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GNU Info File | 1993-10-21 | 48.4 KB | 1,053 lines

This is Info file gcc.info, produced by Makeinfo-1.54 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Copyright (C) 1988, 1989, 1992, 1993 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" 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" 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: VMS Misc, Prev: Global Declarations, Up: VMS Other VMS Issues ================ GNU CC automatically arranges for `main' to return 1 by default if you fail to specify an explicit return value. This will be interpreted by VMS as a status code indicating a normal successful completion. Version 1 of GNU CC did not provide this default. GNU CC on VMS works only with the GNU assembler, GAS. You need version 1.37 or later of GAS in order to produce value debugging information for the VMS debugger. Use the ordinary VMS linker with the object files produced by GAS. Under previous versions of GNU CC, the generated code would occasionally give strange results when linked to the sharable `VAXCRTL' library. Now this should work. A caveat for use of `const' global variables: the `const' modifier must be specified in every external declaration of the variable in all of the source files that use that variable. Otherwise the linker will issue warnings about conflicting attributes for the variable. Your program will still work despite the warnings, but the variable will be placed in writable storage. Although the VMS linker does distinguish between upper and lower case letters in global symbols, most VMS compilers convert all such symbols into upper case and most run-time library routines also have upper case names. To be able to reliably call such routines, GNU CC (by means of the assembler GAS) converts global symbols into upper case like other VMS compilers. However, since the usual practice in C is to distinguish case, GNU CC (via GAS) tries to preserve usual C behavior by augmenting each name that is not all lower case. This means truncating the name to at most 23 characters and then adding more characters at the end which encode the case pattern of those 23. Names which contain at least one dollar sign are an exception; they are converted directly into upper case without augmentation. Name augmentation yields bad results for programs that use precompiled libraries (such as Xlib) which were generated by another compiler. You can use the compiler option `/NOCASE_HACK' to inhibit augmentation; it makes external C functions and variables case-independent as is usual on VMS. Alternatively, you could write all references to the functions and variables in such libraries using lower case; this will work on VMS, but is not portable to other systems. The compiler option `/NAMES' also provides control over global name handling. Function and variable names are handled somewhat differently with GNU C++. The GNU C++ compiler performs "name mangling" on function names, which means that it adds information to the function name to describe the data types of the arguments that the function takes. One result of this is that the name of a function can become very long. Since the VMS linker only recognizes the first 31 characters in a name, special action is taken to ensure that each function and variable has a unique name that can be represented in 31 characters. If the name (plus a name augmentation, if required) is less than 32 characters in length, then no special action is performed. If the name is longer than 31 characters, the assembler (GAS) will generate a hash string based upon the function name, truncate the function name to 23 characters, and append the hash string to the truncated name. If the `/VERBOSE' compiler option is used, the assembler will print both the full and truncated names of each symbol that is truncated. The `/NOCASE_HACK' compiler option should not be used when you are compiling programs that use libg++. libg++ has several instances of objects (i.e. `Filebuf' and `filebuf') which become indistinguishable in a case-insensitive environment. This leads to cases where you need to inhibit augmentation selectively (if you were using libg++ and Xlib in the same program, for example). There is no special feature for doing this, but you can get the result by defining a macro for each mixed case symbol for which you wish to inhibit augmentation. The macro should expand into the lower case equivalent of itself. For example: #define StuDlyCapS studlycaps These macro definitions can be placed in a header file to minimize the number of changes to your source code.  File: gcc.info, Node: Portability, Next: Interface, Prev: VMS, Up: Top GNU CC and Portability ********************** The main goal of GNU CC was to make a good, fast compiler for machines in the class that the GNU system aims to run on: 32-bit machines that address 8-bit bytes and have several general registers. Elegance, theoretical power and simplicity are only secondary. GNU CC gets most of the information about the target machine from a machine description which gives an algebraic formula for each of the machine's instructions. This is a very clean way to describe the target. But when the compiler needs information that is difficult to express in this fashion, I have not hesitated to define an ad-hoc parameter to the machine description. The purpose of portability is to reduce the total work needed on the compiler; it was not of interest for its own sake. GNU CC does not contain machine dependent code, but it does contain code that depends on machine parameters such as endianness (whether the most significant byte has the highest or lowest address of the bytes in a word) and the availability of autoincrement addressing. In the RTL-generation pass, it is often necessary to have multiple strategies for generating code for a particular kind of syntax tree, strategies that are usable for different combinations of parameters. Often I have not tried to address all possible cases, but only the common ones or only the ones that I have encountered. As a result, a new target may require additional strategies. You will know if this happens because the compiler will call `abort'. Fortunately, the new strategies can be added in a machine-independent fashion, and will affect only the target machines that need them.  File: gcc.info, Node: Interface, Next: Passes, Prev: Portability, Up: Top Interfacing to GNU CC Output **************************** GNU CC is normally configured to use the same function calling convention normally in use on the target system. This is done with the machine-description macros described (*note Target Macros::.). However, returning of structure and union values is done differently on some target machines. As a result, functions compiled with PCC returning such types cannot be called from code compiled with GNU CC, and vice versa. This does not cause trouble often because few Unix library routines return structures or unions. GNU CC code returns structures and unions that are 1, 2, 4 or 8 bytes long in the same registers used for `int' or `double' return values. (GNU CC typically allocates variables of such types in registers also.) Structures and unions of other sizes are returned by storing them into an address passed by the caller (usually in a register). The machine-description macros `STRUCT_VALUE' and `STRUCT_INCOMING_VALUE' tell GNU CC where to pass this address. By contrast, PCC on most target machines returns structures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted. This is slower than the method used by GNU CC, and fails to be reentrant. On some target machines, such as RISC machines and the 80386, the standard system convention is to pass to the subroutine the address of where to return the value. On these machines, GNU CC has been configured to be compatible with the standard compiler, when this method is used. It may not be compatible for structures of 1, 2, 4 or 8 bytes. GNU CC uses the system's standard convention for passing arguments. On some machines, the first few arguments are passed in registers; in others, all are passed on the stack. It would be possible to use registers for argument passing on any machine, and this would probably result in a significant speedup. But the result would be complete incompatibility with code that follows the standard convention. So this change is practical only if you are switching to GNU CC as the sole C compiler for the system. We may implement register argument passing on certain machines once we have a complete GNU system so that we can compile the libraries with GNU CC. On some machines (particularly the Sparc), certain types of arguments are passed "by invisible reference". This means that the value is stored in memory, and the address of the memory location is passed to the subroutine. If you use `longjmp', beware of automatic variables. ANSI C says that automatic variables that are not declared `volatile' have undefined values after a `longjmp'. And this is all GNU CC promises to do, because it is very difficult to restore register variables correctly, and one of GNU CC's features is that it can put variables in registers without your asking it to. If you want a variable to be unaltered by `longjmp', and you don't want to write `volatile' because old C compilers don't accept it, just take the address of the variable. If a variable's address is ever taken, even if just to compute it and ignore it, then the variable cannot go in a register: { int careful; &careful; ... } Code compiled with GNU CC may call certain library routines. Most of them handle arithmetic for which there are no instructions. This includes multiply and divide on some machines, and floating point operations on any machine for which floating point support is disabled with `-msoft-float'. Some standard parts of the C library, such as `bcopy' or `memcpy', are also called automatically. The usual function call interface is used for calling the library routines. These library routines should be defined in the library `libgcc.a', which GNU CC automatically searches whenever it links a program. On machines that have multiply and divide instructions, if hardware floating point is in use, normally `libgcc.a' is not needed, but it is searched just in case. Each arithmetic function is defined in `libgcc1.c' to use the corresponding C arithmetic operator. As long as the file is compiled with another C compiler, which supports all the C arithmetic operators, this file will work portably. However, `libgcc1.c' does not work if compiled with GNU CC, because each arithmetic function would compile into a call to itself!  File: gcc.info, Node: Passes, Next: RTL, Prev: Interface, Up: Top Passes and Files of the Compiler ******************************** The overall control structure of the compiler is in `toplev.c'. This file is responsible for initialization, decoding arguments, opening and closing files, and sequencing the passes. The parsing pass is invoked only once, to parse the entire input. The RTL intermediate code for a function is generated as the function is parsed, a statement at a time. Each statement is read in as a syntax tree and then converted to RTL; then the storage for the tree for the statement is reclaimed. Storage for types (and the expressions for their sizes), declarations, and a representation of the binding contours and how they nest, remain until the function is finished being compiled; these are all needed to output the debugging information. Each time the parsing pass reads a complete function definition or top-level declaration, it calls either the function `rest_of_compilation', or the function `rest_of_decl_compilation' in `toplev.c', which are responsible for all further processing necessary, ending with output of the assembler language. All other compiler passes run, in sequence, within `rest_of_compilation'. When that function returns from compiling a function definition, the storage used for that function definition's compilation is entirely freed, unless it is an inline function (*note An Inline Function is As Fast As a Macro: Inline.). Here is a list of all the passes of the compiler and their source files. Also included is a description of where debugging dumps can be requested with `-d' options. * Parsing. This pass reads the entire text of a function definition, constructing partial syntax trees. This and RTL generation are no longer truly separate passes (formerly they were), but it is easier to think of them as separate. The tree representation does not entirely follow C syntax, because it is intended to support other languages as well. Language-specific data type analysis is also done in this pass, and every tree node that represents an expression has a data type attached. Variables are represented as declaration nodes. Constant folding and some arithmetic simplifications are also done during this pass. The language-independent source files for parsing are `stor-layout.c', `fold-const.c', and `tree.c'. There are also header files `tree.h' and `tree.def' which define the format of the tree representation. The source files to parse C are `c-parse.y', `c-decl.c', `c-typeck.c', `c-aux-info.c', `c-convert.c', and `c-lang.c' along with header files `c-lex.h', and `c-tree.h'. The source files for parsing C++ are `cp-parse.y', `cp-class.c', `cp-cvt.c', `cp-decl.c', `cp-decl2.c', `cp-dem.c', `cp-except.c', `cp-expr.c', `cp-init.c', `cp-lex.c', `cp-method.c', `cp-ptree.c', `cp-search.c', `cp-tree.c', `cp-type2.c', and `cp-typeck.c', along with header files `cp-tree.def', `cp-tree.h', and `cp-decl.h'. The special source files for parsing Objective C are `objc-parse.y', `objc-actions.c', `objc-tree.def', and `objc-actions.h'. Certain C-specific files are used for this as well. The file `c-common.c' is also used for all of the above languages. * RTL generation. This is the conversion of syntax tree into RTL code. It is actually done statement-by-statement during parsing, but for most purposes it can be thought of as a separate pass. This is where the bulk of target-parameter-dependent code is found, since often it is necessary for strategies to apply only when certain standard kinds of instructions are available. The purpose of named instruction patterns is to provide this information to the RTL generation pass. Optimization is done in this pass for `if'-conditions that are comparisons, boolean operations or conditional expressions. Tail recursion is detected at this time also. Decisions are made about how best to arrange loops and how to output `switch' statements. The source files for RTL generation include `stmt.c', `calls.c', `expr.c', `explow.c', `expmed.c', `function.c', `optabs.c' and `emit-rtl.c'. Also, the file `insn-emit.c', generated from the machine description by the program `genemit', is used in this pass. The header file `expr.h' is used for communication within this pass. The header files `insn-flags.h' and `insn-codes.h', generated from the machine description by the programs `genflags' and `gencodes', tell this pass which standard names are available for use and which patterns correspond to them. Aside from debugging information output, none of the following passes refers to the tree structure representation of the function (only part of which is saved). The decision of whether the function can and should be expanded inline in its subsequent callers is made at the end of rtl generation. The function must meet certain criteria, currently related to the size of the function and the types and number of parameters it has. Note that this function may contain loops, recursive calls to itself (tail-recursive functions can be inlined!), gotos, in short, all constructs supported by GNU CC. The file `integrate.c' contains the code to save a function's rtl for later inlining and to inline that rtl when the function is called. The header file `integrate.h' is also used for this purpose. The option `-dr' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.rtl' to the input file name. * Jump optimization. This pass simplifies jumps to the following instruction, jumps across jumps, and jumps to jumps. It deletes unreferenced labels and unreachable code, except that unreachable code that contains a loop is not recognized as unreachable in this pass. (Such loops are deleted later in the basic block analysis.) It also converts some code originally written with jumps into sequences of instructions that directly set values from the results of comparisons, if the machine has such instructions. Jump optimization is performed two or three times. The first time is immediately following RTL generation. The second time is after CSE, but only if CSE says repeated jump optimization is needed. The last time is right before the final pass. That time, cross-jumping and deletion of no-op move instructions are done together with the optimizations described above. The source file of this pass is `jump.c'. The option `-dj' causes a debugging dump of the RTL code after this pass is run for the first time. This dump file's name is made by appending `.jump' to the input file name. * Register scan. This pass finds the first and last use of each register, as a guide for common subexpression elimination. Its source is in `regclass.c'. * Jump threading. This pass detects a condition jump that branches to an identical or inverse test. Such jumps can be `threaded' through the second conditional test. The source code for this pass is in `jump.c'. This optimization is only performed if `-fthread-jumps' is enabled. * Common subexpression elimination. This pass also does constant propagation. Its source file is `cse.c'. If constant propagation causes conditional jumps to become unconditional or to become no-ops, jump optimization is run again when CSE is finished. The option `-ds' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.cse' to the input file name. * Loop optimization. This pass moves constant expressions out of loops, and optionally does strength-reduction and loop unrolling as well. Its source files are `loop.c' and `unroll.c', plus the header `loop.h' used for communication between them. Loop unrolling uses some functions in `integrate.c' and the header `integrate.h'. The option `-dL' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.loop' to the input file name. * If `-frerun-cse-after-loop' was enabled, a second common subexpression elimination pass is performed after the loop optimization pass. Jump threading is also done again at this time if it was specified. The option `-dt' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.cse2' to the input file name. * Stupid register allocation is performed at this point in a nonoptimizing compilation. It does a little data flow analysis as well. When stupid register allocation is in use, the next pass executed is the reloading pass; the others in between are skipped. The source file is `stupid.c'. * Data flow analysis (`flow.c'). This pass divides the program into basic blocks (and in the process deletes unreachable loops); then it computes which pseudo-registers are live at each point in the program, and makes the first instruction that uses a value point at the instruction that computed the value. This pass also deletes computations whose results are never used, and combines memory references with add or subtract instructions to make autoincrement or autodecrement addressing. The option `-df' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.flow' to the input file name. If stupid register allocation is in use, this dump file reflects the full results of such allocation. * Instruction combination (`combine.c'). This pass attempts to combine groups of two or three instructions that are related by data flow into single instructions. It combines the RTL expressions for the instructions by substitution, simplifies the result using algebra, and then attempts to match the result against the machine description. The option `-dc' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.combine' to the input file name. * Instruction scheduling (`sched.c'). This pass looks for instructions whose output will not be available by the time that it is used in subsequent instructions. (Memory loads and floating point instructions often have this behavior on RISC machines). It re-orders instructions within a basic block to try to separate the definition and use of items that otherwise would cause pipeline stalls. Instruction scheduling is performed twice. The first time is immediately after instruction combination and the second is immediately after reload. The option `-dS' causes a debugging dump of the RTL code after this pass is run for the first time. The dump file's name is made by appending `.sched' to the input file name. * Register class preferencing. The RTL code is scanned to find out which register class is best for each pseudo register. The source file is `regclass.c'. * Local register allocation (`local-alloc.c'). This pass allocates hard registers to pseudo registers that are used only within one basic block. Because the basic block is linear, it can use fast and powerful techniques to do a very good job. The option `-dl' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.lreg' to the input file name. * Global register allocation (`global.c'). This pass allocates hard registers for the remaining pseudo registers (those whose life spans are not contained in one basic block). * Reloading. This pass renumbers pseudo registers with the hardware registers numbers they were allocated. Pseudo registers that did not get hard registers are replaced with stack slots. Then it finds instructions that are invalid because a value has failed to end up in a register, or has ended up in a register of the wrong kind. It fixes up these instructions by reloading the problematical values temporarily into registers. Additional instructions are generated to do the copying. The reload pass also optionally eliminates the frame pointer and inserts instructions to save and restore call-clobbered registers around calls. Source files are `reload.c' and `reload1.c', plus the header `reload.h' used for communication between them. The option `-dg' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.greg' to the input file name. * Instruction scheduling is repeated here to try to avoid pipeline stalls due to memory loads generated for spilled pseudo registers. The option `-dR' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.sched2' to the input file name. * Jump optimization is repeated, this time including cross-jumping and deletion of no-op move instructions. The option `-dJ' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.jump2' to the input file name. * Delayed branch scheduling. This optional pass attempts to find instructions that can go into the delay slots of other instructions, usually jumps and calls. The source file name is `reorg.c'. The option `-dd' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.dbr' to the input file name. * Conversion from usage of some hard registers to usage of a register stack may be done at this point. Currently, this is supported only for the floating-point registers of the Intel 80387 coprocessor. The source file name is `reg-stack.c'. The options `-dk' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.stack' to the input file name. * Final. This pass outputs the assembler code for the function. It is also responsible for identifying spurious test and compare instructions. Machine-specific peephole optimizations are performed at the same time. The function entry and exit sequences are generated directly as assembler code in this pass; they never exist as RTL. The source files are `final.c' plus `insn-output.c'; the latter is generated automatically from the machine description by the tool `genoutput'. The header file `conditions.h' is used for communication between these files. * Debugging information output. This is run after final because it must output the stack slot offsets for pseudo registers that did not get hard registers. Source files are `dbxout.c' for DBX symbol table format, `sdbout.c' for SDB symbol table format, and `dwarfout.c' for DWARF symbol table format. Some additional files are used by all or many passes: * Every pass uses `machmode.def' and `machmode.h' which define the machine modes. * Several passes use `real.h', which defines the default representation of floating point constants and how to operate on them. * All the passes that work with RTL use the header files `rtl.h' and `rtl.def', and subroutines in file `rtl.c'. The tools `gen*' also use these files to read and work with the machine description RTL. * Several passes refer to the header file `insn-config.h' which contains a few parameters (C macro definitions) generated automatically from the machine description RTL by the tool `genconfig'. * Several passes use the instruction recognizer, which consists of `recog.c' and `recog.h', plus the files `insn-recog.c' and `insn-extract.c' that are generated automatically from the machine description by the tools `genrecog' and `genextract'. * Several passes use the header files `regs.h' which defines the information recorded about pseudo register usage, and `basic-block.h' which defines the information recorded about basic blocks. * `hard-reg-set.h' defines the type `HARD_REG_SET', a bit-vector with a bit for each hard register, and some macros to manipulate it. This type is just `int' if the machine has few enough hard registers; otherwise it is an array of `int' and some of the macros expand into loops. * Several passes use instruction attributes. A definition of the attributes defined for a particular machine is in file `insn-attr.h', which is generated from the machine description by the program `genattr'. The file `insn-attrtab.c' contains subroutines to obtain the attribute values for insns. It is generated from the machine description by the program `genattrtab'.  File: gcc.info, Node: RTL, Next: Machine Desc, Prev: Passes, Up: Top RTL Representation ****************** Most of the work of the compiler is done on an intermediate representation called register transfer language. In this language, the instructions to be output are described, pretty much one by one, in an algebraic form that describes what the instruction does. RTL is inspired by Lisp lists. It has both an internal form, made up of structures that point at other structures, and a textual form that is used in the machine description and in printed debugging dumps. The textual form uses nested parentheses to indicate the pointers in the internal form. * Menu: * RTL Objects:: Expressions vs vectors vs strings vs integers. * Accessors:: Macros to access expression operands or vector elts. * Flags:: Other flags in an RTL expression. * Machine Modes:: Describing the size and format of a datum. * Constants:: Expressions with constant values. * Regs and Memory:: Expressions representing register contents or memory. * Arithmetic:: Expressions representing arithmetic on other expressions. * Comparisons:: Expressions representing comparison of expressions. * Bit Fields:: Expressions representing bit-fields in memory or reg. * Conversions:: Extending, truncating, floating or fixing. * RTL Declarations:: Declaring volatility, constancy, etc. * Side Effects:: Expressions for storing in registers, etc. * Incdec:: Embedded side-effects for autoincrement addressing. * Assembler:: Representing `asm' with operands. * Insns:: Expression types for entire insns. * Calls:: RTL representation of function call insns. * Sharing:: Some expressions are unique; others *must* be copied. * Reading RTL:: Reading textual RTL from a file.  File: gcc.info, Node: RTL Objects, Next: Accessors, Prev: RTL, Up: RTL RTL Object Types ================ RTL uses five kinds of objects: expressions, integers, wide integers, strings and vectors. Expressions are the most important ones. An RTL expression ("RTX", for short) is a C structure, but it is usually referred to with a pointer; a type that is given the typedef name `rtx'. An integer is simply an `int'; their written form uses decimal digits. A wide integer is an integral object whose type is `HOST_WIDE_INT' (*note Config::.); their written form used decimal digits. A string is a sequence of characters. In core it is represented as a `char *' in usual C fashion, and it is written in C syntax as well. However, strings in RTL may never be null. If you write an empty string in a machine description, it is represented in core as a null pointer rather than as a pointer to a null character. In certain contexts, these null pointers instead of strings are valid. Within RTL code, strings are most commonly found inside `symbol_ref' expressions, but they appear in other contexts in the RTL expressions that make up machine descriptions. A vector contains an arbitrary number of pointers to expressions. The number of elements in the vector is explicitly present in the vector. The written form of a vector consists of square brackets (`[...]') surrounding the elements, in sequence and with whitespace separating them. Vectors of length zero are not created; null pointers are used instead. Expressions are classified by "expression codes" (also called RTX codes). The expression code is a name defined in `rtl.def', which is also (in upper case) a C enumeration constant. The possible expression codes and their meanings are machine-independent. The code of an RTX can be extracted with the macro `GET_CODE (X)' and altered with `PUT_CODE (X, NEWCODE)'. The expression code determines how many operands the expression contains, and what kinds of objects they are. In RTL, unlike Lisp, you cannot tell by looking at an operand what kind of object it is. Instead, you must know from its context--from the expression code of the containing expression. For example, in an expression of code `subreg', the first operand is to be regarded as an expression and the second operand as an integer. In an expression of code `plus', there are two operands, both of which are to be regarded as expressions. In a `symbol_ref' expression, there is one operand, which is to be regarded as a string. Expressions are written as parentheses containing the name of the expression type, its flags and machine mode if any, and then the operands of the expression (separated by spaces). Expression code names in the `md' file are written in lower case, but when they appear in C code they are written in upper case. In this manual, they are shown as follows: `const_int'. In a few contexts a null pointer is valid where an expression is normally wanted. The written form of this is `(nil)'.  File: gcc.info, Node: Accessors, Next: Flags, Prev: RTL Objects, Up: RTL Access to Operands ================== For each expression type `rtl.def' specifies the number of contained objects and their kinds, with four possibilities: `e' for expression (actually a pointer to an expression), `i' for integer, `w' for wide integer, `s' for string, and `E' for vector of expressions. The sequence of letters for an expression code is called its "format". Thus, the format of `subreg' is `ei'. 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, the format, and the class 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. `GET_RTX_CLASS (CODE)' A single character representing the type of RTX operation that code CODE performs. The following classes are defined: `o' An RTX code that represents an actual object, such as `reg' or `mem'. `subreg' is not in this class. `<' An RTX code for a comparison. The codes in this class are `NE', `EQ', `LE', `LT', `GE', `GT', `LEU', `LTU', `GEU', `GTU'. `1' An RTX code for a unary arithmetic operation, such as `neg'. `c' An RTX code for a commutative binary operation, other than `NE' and `EQ' (which have class `<'). `2' An RTX code for a noncommutative binary operation, such as `MINUS'. `b' An RTX code for a bitfield operation (`ZERO_EXTRACT' and `SIGN_EXTRACT'). `3' An RTX code for other three input operations, such as `IF_THEN_ELSE'. `i' An RTX code for a machine insn (`INSN', `JUMP_INSN', and `CALL_INSN'). `m' An RTX code for something that matches in insns, such as `MATCH_DUP'. `x' All other RTX codes. 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: gcc.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: `MEM_VOLATILE_P (X)' In `mem' expressions, nonzero for volatile memory references. Stored in the `volatil' field and printed as `/v'. `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. Stored in the `in_struct' field and printed as `/s'. `REG_LOOP_TEST_P' 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_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'. `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'. 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. `SUBREG_PROMOTED_VAR_P' 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'. `SUBREG_PROMOTED_UNSIGNED_P' 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'. `RTX_UNCHANGING_P (X)' Nonzero in a `reg' or `mem' if the value is not changed. (This flag is not set for memory references via pointers to constants. Such pointers only guarantee that the object will not be changed explicitly by the current function. The object might be changed by other functions or by aliasing.) Stored in the `unchanging' field and printed as `/u'. `RTX_INTEGRATED_P (INSN)' Nonzero in an insn if it resulted from an in-line function call. Stored in the `integrated' field and printed as `/i'. This may be deleted; nothing currently depends on it. `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_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'. `LABEL_OUTSIDE_LOOP_P' 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'. `INSN_DELETED_P (INSN)' In an insn, nonzero if the insn has been deleted. Stored in the `volatil' field and printed as `/v'. `INSN_ANNULLED_BRANCH_P (INSN)' 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_FROM_TARGET_P (INSN)' 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 should only be executed if the branch is taken. For annulled branches with this bit clear, the insn should be executed only if the branch is not taken. Stored in the `in_struct' field and printed as `/s'. `CONSTANT_POOL_ADDRESS_P (X)' Nonzero in a `symbol_ref' if it refers to part of the current function's "constants pool". These are addresses close to the beginning of the function, and GNU CC assumes they can be addressed directly (perhaps with the help of base registers). Stored in the `unchanging' field and printed as `/u'. `CONST_CALL_P (X)' In a `call_insn', indicates that the insn represents a call to a const function. Stored in the `unchanging' field and printed as `/u'. `LABEL_PRESERVE_P (X)' In a `code_label', indicates that the label can never be deleted. Labels referenced by a non-local goto will have this bit set. Stored in the `in_struct' field and printed as `/s'. `SCHED_GROUP_P (INSN)' 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'. These are the fields which the above macros refer to: `used' Normally, this flag is used only momentarily, 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::.). In a `symbol_ref', it indicates that an external declaration for the symbol has already been written. In a `reg', it is used by the leaf register renumbering code to ensure that each register is only renumbered once. `volatil' This flag is used in `mem', `symbol_ref' and `reg' expressions and in insns. In RTL dump files, it is printed as `/v'. In a `mem' 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_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 an insn in the delay slot of a branch, 1 means that this insn is from the target of the branch. During instruction scheduling, in an insn, 1 means that this insn must be scheduled as part of a group together with the previous insn. In `reg' expressions, it is 1 if the register has its entire life contained within the test expression of some loopl. 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. In an RTL dump, this flag is represented as `/s'. `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 constants pool. In a `call_insn', 1 means that this instruction is a call to a const function. In an RTL dump, this flag is represented as `/u'. `integrated' In some kinds of expressions, including insns, this flag means the rtl was produced by procedure integration. In a `reg' expression, this flag indicates the register containing 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. ə