This is doc/gccint.info, produced by makeinfo version 4.5 from doc/gccint.texi. INFO-DIR-SECTION Programming START-INFO-DIR-ENTRY * gccint: (gccint). Internals of the GNU Compiler Collection. END-INFO-DIR-ENTRY This file documents the internals of the GNU compilers. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.  File: gccint.info, Node: RTL Classes, Next: Accessors, Prev: RTL Objects, Up: RTL RTL Classes and Formats ======================= The various expression codes are divided into several "classes", which are represented by single characters. You can determine the class of an RTX code with the macro `GET_RTX_CLASS (CODE)'. Currently, `rtx.def' defines these classes: `o' An RTX code that represents an actual object, such as a register (`REG') or a memory location (`MEM', `SYMBOL_REF'). Constants and basic transforms on objects (`ADDRESSOF', `HIGH', `LO_SUM') are also included. Note that `SUBREG' and `STRICT_LOW_PART' are not in this class, but in class `x'. `<' An RTX code for a comparison, such as `NE' or `LT'. `1' An RTX code for a unary arithmetic operation, such as `NEG', `NOT', or `ABS'. This category also includes value extension (sign or zero) and conversions between integer and floating point. `c' An RTX code for a commutative binary operation, such as `PLUS' or `AND'. `NE' and `EQ' are comparisons, so they have class `<'. `2' An RTX code for a non-commutative binary operation, such as `MINUS', `DIV', or `ASHIFTRT'. `b' An RTX code for a bit-field operation. Currently only `ZERO_EXTRACT' and `SIGN_EXTRACT'. These have three inputs and are lvalues (so they can be used for insertion as well). *Note Bit-Fields::. `3' An RTX code for other three input operations. Currently only `IF_THEN_ELSE'. `i' An RTX code for an entire instruction: `INSN', `JUMP_INSN', and `CALL_INSN'. *Note Insns::. `m' An RTX code for something that matches in insns, such as `MATCH_DUP'. These only occur in machine descriptions. `a' An RTX code for an auto-increment addressing mode, such as `POST_INC'. `x' All other RTX codes. This category includes the remaining codes used only in machine descriptions (`DEFINE_*', etc.). It also includes all the codes describing side effects (`SET', `USE', `CLOBBER', etc.) and the non-insns that may appear on an insn chain, such as `NOTE', `BARRIER', and `CODE_LABEL'. For each expression code, `rtl.def' specifies the number of contained objects and their kinds using a sequence of characters called the "format" of the expression code. For example, the format of `subreg' is `ei'. These are the most commonly used format characters: `e' An expression (actually a pointer to an expression). `i' An integer. `w' A wide integer. `s' A string. `E' A vector of expressions. A few other format characters are used occasionally: `u' `u' is equivalent to `e' except that it is printed differently in debugging dumps. It is used for pointers to insns. `n' `n' is equivalent to `i' except that it is printed differently in debugging dumps. It is used for the line number or code number of a `note' insn. `S' `S' indicates a string which is optional. In the RTL objects in core, `S' is equivalent to `s', but when the object is read, from an `md' file, the string value of this operand may be omitted. An omitted string is taken to be the null string. `V' `V' indicates a vector which is optional. In the RTL objects in core, `V' is equivalent to `E', but when the object is read from an `md' file, the vector value of this operand may be omitted. An omitted vector is effectively the same as a vector of no elements. `0' `0' means a slot whose contents do not fit any normal category. `0' slots are not printed at all in dumps, and are often used in special ways by small parts of the compiler. There are macros to get the number of operands and the format of an expression code: `GET_RTX_LENGTH (CODE)' Number of operands of an RTX of code CODE. `GET_RTX_FORMAT (CODE)' The format of an RTX of code CODE, as a C string. Some classes of RTX codes always have the same format. For example, it is safe to assume that all comparison operations have format `ee'. `1' All codes of this class have format `e'. `<' `c' `2' All codes of these classes have format `ee'. `b' `3' All codes of these classes have format `eee'. `i' All codes of this class have formats that begin with `iuueiee'. *Note Insns::. Note that not all RTL objects linked onto an insn chain are of class `i'. `o' `m' `x' You can make no assumptions about the format of these codes.  File: gccint.info, Node: Accessors, Next: Flags, Prev: RTL Classes, Up: RTL Access to Operands ================== Operands of expressions are accessed using the macros `XEXP', `XINT', `XWINT' and `XSTR'. Each of these macros takes two arguments: an expression-pointer (RTX) and an operand number (counting from zero). Thus, XEXP (X, 2) accesses operand 2 of expression X, as an expression. XINT (X, 2) accesses the same operand as an integer. `XSTR', used in the same fashion, would access it as a string. Any operand can be accessed as an integer, as an expression or as a string. You must choose the correct method of access for the kind of value actually stored in the operand. You would do this based on the expression code of the containing expression. That is also how you would know how many operands there are. For example, if X is a `subreg' expression, you know that it has two operands which can be correctly accessed as `XEXP (X, 0)' and `XINT (X, 1)'. If you did `XINT (X, 0)', you would get the address of the expression operand but cast as an integer; that might occasionally be useful, but it would be cleaner to write `(int) XEXP (X, 0)'. `XEXP (X, 1)' would also compile without error, and would return the second, integer operand cast as an expression pointer, which would probably result in a crash when accessed. Nothing stops you from writing `XEXP (X, 28)' either, but this will access memory past the end of the expression with unpredictable results. Access to operands which are vectors is more complicated. You can use the macro `XVEC' to get the vector-pointer itself, or the macros `XVECEXP' and `XVECLEN' to access the elements and length of a vector. `XVEC (EXP, IDX)' Access the vector-pointer which is operand number IDX in EXP. `XVECLEN (EXP, IDX)' Access the length (number of elements) in the vector which is in operand number IDX in EXP. This value is an `int'. `XVECEXP (EXP, IDX, ELTNUM)' Access element number ELTNUM in the vector which is in operand number IDX in EXP. This value is an RTX. It is up to you to make sure that ELTNUM is not negative and is less than `XVECLEN (EXP, IDX)'. All the macros defined in this section expand into lvalues and therefore can be used to assign the operands, lengths and vector elements as well as to access them.  File: gccint.info, Node: Flags, Next: Machine Modes, Prev: Accessors, Up: RTL Flags in an RTL Expression ========================== RTL expressions contain several flags (one-bit bit-fields) that are used in certain types of expression. Most often they are accessed with the following macros, which expand into lvalues: `CONSTANT_POOL_ADDRESS_P (X)' Nonzero in a `symbol_ref' if it refers to part of the current function's constant pool. For most targets these addresses are in a `.rodata' section entirely separate from the function, but for some targets the addresses are close to the beginning of the function. In either case GCC assumes these addresses can be addressed directly, perhaps with the help of base registers. Stored in the `unchanging' field and printed as `/u'. `CONST_OR_PURE_CALL_P (X)' In a `call_insn', `note', or an `expr_list' for notes, indicates that the insn represents a call to a const or pure function. Stored in the `unchanging' field and printed as `/u'. `INSN_ANNULLED_BRANCH_P (X)' In an `insn' in the delay slot of a branch insn, indicates that an annulling branch should be used. See the discussion under `sequence' below. Stored in the `unchanging' field and printed as `/u'. `INSN_DEAD_CODE_P (X)' In an `insn' during the dead-code elimination pass, nonzero if the insn is dead. Stored in the `in_struct' field and printed as `/s'. `INSN_DELETED_P (X)' In an `insn', nonzero if the insn has been deleted. Stored in the `volatil' field and printed as `/v'. `INSN_FROM_TARGET_P (X)' In an `insn' in a delay slot of a branch, indicates that the insn is from the target of the branch. If the branch insn has `INSN_ANNULLED_BRANCH_P' set, this insn will only be executed if the branch is taken. For annulled branches with `INSN_FROM_TARGET_P' clear, the insn will be executed only if the branch is not taken. When `INSN_ANNULLED_BRANCH_P' is not set, this insn will always be executed. Stored in the `in_struct' field and printed as `/s'. `LABEL_OUTSIDE_LOOP_P (X)' In `label_ref' expressions, nonzero if this is a reference to a label that is outside the innermost loop containing the reference to the label. Stored in the `in_struct' field and printed as `/s'. `LABEL_PRESERVE_P (X)' In a `code_label', indicates that the label is referenced by code or data not visible to the RTL of a given function. Labels referenced by a non-local goto will have this bit set. Stored in the `in_struct' field and printed as `/s'. `LABEL_REF_NONLOCAL_P (X)' In `label_ref' and `reg_label' expressions, nonzero if this is a reference to a non-local label. Stored in the `volatil' field and printed as `/v'. `LINK_COST_FREE (X)' In the `LOG_LINKS' `insn_list' during scheduling, nonzero when the cost of executing an instruction through the link is zero, i.e., the link makes the cost free. Stored in the `call' field and printed as `/c'. `LINK_COST_ZERO (X)' In the `LOG_LINKS' `insn_list' during scheduling, nonzero when the cost of executing an instruction through the link varies and is unchanged, i.e., the link has zero additional cost. Stored in the `jump' field and printed as `/j'. `MEM_IN_STRUCT_P (X)' In `mem' expressions, nonzero for reference to an entire structure, union or array, or to a component of one. Zero for references to a scalar variable or through a pointer to a scalar. If both this flag and `MEM_SCALAR_P' are clear, then we don't know whether this `mem' is in a structure or not. Both flags should never be simultaneously set. Stored in the `in_struct' field and printed as `/s'. `MEM_KEEP_ALIAS_SET_P (X)' In `mem' expressions, 1 if we should keep the alias set for this mem unchanged when we access a component. Set to 1, for example, when we are already in a non-addressable component of an aggregate. Stored in the `jump' field and printed as `/j'. `MEM_SCALAR_P (X)' In `mem' expressions, nonzero for reference to a scalar known not to be a member of a structure, union, or array. Zero for such references and for indirections through pointers, even pointers pointing to scalar types. If both this flag and `MEM_IN_STRUCT_P' are clear, then we don't know whether this `mem' is in a structure or not. Both flags should never be simultaneously set. Stored in the `frame_related' field and printed as `/f'. `MEM_VOLATILE_P (X)' In `mem' and `asm_operands' expressions, nonzero for volatile memory references. Stored in the `volatil' field and printed as `/v'. `REG_FUNCTION_VALUE_P (X)' Nonzero in a `reg' if it is the place in which this function's value is going to be returned. (This happens only in a hard register.) Stored in the `integrated' field and printed as `/i'. `REG_LOOP_TEST_P (X)' In `reg' expressions, nonzero if this register's entire life is contained in the exit test code for some loop. Stored in the `in_struct' field and printed as `/s'. `REG_POINTER (X)' Nonzero in a `reg' if the register holds a pointer. Stored in the `frame_related' field and printed as `/f'. `REG_USERVAR_P (X)' In a `reg', nonzero if it corresponds to a variable present in the user's source code. Zero for temporaries generated internally by the compiler. Stored in the `volatil' field and printed as `/v'. The same hard register may be used also for collecting the values of functions called by this one, but `REG_FUNCTION_VALUE_P' is zero in this kind of use. `RTX_FRAME_RELATED_P (X)' Nonzero in an `insn' or `set' which is part of a function prologue and sets the stack pointer, sets the frame pointer, or saves a register. This flag should also be set on an instruction that sets up a temporary register to use in place of the frame pointer. Stored in the `frame_related' field and printed as `/f'. In particular, on RISC targets where there are limits on the sizes of immediate constants, it is sometimes impossible to reach the register save area directly from the stack pointer. In that case, a temporary register is used that is near enough to the register save area, and the Canonical Frame Address, i.e., DWARF2's logical frame pointer, register must (temporarily) be changed to be this temporary register. So, the instruction that sets this temporary register must be marked as `RTX_FRAME_RELATED_P'. If the marked instruction is overly complex (defined in terms of what `dwarf2out_frame_debug_expr' can handle), you will also have to create a `REG_FRAME_RELATED_EXPR' note and attach it to the instruction. This note should contain a simple expression of the computation performed by this instruction, i.e., one that `dwarf2out_frame_debug_expr' can handle. This flag is required for exception handling support on targets with RTL prologues. `RTX_INTEGRATED_P (X)' Nonzero in an `insn', `insn_list', or `const' if it resulted from an in-line function call. Stored in the `integrated' field and printed as `/i'. `RTX_UNCHANGING_P (X)' Nonzero in a `reg' or `mem' if the memory is set at most once, anywhere. This does not mean that it is function invariant. Stored in the `unchanging' field and printed as `/u'. `SCHED_GROUP_P (X)' During instruction scheduling, in an `insn', indicates that the previous insn must be scheduled together with this insn. This is used to ensure that certain groups of instructions will not be split up by the instruction scheduling pass, for example, `use' insns before a `call_insn' may not be separated from the `call_insn'. Stored in the `in_struct' field and printed as `/s'. `SET_IS_RETURN_P (X)' For a `set', nonzero if it is for a return. Stored in the `jump' field and printed as `/j'. `SIBLING_CALL_P (X)' For a `call_insn', nonzero if the insn is a sibling call. Stored in the `jump' field and printed as `/j'. `STRING_POOL_ADDRESS_P (X)' For a `symbol_ref' expression, nonzero if it addresses this function's string constant pool. Stored in the `frame_related' field and printed as `/f'. `SUBREG_PROMOTED_UNSIGNED_P (X)' Nonzero in a `subreg' that has `SUBREG_PROMOTED_VAR_P' nonzero if the object being referenced is kept zero-extended and zero if it is kept sign-extended. Stored in the `unchanging' field and printed as `/u'. `SUBREG_PROMOTED_VAR_P (X)' Nonzero in a `subreg' if it was made when accessing an object that was promoted to a wider mode in accord with the `PROMOTED_MODE' machine description macro (*note Storage Layout::). In this case, the mode of the `subreg' is the declared mode of the object and the mode of `SUBREG_REG' is the mode of the register that holds the object. Promoted variables are always either sign- or zero-extended to the wider mode on every assignment. Stored in the `in_struct' field and printed as `/s'. `SYMBOL_REF_FLAG (X)' In a `symbol_ref', this is used as a flag for machine-specific purposes. Stored in the `volatil' field and printed as `/v'. `SYMBOL_REF_USED (X)' In a `symbol_ref', indicates that X has been used. This is normally only used to ensure that X is only declared external once. Stored in the `used' field. `SYMBOL_REF_WEAK (X)' In a `symbol_ref', indicates that X has been declared weak. Stored in the `integrated' field and printed as `/i'. These are the fields to which the above macros refer: `call' In the `LOG_LINKS' of an `insn_list' during scheduling, 1 means that the cost of executing an instruction through the link is zero. In an RTL dump, this flag is represented as `/c'. `frame_related' In an `insn' or `set' expression, 1 means that it is part of a function prologue and sets the stack pointer, sets the frame pointer, saves a register, or sets up a temporary register to use in place of the frame pointer. In `reg' expressions, 1 means that the register holds a pointer. In `symbol_ref' expressions, 1 means that the reference addresses this function's string constant pool. In `mem' expressions, 1 means that the reference is to a scalar. In an RTL dump, this flag is represented as `/f'. `in_struct' In `mem' expressions, it is 1 if the memory datum referred to is all or part of a structure or array; 0 if it is (or might be) a scalar variable. A reference through a C pointer has 0 because the pointer might point to a scalar variable. This information allows the compiler to determine something about possible cases of aliasing. In `reg' expressions, it is 1 if the register has its entire life contained within the test expression of some loop. In `subreg' expressions, 1 means that the `subreg' is accessing an object that has had its mode promoted from a wider mode. In `label_ref' expressions, 1 means that the referenced label is outside the innermost loop containing the insn in which the `label_ref' was found. In `code_label' expressions, it is 1 if the label may never be deleted. This is used for labels which are the target of non-local gotos. Such a label that would have been deleted is replaced with a `note' of type `NOTE_INSN_DELETED_LABEL'. In an `insn' during dead-code elimination, 1 means that the insn is dead code. In an `insn' during reorg for an insn in the delay slot of a branch, 1 means that this insn is from the target of the branch. In an `insn' during instruction scheduling, 1 means that this insn must be scheduled as part of a group together with the previous insn. In an RTL dump, this flag is represented as `/s'. `integrated' In an `insn', `insn_list', or `const', 1 means the RTL was produced by procedure integration. In `reg' expressions, 1 means the register contains the value to be returned by the current function. On machines that pass parameters in registers, the same register number may be used for parameters as well, but this flag is not set on such uses. In `symbol_ref' expressions, 1 means the referenced symbol is weak. In an RTL dump, this flag is represented as `/i'. `jump' In a `mem' expression, 1 means we should keep the alias set for this mem unchanged when we access a component. In a `set', 1 means it is for a return. In a `call_insn', 1 means it is a sibling call. In the `LOG_LINKS' of an `insn_list' during scheduling, 1 means the cost of executing an instruction through the link varies and is unchanging. In an RTL dump, this flag is represented as `/j'. `unchanging' In `reg' and `mem' expressions, 1 means that the value of the expression never changes. In `subreg' expressions, it is 1 if the `subreg' references an unsigned object whose mode has been promoted to a wider mode. In an `insn', 1 means that this is an annulling branch. In a `symbol_ref' expression, 1 means that this symbol addresses something in the per-function constant pool. In a `call_insn', `note', or an `expr_list' of notes, 1 means that this instruction is a call to a const or pure function. In an RTL dump, this flag is represented as `/u'. `used' This flag is used directly (without an access macro) at the end of RTL generation for a function, to count the number of times an expression appears in insns. Expressions that appear more than once are copied, according to the rules for shared structure (*note Sharing::). For a `reg', it is used directly (without an access macro) by the leaf register renumbering code to ensure that each register is only renumbered once. In a `symbol_ref', it indicates that an external declaration for the symbol has already been written. `volatil' In a `mem' or `asm_operands' expression, it is 1 if the memory reference is volatile. Volatile memory references may not be deleted, reordered or combined. In a `symbol_ref' expression, it is used for machine-specific purposes. In a `reg' expression, it is 1 if the value is a user-level variable. 0 indicates an internal compiler temporary. In an `insn', 1 means the insn has been deleted. In `label_ref' and `reg_label' expressions, 1 means a reference to a non-local label. In an RTL dump, this flag is represented as `/v'.  File: gccint.info, Node: Machine Modes, Next: Constants, Prev: Flags, Up: RTL Machine Modes ============= A machine mode describes a size of data object and the representation used for it. In the C code, machine modes are represented by an enumeration type, `enum machine_mode', defined in `machmode.def'. Each RTL expression has room for a machine mode and so do certain kinds of tree expressions (declarations and types, to be precise). In debugging dumps and machine descriptions, the machine mode of an RTL expression is written after the expression code with a colon to separate them. The letters `mode' which appear at the end of each machine mode name are omitted. For example, `(reg:SI 38)' is a `reg' expression with machine mode `SImode'. If the mode is `VOIDmode', it is not written at all. Here is a table of machine modes. The term "byte" below refers to an object of `BITS_PER_UNIT' bits (*note Storage Layout::). `BImode' "Bit" mode represents a single bit, for predicate registers. `QImode' "Quarter-Integer" mode represents a single byte treated as an integer. `HImode' "Half-Integer" mode represents a two-byte integer. `PSImode' "Partial Single Integer" mode represents an integer which occupies four bytes but which doesn't really use all four. On some machines, this is the right mode to use for pointers. `SImode' "Single Integer" mode represents a four-byte integer. `PDImode' "Partial Double Integer" mode represents an integer which occupies eight bytes but which doesn't really use all eight. On some machines, this is the right mode to use for certain pointers. `DImode' "Double Integer" mode represents an eight-byte integer. `TImode' "Tetra Integer" (?) mode represents a sixteen-byte integer. `OImode' "Octa Integer" (?) mode represents a thirty-two-byte integer. `QFmode' "Quarter-Floating" mode represents a quarter-precision (single byte) floating point number. `HFmode' "Half-Floating" mode represents a half-precision (two byte) floating point number. `TQFmode' "Three-Quarter-Floating" (?) mode represents a three-quarter-precision (three byte) floating point number. `SFmode' "Single Floating" mode represents a four byte floating point number. In the common case, of a processor with IEEE arithmetic and 8-bit bytes, this is a single-precision IEEE floating point number; it can also be used for double-precision (on processors with 16-bit bytes) and single-precision VAX and IBM types. `DFmode' "Double Floating" mode represents an eight byte floating point number. In the common case, of a processor with IEEE arithmetic and 8-bit bytes, this is a double-precision IEEE floating point number. `XFmode' "Extended Floating" mode represents a twelve byte floating point number. This mode is used for IEEE extended floating point. On some systems not all bits within these bytes will actually be used. `TFmode' "Tetra Floating" mode represents a sixteen byte floating point number. This gets used for both the 96-bit extended IEEE floating-point types padded to 128 bits, and true 128-bit extended IEEE floating-point types. `CCmode' "Condition Code" mode represents the value of a condition code, which is a machine-specific set of bits used to represent the result of a comparison operation. Other machine-specific modes may also be used for the condition code. These modes are not used on machines that use `cc0' (see *note Condition Code::). `BLKmode' "Block" mode represents values that are aggregates to which none of the other modes apply. In RTL, only memory references can have this mode, and only if they appear in string-move or vector instructions. On machines which have no such instructions, `BLKmode' will not appear in RTL. `VOIDmode' Void mode means the absence of a mode or an unspecified mode. For example, RTL expressions of code `const_int' have mode `VOIDmode' because they can be taken to have whatever mode the context requires. In debugging dumps of RTL, `VOIDmode' is expressed by the absence of any mode. `QCmode, HCmode, SCmode, DCmode, XCmode, TCmode' These modes stand for a complex number represented as a pair of floating point values. The floating point values are in `QFmode', `HFmode', `SFmode', `DFmode', `XFmode', and `TFmode', respectively. `CQImode, CHImode, CSImode, CDImode, CTImode, COImode' These modes stand for a complex number represented as a pair of integer values. The integer values are in `QImode', `HImode', `SImode', `DImode', `TImode', and `OImode', respectively. The machine description defines `Pmode' as a C macro which expands into the machine mode used for addresses. Normally this is the mode whose size is `BITS_PER_WORD', `SImode' on 32-bit machines. The only modes which a machine description must support are `QImode', and the modes corresponding to `BITS_PER_WORD', `FLOAT_TYPE_SIZE' and `DOUBLE_TYPE_SIZE'. The compiler will attempt to use `DImode' for 8-byte structures and unions, but this can be prevented by overriding the definition of `MAX_FIXED_MODE_SIZE'. Alternatively, you can have the compiler use `TImode' for 16-byte structures and unions. Likewise, you can arrange for the C type `short int' to avoid using `HImode'. Very few explicit references to machine modes remain in the compiler and these few references will soon be removed. Instead, the machine modes are divided into mode classes. These are represented by the enumeration type `enum mode_class' defined in `machmode.h'. The possible mode classes are: `MODE_INT' Integer modes. By default these are `BImode', `QImode', `HImode', `SImode', `DImode', `TImode', and `OImode'. `MODE_PARTIAL_INT' The "partial integer" modes, `PQImode', `PHImode', `PSImode' and `PDImode'. `MODE_FLOAT' Floating point modes. By default these are `QFmode', `HFmode', `TQFmode', `SFmode', `DFmode', `XFmode' and `TFmode'. `MODE_COMPLEX_INT' Complex integer modes. (These are not currently implemented). `MODE_COMPLEX_FLOAT' Complex floating point modes. By default these are `QCmode', `HCmode', `SCmode', `DCmode', `XCmode', and `TCmode'. `MODE_FUNCTION' Algol or Pascal function variables including a static chain. (These are not currently implemented). `MODE_CC' Modes representing condition code values. These are `CCmode' plus any modes listed in the `EXTRA_CC_MODES' macro. *Note Jump Patterns::, also see *Note Condition Code::. `MODE_RANDOM' This is a catchall mode class for modes which don't fit into the above classes. Currently `VOIDmode' and `BLKmode' are in `MODE_RANDOM'. Here are some C macros that relate to machine modes: `GET_MODE (X)' Returns the machine mode of the RTX X. `PUT_MODE (X, NEWMODE)' Alters the machine mode of the RTX X to be NEWMODE. `NUM_MACHINE_MODES' Stands for the number of machine modes available on the target machine. This is one greater than the largest numeric value of any machine mode. `GET_MODE_NAME (M)' Returns the name of mode M as a string. `GET_MODE_CLASS (M)' Returns the mode class of mode M. `GET_MODE_WIDER_MODE (M)' Returns the next wider natural mode. For example, the expression `GET_MODE_WIDER_MODE (QImode)' returns `HImode'. `GET_MODE_SIZE (M)' Returns the size in bytes of a datum of mode M. `GET_MODE_BITSIZE (M)' Returns the size in bits of a datum of mode M. `GET_MODE_MASK (M)' Returns a bitmask containing 1 for all bits in a word that fit within mode M. This macro can only be used for modes whose bitsize is less than or equal to `HOST_BITS_PER_INT'. `GET_MODE_ALIGNMENT (M)' Return the required alignment, in bits, for an object of mode M. `GET_MODE_UNIT_SIZE (M)' Returns the size in bytes of the subunits of a datum of mode M. This is the same as `GET_MODE_SIZE' except in the case of complex modes. For them, the unit size is the size of the real or imaginary part. `GET_MODE_NUNITS (M)' Returns the number of units contained in a mode, i.e., `GET_MODE_SIZE' divided by `GET_MODE_UNIT_SIZE'. `GET_CLASS_NARROWEST_MODE (C)' Returns the narrowest mode in mode class C. The global variables `byte_mode' and `word_mode' contain modes whose classes are `MODE_INT' and whose bitsizes are either `BITS_PER_UNIT' or `BITS_PER_WORD', respectively. On 32-bit machines, these are `QImode' and `SImode', respectively.  File: gccint.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 (GCC does not provide a mechanism to represent even larger constants). In the latter case, M will be `VOIDmode'. `(const_vector:M [X0 X1 ...])' Represents a vector constant. The square brackets stand for the vector containing the constant elements. X0, X1 and so on are the `const_int' or `const_double' elements. The number of units in a `const_vector' is obtained with the macro `CONST_VECTOR_NUNITS' as in `CONST_VECTOR_NUNITS (V)'. Individual elements in a vector constant are accessed with the macro `CONST_VECTOR_ELT' as in `CONST_VECTOR_ELT (V, N)' where V is the vector constant and N is the element desired. 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'. If mode MODE is of mode class `MODE_FLOAT', it returns a `CONST_DOUBLE' expression in mode MODE. Otherwise, it returns a `CONST_VECTOR' 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'. The `CONST1_RTX' and `CONST2_RTX' macros are undefined for vector modes. `(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' or a `note' of type `NOTE_INSN_DELETED_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: gccint.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 BYTENUM)' `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-part `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, BYTENUM 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 BYTENUM offset 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 byte number zero is part of the most significant word; otherwise, it is part of the least significant word. The compilation parameter `BYTES_BIG_ENDIAN', if set to 1, says that byte number zero is the most significant byte within a word; otherwise, it is the least significant byte within a word. On a few targets, `FLOAT_WORDS_BIG_ENDIAN' disagrees with `WORDS_BIG_ENDIAN'. However, most parts of the compiler treat floating point values as if they had the same endianness as integer values. This works because they handle them solely as a collection of integer values, with no particular numerical value. Only real.c and the runtime libraries care about `FLOAT_WORDS_BIG_ENDIAN'. 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_BYTE' 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 ALIAS)' 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. ALIAS specifies an alias set for the reference. In general two items are in different alias sets if they cannot reference the same memory address. `(addressof:M REG)' This RTX represents a request for the address of register REG. Its mode is always `Pmode'. If there are any `addressof' expressions left in the function after CSE, REG is forced into the stack and the `addressof' expression is replaced with a `plus' expression for the address of its stack slot.