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GCC 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 GCC did not provide this default.
GCC 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 GCC, 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, GCC (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, GCC (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.
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.
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.in', `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.
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 GCC. 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 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.
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.
The source file for this pass is gcse.c.
The option `-dG' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.gcse' to the input file name.
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.
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.
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.
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.
The option `-dN' causes a debugging dump of the RTL code after this pass. This dump file's name is made by appending `.regmove' to the input file name.
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.
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.
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.
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.
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.
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.
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.
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.
Some additional files are used by all or many passes:
gen*
also use these files to read and work with the machine
description RTL.
genconfig
.
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.
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