the macro `CONST_DOUBLE_LOW' and I1 with `CONST_DOUBLE_HIGH'. If the constant is floating point (regardless of its precision), then the number of integers used to store the value depends on the size of `REAL_VALUE_TYPE' (*note Cross-compilation::.). The integers represent a floating point number, but not precisely in the target machine's or host machine's floating point format. To convert them to the precise bit pattern used by the target machine, use the macro `REAL_VALUE_TO_TARGET_DOUBLE' and friends (*note Data Output::.). The macro `CONST0_RTX (MODE)' refers to an expression with value 0 in mode MODE. If mode MODE is of mode class `MODE_INT', it returns `const0_rtx'. Otherwise, it returns a `CONST_DOUBLE' expression in mode MODE. Similarly, the macro `CONST1_RTX (MODE)' refers to an expression with value 1 in mode MODE and similarly for `CONST2_RTX'. `(const_string STR)' Represents a constant string with value STR. Currently this is used only for insn attributes (*note Insn Attributes::.) since constant strings in C are placed in memory. `(symbol_ref:MODE SYMBOL)' Represents the value of an assembler label for data. SYMBOL is a string that describes the name of the assembler label. If it starts with a `*', the label is the rest of SYMBOL not including the `*'. Otherwise, the label is SYMBOL, usually prefixed with `_'. The `symbol_ref' contains a mode, which is usually `Pmode'. Usually that is the only mode for which a symbol is directly valid. `(label_ref LABEL)' Represents the value of an assembler label for code. It contains one operand, an expression, which must be a `code_label' that appears in the instruction sequence to identify the place where the label should go. The reason for using a distinct expression type for code label references is so that jump optimization can distinguish them. `(const:M EXP)' Represents a constant that is the result of an assembly-time arithmetic computation. The operand, EXP, is an expression that contains only constants (`const_int', `symbol_ref' and `label_ref' expressions) combined with `plus' and `minus'. However, not all combinations are valid, since the assembler cannot do arbitrary arithmetic on relocatable symbols. M should be `Pmode'. `(high:M EXP)' Represents the high-order bits of EXP, usually a `symbol_ref'. The number of bits is machine-dependent and is normally the number of bits specified in an instruction that initializes the high order bits of a register. It is used with `lo_sum' to represent the typical two-instruction sequence used in RISC machines to reference a global memory location. M should be `Pmode'. File: gcc.info, Node: Regs and Memory, Next: Arithmetic, Prev: Constants, Up: RTL Registers and Memory ==================== Here are the RTL expression types for describing access to machine registers and to main memory. `(reg:M N)' For small values of the integer N (those that are less than `FIRST_PSEUDO_REGISTER'), this stands for a reference to machine register number N: a "hard register". For larger values of N, it stands for a temporary value or "pseudo register". The compiler's strategy is to generate code assuming an unlimited number of such pseudo registers, and later convert them into hard registers or into memory references. M is the machine mode of the reference. It is necessary because machines can generally refer to each register in more than one mode. For example, a register may contain a full word but there may be instructions to refer to it as a half word or as a single byte, as well as instructions to refer to it as a floating point number of various precisions. Even for a register that the machine can access in only one mode, the mode must always be specified. The symbol `FIRST_PSEUDO_REGISTER' is defined by the machine description, since the number of hard registers on the machine is an invariant characteristic of the machine. Note, however, that not all of the machine registers must be general registers. All the machine registers that can be used for storage of data are given hard register numbers, even those that can be used only in certain instructions or can hold only certain types of data. A hard register may be accessed in various modes throughout one function, but each pseudo register is given a natural mode and is accessed only in that mode. When it is necessary to describe an access to a pseudo register using a nonnatural mode, a `subreg' expression is used. A `reg' expression with a machine mode that specifies more than one word of data may actually stand for several consecutive registers. If in addition the register number specifies a hardware register, then it actually represents several consecutive hardware registers starting with the specified one. Each pseudo register number used in a function's RTL code is represented by a unique `reg' expression. Some pseudo register numbers, those within the range of `FIRST_VIRTUAL_REGISTER' to `LAST_VIRTUAL_REGISTER' only appear during the RTL generation phase and are eliminated before the optimization phases. These represent locations in the stack frame that cannot be determined until RTL generation for the function has been completed. The following virtual register numbers are defined: `VIRTUAL_INCOMING_ARGS_REGNUM' This points to the first word of the incoming arguments passed on the stack. Normally these arguments are placed there by the caller, but the callee may have pushed some arguments that were previously passed in registers. When RTL generation is complete, this virtual register is replaced by the sum of the register given by `ARG_POINTER_REGNUM' and the value of `FIRST_PARM_OFFSET'. `VIRTUAL_STACK_VARS_REGNUM' If `FRAME_GROWS_DOWNWARD' is defined, this points to immediately above the first variable on the stack. Otherwise, it points to the first variable on the stack. `VIRTUAL_STACK_VARS_REGNUM' is replaced with the sum of the register given by `FRAME_POINTER_REGNUM' and the value `STARTING_FRAME_OFFSET'. `VIRTUAL_STACK_DYNAMIC_REGNUM' This points to the location of dynamically allocated memory on the stack immediately after the stack pointer has been adjusted by the amount of memory desired. This virtual register is replaced by the sum of the register given by `STACK_POINTER_REGNUM' and the value `STACK_DYNAMIC_OFFSET'. `VIRTUAL_OUTGOING_ARGS_REGNUM' This points to the location in the stack at which outgoing arguments should be written when the stack is pre-pushed (arguments pushed using push insns should always use `STACK_POINTER_REGNUM'). This virtual register is replaced by the sum of the register given by `STACK_POINTER_REGNUM' and the value `STACK_POINTER_OFFSET'. `(subreg:M REG WORDNUM)' `subreg' expressions are used to refer to a register in a machine mode other than its natural one, or to refer to one register of a multi-word `reg' that actually refers to several registers. Each pseudo-register has a natural mode. If it is necessary to operate on it in a different mode--for example, to perform a fullword move instruction on a pseudo-register that contains a single byte--the pseudo-register must be enclosed in a `subreg'. In such a case, WORDNUM is zero. Usually M is at least as narrow as the mode of REG, in which case it is restricting consideration to only the bits of REG that are in M. Sometimes M is wider than the mode of REG. These `subreg' expressions are often called "paradoxical". They are used in cases where we want to refer to an object in a wider mode but do not care what value the additional bits have. The reload pass ensures that paradoxical references are only made to hard registers. The other use of `subreg' is to extract the individual registers of a multi-register value. Machine modes such as `DImode' and `TImode' can indicate values longer than a word, values which usually require two or more consecutive registers. To access one of the registers, use a `subreg' with mode `SImode' and a WORDNUM that says which register. Storing in a non-paradoxical `subreg' has undefined results for bits belonging to the same word as the `subreg'. This laxity makes it easier to generate efficient code for such instructions. To represent an instruction that preserves all the bits outside of those in the `subreg', use `strict_low_part' around the `subreg'. The compilation parameter `WORDS_BIG_ENDIAN', if set to 1, says that word number zero is the most significant part; otherwise, it is the least significant part. Between the combiner pass and the reload pass, it is possible to have a paradoxical `subreg' which contains a `mem' instead of a `reg' as its first operand. After the reload pass, it is also possible to have a non-paradoxical `subreg' which contains a `mem'; this usually occurs when the `mem' is a stack slot which replaced a pseudo register. Note that it is not valid to access a `DFmode' value in `SFmode' using a `subreg'. On some machines the most significant part of a `DFmode' value does not have the same format as a single-precision floating value. It is also not valid to access a single word of a multi-word value in a hard register when less registers can hold the value than would be expected from its size. For example, some 32-bit machines have floating-point registers that can hold an entire `DFmode' value. If register 10 were such a register `(subreg:SI (reg:DF 10) 1)' would be invalid because there is no way to convert that reference to a single machine register. The reload pass prevents `subreg' expressions such as these from being formed. The first operand of a `subreg' expression is customarily accessed with the `SUBREG_REG' macro and the second operand is customarily accessed with the `SUBREG_WORD' macro. `(scratch:M)' This represents a scratch register that will be required for the execution of a single instruction and not used subsequently. It is converted into a `reg' by either the local register allocator or the reload pass. `scratch' is usually present inside a `clobber' operation (*note Side Effects::.). `(cc0)' This refers to the machine's condition code register. It has no operands and may not have a machine mode. There are two ways to use it: * To stand for a complete set of condition code flags. This is best on most machines, where each comparison sets the entire series of flags. With this technique, `(cc0)' may be validly used in only two contexts: as the destination of an assignment (in test and compare instructions) and in comparison operators comparing against zero (`const_int' with value zero; that is to say, `const0_rtx'). * To stand for a single flag that is the result of a single condition. This is useful on machines that have only a single flag bit, and in which comparison instructions must specify the condition to test. With this technique, `(cc0)' may be validly used in only two contexts: as the destination of an assignment (in test and compare instructions) where the source is a comparison operator, and as the first operand of `if_then_else' (in a conditional branch). There is only one expression object of code `cc0'; it is the value of the variable `cc0_rtx'. Any attempt to create an expression of code `cc0' will return `cc0_rtx'. Instructions can set the condition code implicitly. On many machines, nearly all instructions set the condition code based on the value that they compute or store. It is not necessary to record these actions explicitly in the RTL because the machine description includes a prescription for recognizing the instructions that do so (by means of the macro `NOTICE_UPDATE_CC'). *Note Condition Code::. Only instructions whose sole purpose is to set the condition code, and instructions that use the condition code, need mention `(cc0)'. On some machines, the condition code register is given a register number and a `reg' is used instead of `(cc0)'. This is usually the preferable approach if only a small subset of instructions modify the condition code. Other machines store condition codes in general registers; in such cases a pseudo register should be used. Some machines, such as the Sparc and RS/6000, have two sets of arithmetic instructions, one that sets and one that does not set the condition code. This is best handled by normally generating the instruction that does not set the condition code, and making a pattern that both performs the arithmetic and sets the condition code register (which would not be `(cc0)' in this case). For examples, search for `addcc' and `andcc' in `sparc.md'. `(pc)' This represents the machine's program counter. It has no operands and may not have a machine mode. `(pc)' may be validly used only in certain specific contexts in jump instructions. There is only one expression object of code `pc'; it is the value of the variable `pc_rtx'. Any attempt to create an expression of code `pc' will return `pc_rtx'. All instructions that do not jump alter the program counter implicitly by incrementing it, but there is no need to mention this in the RTL. `(mem:M ADDR)' This RTX represents a reference to main memory at an address represented by the expression ADDR. M specifies how large a unit of memory is accessed. File: gcc.info, Node: Arithmetic, Next: Comparisons, Prev: Regs and Memory, Up: RTL RTL Expressions for Arithmetic ============================== Unless otherwise specified, all the operands of arithmetic expressions must be valid for mode M. An operand is valid for mode M if it has mode M, or if it is a `const_int' or `const_double' and M is a mode of class `MODE_INT'. For commutative binary operations, constants should be placed in the second operand. `(plus:M X Y)' Represents the sum of the values represented by X and Y carried out in machine mode M. `(lo_sum:M X Y)' Like `plus', except that it represents that sum of X and the low-order bits of Y. The number of low order bits is machine-dependent but is normally the number of bits in a `Pmode' item minus the number of bits set by the `high' code (*note Constants::.). M should be `Pmode'. `(minus:M X Y)' Like `plus' but represents subtraction. `(compare:M X Y)' Represents the result of subtracting Y from X for purposes of comparison. The result is computed without overflow, as if with infinite precision. Of course, machines can't really subtract with infinite precision. However, they can pretend to do so when only the sign of the result will be used, which is the case when the result is stored in the condition code. And that is the only way this kind of expression may validly be used: as a value to be stored in the condition codes. The mode M is not related to the modes of X and Y, but instead is the mode of the condition code value. If `(cc0)' is used, it is `VOIDmode'. Otherwise it is some mode in class `MODE_CC', often `CCmode'. *Note Condition Code::. Normally, X and Y must have the same mode. Otherwise, `compare' is valid only if the mode of X is in class `MODE_INT' and Y is a `const_int' or `const_double' with mode `VOIDmode'. The mode of X determines what mode the comparison is to be done in; thus it must not be `VOIDmode'. If one of the operands is a constant, it should be placed in the second operand and the comparison code adjusted as appropriate. A `compare' specifying two `VOIDmode' constants is not valid since there is no way to know in what mode the comparison is to be performed; the comparison must either be folded during the compilation or the first operand must be loaded into a register while its mode is still known. `(neg:M X)' Represents the negation (subtraction from zero) of the value represented by X, carried out in mode M. `(mult:M X Y)' Represents the signed product of the values represented by X and Y carried out in machine mode M. Some machines support a multiplication that generates a product wider than the operands. Write the pattern for this as (mult:M (sign_extend:M X) (sign_extend:M Y)) where M is wider than the modes of X and Y, which need not be the same. Write patterns for unsigned widening multiplication similarly using `zero_extend'. `(div:M X Y)' Represents the quotient in signed division of X by Y, carried out in machine mode M. If M is a floating point mode, it represents the exact quotient; otherwise, the integerized quotient. Some machines have division instructions in which the operands and quotient widths are not all the same; you should represent such instructions using `truncate' and `sign_extend' as in, (truncate:M1 (div:M2 X (sign_extend:M2 Y))) `(udiv:M X Y)' Like `div' but represents unsigned division. `(mod:M X Y)' `(umod:M X Y)' Like `div' and `udiv' but represent the remainder instead of the quotient. `(smin:M X Y)' `(smax:M X Y)' Represents the smaller (for `smin') or larger (for `smax') of X and Y, interpreted as signed integers in mode M. `(umin:M X Y)' `(umax:M X Y)' Like `smin' and `smax', but the values are interpreted as unsigned integers. `(not:M X)' Represents the bitwise complement of the value represented by X, carried out in mode M, which must be a fixed-point machine mode. `(and:M X Y)' Represents the bitwise logical-and of the values represented by X and Y, carried out in machine mode M, which must be a fixed-point machine mode. `(ior:M X Y)' Represents the bitwise inclusive-or of the values represented by X and Y, carried out in machine mode M, which must be a fixed-point mode. `(xor:M X Y)' Represents the bitwise exclusive-or of the values represented by X and Y, carried out in machine mode M, which must be a fixed-point mode. `(ashift:M X C)' Represents the result of arithmetically shifting X left by C places. X have mode M, a fixed-point machine mode. C be a fixed-point mode or be a constant with mode `VOIDmode'; which mode is determined by the mode called for in the machine description entry for the left-shift instruction. For example, on the Vax, the mode of C is `QImode' regardless of M. `(lshiftrt:M X C)' `(ashiftrt:M X C)' Like `ashift' but for right shift. Unlike the case for left shift, these two operations are distinct. `(rotate:M X C)' `(rotatert:M X C)' Similar but represent left and right rotate. If C is a constant, use `rotate'. `(abs:M X)' Represents the absolute value of X, computed in mode M. `(sqrt:M X)' Represents the square root of X, computed in mode M. Most often M will be a floating point mode. `(ffs:M X)' Represents one plus the index of the least significant 1-bit in X, represented as an integer of mode M. (The value is zero if X is zero.) The mode of X need not be M; depending on the target machine, various mode combinations may be valid. File: gcc.info, Node: Comparisons, Next: Bit Fields, Prev: Arithmetic, Up: RTL Comparison Operations ===================== Comparison operators test a relation on two operands and are considered to represent a machine-dependent nonzero value described by, but not necessarily equal to, `STORE_FLAG_VALUE' (*note Misc::.) if the relation holds, or zero if it does not. The mode of the comparison operation is independent of the mode of the data being compared. If the comparison operation is being tested (e.g., the first operand of an `if_then_else'), the mode must be `VOIDmode'. If the comparison operation is producing data to be stored in some variable, the mode must be in class `MODE_INT'. All comparison operations producing data must use the same mode, which is machine-specific. There are two ways that comparison operations may be used. The comparison operators may be used to compare the condition codes `(cc0)' against zero, as in `(eq (cc0) (const_int 0))'. Such a construct actually refers to the result of the preceding instruction in which the condition codes were set. The instructing setting the condition code must be adjacent to the instruction using the condition code; only `note' insns may separate them. Alternatively, a comparison operation may directly compare two data objects. The mode of the comparison is determined by the operands; they must both be valid for a common machine mode. A comparison with both operands constant would be invalid as the machine mode could not be deduced from it, but such a comparison should never exist in RTL due to constant folding. In the example above, if `(cc0)' were last set to `(compare X Y)', the comparison operation is identical to `(eq X Y)'. Usually only one style of comparisons is supported on a particular machine, but the combine pass will try to merge the operations to produce the `eq' shown in case it exists in the context of the particular insn involved. Inequality comparisons come in two flavors, signed and unsigned. Thus, there are distinct expression codes `gt' and `gtu' for signed and unsigned greater-than. These can produce different results for the same pair of integer values: for example, 1 is signed greater-than -1 but not unsigned greater-than, because -1 when regarded as unsigned is actually `0xffffffff' which is greater than 1. The signed comparisons are also used for floating point values. Floating point comparisons are distinguished by the machine modes of the operands. `(eq:M X Y)' 1 if the values represented by X and Y are equal, otherwise 0. `(ne:M X Y)' 1 if the values represented by X and Y are not equal, otherwise 0. `(gt:M X Y)' 1 if the X is greater than Y. If they are fixed-point, the comparison is done in a signed sense. `(gtu:M X Y)' Like `gt' but does unsigned comparison, on fixed-point numbers only. `(lt:M X Y)' `(ltu:M X Y)' Like `gt' and `gtu' but test for "less than". `(ge:M X Y)' `(geu:M X Y)' Like `gt' and `gtu' but test for "greater than or equal". `(le:M X Y)' `(leu:M X Y)' Like `gt' and `gtu' but test for "less than or equal". `(if_then_else COND THEN ELSE)' This is not a comparison operation but is listed here because it is always used in conjunction with a comparison operation. To be precise, COND is a comparison expression. This expression represents a choice, according to COND, between the value represented by THEN and the one represented by ELSE. On most machines, `if_then_else' expressions are valid only to express conditional jumps. `(cond [TEST1 VALUE1 TEST2 VALUE2 ...] DEFAULT)' Similar to `if_then_else', but more general. Each of TEST1, TEST2, ... is performed in turn. The result of this expression is the VALUE corresponding to the first non-zero test, or DEFAULT if none of the tests are non-zero expressions. This is currently not valid for instruction patterns and is supported only for insn attributes. *Note Insn Attributes::. File: gcc.info, Node: Bit Fields, Next: Conversions, Prev: Comparisons, Up: RTL Bit Fields ========== Special expression codes exist to represent bitfield instructions. These types of expressions are lvalues in RTL; they may appear on the left side of an assignment, indicating insertion of a value into the specified bit field. `(sign_extract:M LOC SIZE POS)' This represents a reference to a sign-extended bit field contained or starting in LOC (a memory or register reference). The bit field is SIZE bits wide and starts at bit POS. The compilation option `BITS_BIG_ENDIAN' says which end of the memory unit POS counts from. If LOC is in memory, its mode must be a single-byte integer mode. If LOC is in a register, the mode to use is specified by the operand of the `insv' or `extv' pattern (*note Standard Names::.) and is usually a full-word integer mode. The mode of POS is machine-specific and is also specified in the `insv' or `extv' pattern. The mode M is the same as the mode that would be used for LOC if it were a register. `(zero_extract:M LOC SIZE POS)' Like `sign_extract' but refers to an unsigned or zero-extended bit field. The same sequence of bits are extracted, but they are filled to an entire word with zeros instead of by sign-extension. File: gcc.info, Node: Conversions, Next: RTL Declarations, Prev: Bit Fields, Up: RTL Conversions =========== All conversions between machine modes must be represented by explicit conversion operations. For example, an expression which is the sum of a byte and a full word cannot be written as `(plus:SI (reg:QI 34) (reg:SI 80))' because the `plus' operation requires two operands of the same machine mode. Therefore, the byte-sized operand is enclosed in a conversion operation, as in (plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80)) The conversion operation is not a mere placeholder, because there may be more than one way of converting from a given starting mode to the desired final mode. The conversion operation code says how to do For all conversion operations, X must not be `VOIDmode' because the mode in which to do the conversion would not be known. The conversion must either be done at compile-time or X must be placed into a register. `(sign_extend:M X)' Represents the result of sign-extending the value X to machine mode M. M must be a fixed-point mode and X a fixed-point value of a mode narrower than M. `(zero_extend:M X)' Represents the result of zero-extending the value X to machine mode M. M must be a fixed-point mode and X a fixed-point value of a mode narrower than M. `(float_extend:M X)' Represents the result of extending the value X to machine mode M. m must be a floating point mode and X a floating point value of a mode narrower than M. `(truncate:M X)' Represents the result of truncating the value X to machine mode M. M must be a fixed-point mode and X a fixed-point value of a mode wider than M. `(float_truncate:M X)' Represents the result of truncating the value X to machine mode M. M must be a floating point mode and X a floating point value of a mode wider than M. `(float:M X)' Represents the result of converting fixed point value X, regarded as signed, to floating point mode M. `(unsigned_float:M X)' Represents the result of converting fixed point value X, regarded as unsigned, to floating point mode M. `(fix:M X)' When M is a fixed point mode, represents the result of converting floating point value X to mode M, regarded as signed. How rounding is done is not specified, so this operation may be used validly in compiling C code only for integer-valued operands. `(unsigned_fix:M X)' Represents the result of converting floating point value X to fixed point mode M, regarded as unsigned. How rounding is done is not specified. `(fix:M X)' When M is a floating point mode, represents the result of converting floating point value X (valid for mode M) to an integer, still represented in floating point mode M, by rounding towards zero. File: gcc.info, Node: RTL Declarations, Next: Side Effects, Prev: Conversions, Up: RTL Declarations ============ Declaration expression codes do not represent arithmetic operations but rather state assertions about their operands. `(strict_low_part (subreg:M (reg:N R) 0))' This expression code is used in only one context: as the destination operand of a `set' expression. In addition, the operand of this expression must be a non-paradoxical `subreg' expression. The presence of `strict_low_part' says that the part of the register which is meaningful in mode N, but is not part of mode M, is not to be altered. Normally, an assignment to such a subreg is allowed to have undefined effects on the rest of the register when M is less than a word. File: gcc.info, Node: Side Effects, Next: Incdec, Prev: RTL Declarations, Up: RTL Side Effect Expressions ======================= The expression codes described so far represent values, not actions. But machine instructions never produce values; they are meaningful only for their side effects on the state of the machine. Special expression codes are used to represent side effects. The body of an instruction is always one of these side effect codes; the codes described above, which represent values, appear only as the operands of these. `(set LVAL X)' Represents the action of storing the value of X into the place represented by LVAL. LVAL must be an expression representing a place that can be stored in: `reg' (or `subreg' or `strict_low_part'), `mem', `pc' or `cc0'. If LVAL is a `reg', `subreg' or `mem', it has a machine mode; then X must be valid for that mode. If LVAL is a `reg' whose machine mode is less than the full width of the register, then it means that the part of the register specified by the machine mode is given the specified value and the rest of the register receives an undefined value. Likewise, if LVAL is a `subreg' whose machine mode is narrower than the mode of the register, the rest of the register can be changed in an undefined way. If LVAL is a `strict_low_part' of a `subreg', then the part of the register specified by the machine mode of the `subreg' is given the value X and the rest of the register is not changed. If LVAL is `(cc0)', it has no machine mode, and X may be either a `compare' expression or a value that may have any mode. The latter case represents a "test" instruction. The expression `(set (cc0) (reg:M N))' is equivalent to `(set (cc0) (compare (reg:M N) (const_int 0)))'. Use the former expression to save space during the compilation. If LVAL is `(pc)', we have a jump instruction, and the possibilities for X are very limited. It may be a `label_ref' expression (unconditional jump). It may be an `if_then_else' (conditional jump), in which case either the second or the third operand must be `(pc)' (for the case which does not jump) and the other of the two must be a `label_ref' (for the case which does jump). X may also be a `mem' or `(plus:SI (pc) Y)', where Y may be a `reg' or a `mem'; these unusual patterns are used to represent jumps through branch tables. If LVAL is neither `(cc0)' nor `(pc)', the mode of LVAL must not be `VOIDmode' and the mode of X must be valid for the mode of LVAL. LVAL is customarily accessed with the `SET_DEST' macro and X with the `SET_SRC' macro. `(return)' As the sole expression in a pattern, represents a return from the current function, on machines where this can be done with one instruction, such as Vaxes. On machines where a multi-instruction "epilogue" must be executed in order to return from the function, returning is done by jumping to a label which precedes the epilogue, and the `return' expression code is never used. Inside an `if_then_else' expression, represents the value to be placed in `pc' to return to the caller. Note that an insn pattern of `(return)' is logically equivalent to `(set (pc) (return))', but the latter form is never used. `(call FUNCTION NARGS)' Represents a function call. FUNCTION is a `mem' expression whose address is the address of the function to be called. NARGS is an expression which can be used for two purposes: on some machines it represents the number of bytes of stack argument; on others, it represents the number of argument registers. Each machine has a standard machine mode which FUNCTION must have. The machine description defines macro `FUNCTION_MODE' to expand into the requisite mode name. The purpose of this mode is to specify what kind of addressing is allowed, on machines where the allowed kinds of addressing depend on the machine mode being addressed. `(clobber X)' Represents the storing or possible storing of an unpredictable, undescribed value into X, which must be a `reg', `scratch' or `mem' expression. One place this is used is in string instructions that store standard values into particular hard registers. It may not be worth the trouble to describe the values that are stored, but it is essential to inform the compiler that the registers will be altered, lest it attempt to keep data in them across the string instruction. If X is `(mem:BLK (const_int 0))', it means that all memory locations must be presumed clobbered. Note that the machine description classifies certain hard registers as "call-clobbered". All function call instructions are assumed by default to clobber these registers, so there is no need to use `clobber' expressions to indicate this fact. Also, each function call is assumed to have the potential to alter any memory location, unless the function is declared `const'. If the last group of expressions in a `parallel' are each a `clobber' expression whose arguments are `reg' or `match_scratch' (*note RTL Template::.) expressions, the combiner phase can add the appropriate `clobber' expressions to an insn it has constructed when doing so will cause a pattern to be matched. This feature can be used, for example, on a machine that whose multiply and add instructions don't use an MQ register but which has an add-accumulate instruction that does clobber the MQ register. Similarly, a combined instruction might require a temporary register while the constituent instructions might not. When a `clobber' expression for a register appears inside a `parallel' with other side effects, the register allocator guarantees that the register is unoccupied both before and after that insn. However, the reload phase may allocate a register used for one of the inputs unless the `&' constraint is specified for the selected alternative (*note Modifiers::.). You can clobber either a specific hard register, a pseudo register, or a `scratch' expression; in the latter two cases, GNU CC will allocate a hard register that is available there for use as a temporary. For instructions that require a temporary register, you should use `scratch' instead of a pseudo-register because this will allow the combiner phase to add the `clobber' when required. You do this by coding (`clobber' (`match_scratch' ...)). If you do clobber a pseudo register, use one which appears nowhere else--generate a new one each time. Otherwise, you may confuse CSE. There is one other known use for clobbering a pseudo register in a `parallel': when one of the input operands of the insn is also clobbered by the insn. In this case, using the same pseudo register in the clobber and elsewhere in the insn produces the expected results. `(use X)' Represents the use of the value of X. It indicates that the value in X at this point in the program is needed, even though it may not be apparent why this is so. Therefore, the compiler will not attempt to delete previous instructions whose only effect is to store a value in X. X must be a `reg' expression. During the delayed branch scheduling phase, X may be an insn. This indicates that X previously was located at this place in the code and its data dependencies need to be taken into account. These `use' insns will be deleted before the delayed branch scheduling phase exits. `(parallel [X0 X1 ...])' Represents several side effects performed in parallel. The square brackets stand for a vector; the operand of `parallel' is a vector of expressions. X0, X1 and so on are individual side effect expressions--expressions of code `set', `call', `return', `clobber' or `use'. "In parallel" means that first all the values used in the individual side-effects are computed, and second all the actual side-effects are performed. For example, (parallel [(set (reg:SI 1) (mem:SI (reg:SI 1))) (set (mem:SI (reg:SI 1)) (reg:SI 1))]) says unambiguously that the values of hard register 1 and the memory location addressed by it are interchanged. In both places where `(reg:SI 1)' appears as a memory address it refers to the value in register 1 *before* the execution of the insn. It follows that it is *incorrect* to use `parallel' and expect the result of one `set' to be available for the next one. For example, people sometimes attempt to represent a jump-if-zero instruction this way: (parallel [(set (cc0) (reg:SI 34)) (set (pc) (if_then_else (eq (cc0) (const_int 0)) (label_ref ...) (pc)))]) But this is incorrect, because it says that the jump condition depends on the condition code value *before* this instruction, not on the new value that is set by this instruction. Peephole optimization, which takes place together with final assembly code output, can produce insns whose patterns consist of a `parallel' whose elements are the operands needed to output the resulting assembler code--often `reg', `mem' or constant expressions. This would not be well-formed RTL at any other stage in compilation, but it is ok then because no further optimization remains to be done. However, the definition of the macro `NOTICE_UPDATE_CC', if any, must deal with such insns if you define any peephole optimizations. `(sequence [INSNS ...])' Represents a sequence of insns. Each of the INSNS that appears in the vector is suitable for appearing in the chain of insns, so it must be an `insn', `jump_insn', `call_insn', `code_label', `barrier' or `note'. A `sequence' RTX is never placed in an actual insn during RTL generation. It represents the sequence of insns that result from a `define_expand' *before* those insns are passed to `emit_insn' to insert them in the chain of insns. When actually inserted, the individual sub-insns are separated out and the `sequence' is forgotten. After delay-slot scheduling is completed, an insn and all the insns that reside in its delay slots are grouped together into a `sequence'. The insn requiring the delay slot is the first insn in the vector; subsequent insns are to be placed in the delay slot. `INSN_ANNULLED_BRANCH_P' is set on an insn in a delay slot to indicate that a branch insn should be used that will conditionally annul the effect of the insns in the delay slots. In such a case, `INSN_FROM_TARGET_P' indicates that the insn is from the target of the branch and should be executed only if the branch is taken; otherwise the insn should be executed only if the branch is not taken. *Note Delay Slots::. These expression codes appear in place of a side effect, as the body of an insn, though strictly speaking they do not always describe side effects as such: `(asm_input S)' Represents literal assembler code as described by the string S. `(unspec [OPERANDS ...] INDEX)' `(unspec_volatile [OPERANDS ...] INDEX)' Represents a machine-specific operation on OPERANDS. INDEX selects between multiple machine-specific operations. `unspec_volatile' is used for volatile operations and operations that may trap; `unspec' is used for other operations. These codes may appear inside a `pattern' of an insn, inside a `parallel', or inside an expression. `(addr_vec:M [LR0 LR1 ...])' Represents a table of jump addresses. The vector elements LR0, etc., are `label_ref' expressions. The mode M specifies how much space is given to each address; normally M would be `Pmode'. `(addr_diff_vec:M BASE [LR0 LR1 ...])' Represents a table of jump addresses expressed as offsets from BASE. The vector elements LR0, etc., are `label_ref' expressions and so is BASE. The mode M specifies how much space is given to each address-difference. File: gcc.info, Node: Incdec, Next: Assembler, Prev: Side Effects, Up: RTL Embedded Side-Effects on Addresses ================================== Four special side-effect expression codes appear as memory addresses. `(pre_dec:M X)' Represents the side effect of decrementing X by a standard amount and represents also the value that X has after being decremented. x must be a `reg' or `mem', but most machines allow only a `reg'. m must be the machine mode for pointers on the machine in use. The amount X is decremented by is the length in bytes of the machine mode of the containing memory reference of which this expression serves as the address. Here is an example of its use: (mem:DF (pre_dec:SI (reg:SI 39))) This says to decrement pseudo register 39 by the length of a `DFmode' value and use the result to address a `DFmode' value. `(pre_inc:M X)' Similar, but specifies incrementing X instead of decrementing it. `(post_dec:M X)' Represents the same side effect as `pre_dec' but a different value. The value represented here is the value X has before being decremented. `(post_inc:M X)' Similar, but specifies incrementing X instead of decrementing it. These embedded side effect expressions must be used with care. Instruction patterns may not use them. Until the `flow' pass of the compiler, they may occur only to represent pushes onto the stack. The `flow' pass finds cases where registers are incremented or decremented in one instruction and used as an address shortly before or after; these cases are then transformed to use pre- or post-increment or -decrement. If a register used as the operand of these expressions is used in another address in an insn, the original value of the register is used. Uses of the register outside of an address are not permitted within the same insn as a use in an embedded side effect expression because such insns behave differently on different machines and hence must be treated as ambiguous and disallowed. An instruction that can be represented with an embedded side effect could also be represented using `parallel' containing an additional `set' to describe how the address register is altered. This is not done because machines that allow these operations at all typically allow them wherever a memory address is called for. Describing them as additional parallel stores would require doubling the number of entries in the machine description. File: gcc.info, Node: Assembler, Next: Insns, Prev: Incdec, Up: RTL Assembler Instructions as Expressions ===================================== The RTX code `asm_operands' represents a value produced by a user-specified assembler instruction. It is used to represent an `asm' statement with arguments. An `asm' statement with a single output operand, like this: asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z)); is represented using a single `asm_operands' RTX which represents the value that is stored in `outputvar': (set RTX-FOR-OUTPUTVAR (asm_operands "foo %1,%2,%0" "a" 0 [RTX-FOR-ADDITION-RESULT RTX-FOR-*Z] [(asm_input:M1 "g") (asm_input:M2 "di")])) Here the operands of the `asm_operands' RTX are the assembler template string, the output-operand's constraint, the index-number of the output operand among the output operands specified, a vector of input operand RTX's, and a vector of input-operand modes and constraints. The mode M1 is the mode of the sum `x+y'; M2 is that of `*z'. When an `asm' statement has multiple output values, its insn has several such `set' RTX's inside of a `parallel'. Each `set' contains a `asm_operands'; all of these share the same assembler template and vectors, but each contains the constraint for the respective output operand. They are also distinguished by the output-operand index number, which is 0, 1, ... for successive output operands.