4.4BSD/usr/src/contrib/gcc-2.3.3/gcc.info-2
This is Info file gcc.info, produced by Makeinfo-1.49 from the input
file gcc.texi.
This file documents the use and the internals of the GNU compiler.
Copyright (C) 1988, 1989, 1992 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: Dialect Options, Next: Warning Options, Prev: Overall Options, Up: Invoking GCC
Options Controlling Dialect
===========================
The following options control the dialect of C or C++ that the
compiler accepts:
`-ansi'
Support all ANSI standard C programs.
This turns off certain features of GNU C that are incompatible
with ANSI C, such as the `asm', `inline' and `typeof' keywords, and
predefined macros such as `unix' and `vax' that identify the type
of system you are using. It also enables the undesirable and
rarely used ANSI trigraph feature, and disallows `$' as part of
identifiers.
The alternate keywords `__asm__', `__extension__', `__inline__'
and `__typeof__' continue to work despite `-ansi'. You would not
want to use them in an ANSI C program, of course, but it useful to
put them in header files that might be included in compilations
done with `-ansi'. Alternate predefined macros such as `__unix__'
and `__vax__' are also available, with or without `-ansi'.
The `-ansi' option does not cause non-ANSI programs to be rejected
gratuitously. For that, `-pedantic' is required in addition to
`-ansi'. *Note Warning Options::.
The macro `__STRICT_ANSI__' is predefined when the `-ansi' option
is used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the
ANSI standard doesn't call for; this is to avoid interfering with
any programs that might use these names for other things.
The functions `alloca', `abort', `exit', and `_exit' are not
builtin functions when `-ansi' is used.
`-fall-virtual'
Treat certain member functions as virtual, implicitly (C++ only).
This applies to all member functions declared in the same class
with a "method-call" operator method (except for constructor
functions and `new' or `delete' member operators). In effect, all
of these methods become "implicitly virtual."
This does not mean that all calls to these methods will be made
through the internal table of virtual functions. There are some
circumstances under which it is obvious that a call to a given
virtual function can be made directly, and in these cases the
calls still go direct.
The effect of making all methods of a class with a declared
`operator->()()' implicitly virtual using `-fall-virtual' extends
also to all non-constructor methods of any class derived from such
a class.
`-fdollars-in-identifiers'
Permit the use of `$' in identifiers (C++ only). You can also use
`-fno-dollars-in-identifiers' to explicitly prohibit use of `$'.
(GNU C++ allows `$' by default on some target systems but not
others.)
`-fenum-int-equiv'
Permit implicit conversion of `int' to enumeration types (C++
only). Normally GNU C++ allows conversion of `enum' to `int', but
not the other way around.
`-fno-asm'
Do not recognize `asm', `inline' or `typeof' as a keyword. These
words may then be used as identifiers. You can use `__asm__',
`__inline__' and `__typeof__' instead. `-ansi' implies `-fno-asm'.
`-fno-builtin'
Don't recognize built-in functions that do not begin with two
leading underscores. Currently, the functions affected include
`_exit', `abort', `abs', `alloca', `cos', `exit', `fabs', `labs',
`memcmp', `memcpy', `sin', `sqrt', `strcmp', `strcpy', and
`strlen'.
The `-ansi' option prevents `alloca' and `_exit' from being
builtin functions.
`-fno-strict-prototype'
Treat a function declaration with no arguments, such as `int foo
();', as C would treat it--as saying nothing about the number of
arguments or their types (C++ only). Normally, such a declaration
in C++ means that the function `foo' takes no arguments.
`-fthis-is-variable'
Permit assignment to `this' (C++ only). The incorporation of
user-defined free store management into C++ has made assignment to
`this' an anachronism. Therefore, by default it is invalid to
assign to `this' within a class member function. However, for
backwards compatibility, you can make it valid with
`-fthis-is-variable'.
`-trigraphs'
Support ANSI C trigraphs. You don't want to know about this
brain-damage. The `-ansi' option implies `-trigraphs'.
`-traditional'
Attempt to support some aspects of traditional C compilers.
Specifically:
* All `extern' declarations take effect globally even if they
are written inside of a function definition. This includes
implicit declarations of functions.
* The keywords `typeof', `inline', `signed', `const' and
`volatile' are not recognized. (You can still use the
alternative keywords such as `__typeof__', `__inline__', and
so on.)
* Comparisons between pointers and integers are always allowed.
* Integer types `unsigned short' and `unsigned char' promote to
`unsigned int'.
* Out-of-range floating point literals are not an error.
* String "constants" are not necessarily constant; they are
stored in writable space, and identical looking constants are
allocated separately. (This is the same as the effect of
`-fwritable-strings'.)
* All automatic variables not declared `register' are preserved
by `longjmp'. Ordinarily, GNU C follows ANSI C: automatic
variables not declared `volatile' may be clobbered.
* In the preprocessor, comments convert to nothing at all,
rather than to a space. This allows traditional token
concatenation.
* In the preprocessor, macro arguments are recognized within
string constants in a macro definition (and their values are
stringified, though without additional quote marks, when they
appear in such a context). The preprocessor always considers
a string constant to end at a newline.
* The predefined macro `__STDC__' is not defined when you use
`-traditional', but `__GNUC__' is (since the GNU extensions
which `__GNUC__' indicates are not affected by
`-traditional'). If you need to write header files that work
differently depending on whether `-traditional' is in use, by
testing both of these predefined macros you can distinguish
four situations: GNU C, traditional GNU C, other ANSI C
compilers, and other old C compilers.
You may wish to use `-fno-builtin' as well as `-traditional' if
your program uses names that are normally GNU C builtin functions
for other purposes of its own.
`-traditional-cpp'
Attempt to support some aspects of traditional C preprocessors.
This includes the last three items in the table immediately above,
but none of the other effects of `-traditional'.
`-fcond-mismatch'
Allow conditional expressions with mismatched types in the second
and third arguments. The value of such an expression is void.
`-funsigned-char'
Let the type `char' be unsigned, like `unsigned char'.
Each kind of machine has a default for what `char' should be. It
is either like `unsigned char' by default or like `signed char' by
default.
Ideally, a portable program should always use `signed char' or
`unsigned char' when it depends on the signedness of an object.
But many programs have been written to use plain `char' and expect
it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let
you make such a program work with the opposite default.
The type `char' is always a distinct type from each of `signed
char' or `unsigned char', even though its behavior is always just
like one of those two.
`-fsigned-char'
Let the type `char' be signed, like `signed char'.
Note that this is equivalent to `-fno-unsigned-char', which is the
negative form of `-funsigned-char'. Likewise, `-fno-signed-char'
is equivalent to `-funsigned-char'.
`-fsigned-bitfields'
`-funsigned-bitfields'
`-fno-signed-bitfields'
`-fno-unsigned-bitfields'
These options control whether a bitfield is signed or unsigned,
when the declaration does not use either `signed' or `unsigned'.
By default, such a bitfield is signed, because this is consistent:
the basic integer types such as `int' are signed types.
However, when `-traditional' is used, bitfields are all unsigned
no matter what.
`-fwritable-strings'
Store string constants in the writable data segment and don't
uniquize them. This is for compatibility with old programs which
assume they can write into string constants. `-traditional' also
has this effect.
Writing into string constants is a very bad idea; "constants"
should be constant.
File: gcc.info, Node: Warning Options, Next: Debugging Options, Prev: Dialect Options, Up: Invoking GCC
Options to Request or Suppress Warnings
=======================================
Warnings are diagnostic messages that report constructions which are
not inherently erroneous but which are risky or suggest there may have
been an error.
You can request many specific warnings with options beginning `-W',
for example `-Wimplicit' to request warnings on implicit declarations.
Each of these specific warning options also has a negative form
beginning `-Wno-' to turn off warnings; for example, `-Wno-implicit'.
This manual lists only one of the two forms, whichever is not the
default.
These options control the amount and kinds of warnings produced by
GNU CC:
`-fsyntax-only'
Check the code for syntax errors, but don't emit any output.
`-w'
Inhibit all warning messages.
`-Wno-import'
Inhibit warning messages about the use of `#import'.
`-pedantic'
Issue all the warnings demanded by strict ANSI standard C; reject
all programs that use forbidden extensions.
Valid ANSI standard C programs should compile properly with or
without this option (though a rare few will require `-ansi').
However, without this option, certain GNU extensions and
traditional C features are supported as well. With this option,
they are rejected.
`-pedantic' does not cause warning messages for use of the
alternate keywords whose names begin and end with `__'. Pedantic
warnings are also disabled in the expression that follows
`__extension__'. However, only system header files should use
these escape routes; application programs should avoid them. *Note
Alternate Keywords::.
This option is not intended to be useful; it exists only to satisfy
pedants who would otherwise claim that GNU CC fails to support the
ANSI standard.
Some users try to use `-pedantic' to check programs for strict ANSI
C conformance. They soon find that it does not do quite what they
want: it finds some non-ANSI practices, but not all--only those
for which ANSI C *requires* a diagnostic.
A feature to report any failure to conform to ANSI C might be
useful in some instances, but would require considerable
additional work and would be quite different from `-pedantic'. We
recommend, rather, that users take advantage of the extensions of
GNU C and disregard the limitations of other compilers. Aside
from certain supercomputers and obsolete small machines, there is
less and less reason ever to use any other C compiler other than
for bootstrapping GNU CC.
`-pedantic-errors'
Like `-pedantic', except that errors are produced rather than
warnings.
`-W'
Print extra warning messages for these events:
* A nonvolatile automatic variable might be changed by a call to
`longjmp'. These warnings as well are possible only in
optimizing compilation.
The compiler sees only the calls to `setjmp'. It cannot know
where `longjmp' will be called; in fact, a signal handler
could call it at any point in the code. As a result, you may
get a warning even when there is in fact no problem because
`longjmp' cannot in fact be called at the place which would
cause a problem.
* A function can return either with or without a value.
(Falling off the end of the function body is considered
returning without a value.) For example, this function would
evoke such a warning:
foo (a)
{
if (a > 0)
return a;
}
* An expression-statement contains no side effects.
* An unsigned value is compared against zero with `>' or `<='.
* A comparison like `x<=y<=z' appears; this is equivalent to
`(x<=y ? 1 : 0) <= z', which is a different interpretation
from that of ordinary mathematical notation.
* Storage-class specifiers like `static' are not the first
things in a declaration. According to the C Standard, this
usage is obsolescent.
* An aggregate has a partly bracketed initializer. For example,
the following code would evoke such a warning, because braces
are missing around the initializer for `x.h':
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
`-Wenum-clash'
Warn about conversion between different enumeration types (C++
only).
`-Wimplicit'
Warn whenever a function or parameter is implicitly declared.
`-Wreturn-type'
Warn whenever a function is defined with a return-type that
defaults to `int'. Also warn about any `return' statement with no
return-value in a function whose return-type is not `void'.
`-Wunused'
Warn whenever a local variable is unused aside from its
declaration, whenever a function is declared static but never
defined, and whenever a statement computes a result that is
explicitly not used.
If you want to prevent a warning for a particular variable, you
can use this macro:
#define USE(var) \
static void * use_##var = (&use_##var, (void *) &var)
USE (string);
`-Wswitch'
Warn whenever a `switch' statement has an index of enumeral type
and lacks a `case' for one or more of the named codes of that
enumeration. (The presence of a `default' label prevents this
warning.) `case' labels outside the enumeration range also
provoke warnings when this option is used.
`-Wcomment'
Warn whenever a comment-start sequence `/*' appears in a comment.
`-Wtrigraphs'
Warn if any trigraphs are encountered (assuming they are enabled).
`-Wformat'
Check calls to `printf' and `scanf', etc., to make sure that the
arguments supplied have types appropriate to the format string
specified.
`-Wchar-subscripts'
Warn if an array subscript has type `char'. This is a common cause
of error, as programmers often forget that this type is signed on
some machines.
`-Wuninitialized'
An automatic variable is used without first being initialized.
These warnings are possible only in optimizing compilation,
because they require data flow information that is computed only
when optimizing. If you don't specify `-O', you simply won't get
these warnings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a variable
that is declared `volatile', or whose address is taken, or whose
size is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in registers.
Note that there may be no warning about a variable that is used
only to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the
warnings are printed.
These warnings are made optional because GNU CC is not smart
enough to see all the reasons why the code might be correct
despite appearing to have an error. Here is one example of how
this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of `y' is always 1, 2 or 3, then `x' is always
initialized, but GNU CC doesn't know this. Here is another common
case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because `save_y' is used only if it is set.
Some spurious warnings can be avoided if you declare as `volatile'
all the functions you use that never return. *Note Function
Attributes::.
`-Wparentheses'
Warn if parentheses are omitted in certain contexts.
`-Wall'
All of the above `-W' options combined. These are all the options
which pertain to usage that we recommend avoiding and that we
believe is easy to avoid, even in conjunction with macros.
The remaining `-W...' options are not implied by `-Wall' because
they warn about constructions that we consider reasonable to use, on
occasion, in clean programs.
`-Wtraditional'
Warn about certain constructs that behave differently in
traditional and ANSI C.
* Macro arguments occurring within string constants in the
macro body. These would substitute the argument in
traditional C, but are part of the constant in ANSI C.
* A function declared external in one block and then used after
the end of the block.
* A `switch' statement has an operand of type `long'.
`-Wshadow'
Warn whenever a local variable shadows another local variable.
`-Wid-clash-LEN'
Warn whenever two distinct identifiers match in the first LEN
characters. This may help you prepare a program that will compile
with certain obsolete, brain-damaged compilers.
`-Wpointer-arith'
Warn about anything that depends on the "size of" a function type
or of `void'. GNU C assigns these types a size of 1, for
convenience in calculations with `void *' pointers and pointers to
functions.
`-Wcast-qual'
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a `const char *' is
cast to an ordinary `char *'.
`-Wcast-align'
Warn whenever a pointer is cast such that the required alignment
of the target is increased. For example, warn if a `char *' is
cast to an `int *' on machines where integers can only be accessed
at two- or four-byte boundaries.
`-Wwrite-strings'
Give string constants the type `const char[LENGTH]' so that
copying the address of one into a non-`const' `char *' pointer
will get a warning. These warnings will help you find at compile
time code that can try to write into a string constant, but only
if you have been very careful about using `const' in declarations
and prototypes. Otherwise, it will just be a nuisance; this is
why we did not make `-Wall' request these warnings.
`-Wconversion'
Warn if a prototype causes a type conversion that is different
from what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating
and vice versa, and conversions changing the width or signedness
of a fixed point argument except when the same as the default
promotion.
`-Waggregate-return'
Warn if any functions that return structures or unions are defined
or called. (In languages where you can return an array, this also
elicits a warning.)
`-Wstrict-prototypes'
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration which specifies the
argument types.)
`-Wmissing-prototypes'
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. The aim is to detect global functions that
fail to be declared in header files.
`-Wredundant-decls'
Warn if anything is declared more than once in the same scope,
even in cases where multiple declaration is valid and changes
nothing.
`-Wnested-externs'
Warn if an `extern' declaration is encountered within an function.
`-Winline'
Warn if a function can not be inlined, and either it was declared
as inline, or else the `-finline-functions' option was given.
`-Werror'
Make all warnings into errors.
File: gcc.info, Node: Debugging Options, Next: Optimize Options, Prev: Warning Options, Up: Invoking GCC
Options for Debugging Your Program or GNU CC
============================================
GNU CC has various special options that are used for debugging
either your program or GCC:
`-g'
Produce debugging information in the operating system's native
format (stabs, COFF, XCOFF, or DWARF). GDB can work with this
debugging information.
On most systems that use stabs format, `-g' enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but will probably make other
debuggers crash or refuse to read the program. If you want to
control for certain whether to generate the extra information, use
`-gstabs+', `-gstabs', `-gxcoff+', `-gxcoff', `-gdwarf+', or
`-gdwarf' (see below).
Unlike most other C compilers, GNU CC allows you to use `-g' with
`-O'. The shortcuts taken by optimized code may occasionally
produce surprising results: some variables you declared may not
exist at all; flow of control may briefly move where you did not
expect it; some statements may not be executed because they
compute constant results or their values were already at hand;
some statements may execute in different places because they were
moved out of loops.
Nevertheless it proves possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might
have bugs.
The following options are useful when GNU CC is generated with the
capability for more than one debugging format.
`-ggdb'
Produce debugging information in the native format (if that is
supported), including GDB extensions if at all possible.
`-gstabs'
Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by
DBX on most BSD systems.
`-gstabs+'
Produce debugging information in stabs format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program.
`-gcoff'
Produce debugging information in COFF format (if that is
supported). This is the format used by SDB on most System V
systems prior to System V Release 4.
`-gxcoff'
Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
`-gxcoff+'
Produce debugging information in XCOFF format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program.
`-gdwarf'
Produce debugging information in DWARF format (if that is
supported). This is the format used by SDB on most System V
Release 4 systems.
`-gdwarf+'
Produce debugging information in DWARF format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program.
`-gLEVEL'
`-ggdbLEVEL'
`-gstabsLEVEL'
`-gcoffLEVEL'
`-gxcoffLEVEL'
`-gdwarfLEVEL'
Request debugging information and also use LEVEL to specify how
much information. The default level is 2.
Level 1 produces minimal information, enough for making backtraces
in parts of the program that you don't plan to debug. This
includes descriptions of functions and external variables, but no
information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support macro
expansion when you use `-g3'.
`-p'
Generate extra code to write profile information suitable for the
analysis program `prof'.
`-pg'
Generate extra code to write profile information suitable for the
analysis program `gprof'.
`-a'
Generate extra code to write profile information for basic blocks,
which will record the number of times each basic block is executed.
This data could be analyzed by a program like `tcov'. Note,
however, that the format of the data is not what `tcov' expects.
Eventually GNU `gprof' should be extended to process this data.
`-dLETTERS'
Says to make debugging dumps during compilation at times specified
by LETTERS. This is used for debugging the compiler. The file
names for most of the dumps are made by appending a word to the
source file name (e.g. `foo.c.rtl' or `foo.c.jump'). Here are the
possible letters for use in LETTERS, and their meanings:
`M'
Dump all macro definitions, at the end of preprocessing, and
write no output.
`N'
Dump all macro names, at the end of preprocessing.
`D'
Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
`y'
Dump debugging information during parsing, to standard error.
`r'
Dump after RTL generation, to `FILE.rtl'.
`x'
Just generate RTL for a function instead of compiling it.
Usually used with `r'.
`j'
Dump after first jump optimization, to `FILE.jump'.
`s'
Dump after CSE (including the jump optimization that sometimes
follows CSE), to `FILE.cse'.
`L'
Dump after loop optimization, to `FILE.loop'.
`t'
Dump after the second CSE pass (including the jump
optimization that sometimes follows CSE), to `FILE.cse2'.
`f'
Dump after flow analysis, to `FILE.flow'.
`c'
Dump after instruction combination, to `FILE.combine'.
`S'
Dump after the first instruction scheduling pass, to
`FILE.sched'.
`l'
Dump after local register allocation, to
`FILE.lreg'.
`g'
Dump after global register allocation, to
`FILE.greg'.
`R'
Dump after the second instruction scheduling pass, to
`FILE.sched2'.
`J'
Dump after last jump optimization, to `FILE.jump2'.
`d'
Dump after delayed branch scheduling, to `FILE.dbr'.
`k'
Dump after conversion from registers to stack, to
`FILE.stack'.
`a'
Produce all the dumps listed above.
`m'
Print statistics on memory usage, at the end of the run, to
standard error.
`p'
Annotate the assembler output with a comment indicating which
pattern and alternative was used.
`-fpretend-float'
When running a cross-compiler, pretend that the target machine
uses the same floating point format as the host machine. This
causes incorrect output of the actual floating constants, but the
actual instruction sequence will probably be the same as GNU CC
would make when running on the target machine.
`-save-temps'
Store the usual "temporary" intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling `foo.c' with `-c -save-temps' would produce
files `foo.i' and `foo.s', as well as `foo.o'.
File: gcc.info, Node: Optimize Options, Next: Preprocessor Options, Prev: Debugging Options, Up: Invoking GCC
Options That Control Optimization
=================================
These options control various sorts of optimizations:
`-O'
`-O1'
Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.
Without `-O', the compiler's goal is to reduce the cost of
compilation and to make debugging produce the expected results.
Statements are independent: if you stop the program with a
breakpoint between statements, you can then assign a new value to
any variable or change the program counter to any other statement
in the function and get exactly the results you would expect from
the source code.
Without `-O', only variables declared `register' are allocated in
registers. The resulting compiled code is a little worse than
produced by PCC without `-O'.
With `-O', the compiler tries to reduce code size and execution
time.
When `-O' is specified, `-fthread-jumps' and `-fdelayed-branch'
are turned on. On some machines other flags may also be turned on.
`-O2'
Optimize even more. Nearly all supported optimizations that do not
involve a space-speed tradeoff are performed. As compared to `-O',
this option increases both compilation time and the performance of
the generated code.
`-O2' turns on all `-fFLAG' options that enable more optimization,
except for `-funroll-loops', `-funroll-all-loops' and
`-fomit-frame-pointer'.
`-O0'
Do not optimize.
If you use multiple `-O' options, with or without level numbers,
the last such option is the one that is effective.
Options of the form `-fFLAG' specify machine-independent flags.
Most flags have both positive and negative forms; the negative form of
`-ffoo' would be `-fno-foo'. In the table below, only one of the forms
is listed--the one which is not the default. You can figure out the
other form by either removing `no-' or adding it.
`-ffloat-store'
Do not store floating point variables in registers, and inhibit
other options that might change whether a floating point value is
taken from a register or memory.
This option prevents undesirable excess precision on machines such
as the 68000 where the floating registers (of the 68881) keep more
precision than a `double' is supposed to have. For most programs,
the excess precision does only good, but a few programs rely on the
precise definition of IEEE floating point. Use `-ffloat-store' for
such programs.
`-fno-defer-pop'
Always pop the arguments to each function call as soon as that
function returns. For machines which must pop arguments after a
function call, the compiler normally lets arguments accumulate on
the stack for several function calls and pops them all at once.
`-fforce-mem'
Force memory operands to be copied into registers before doing
arithmetic on them. This may produce better code by making all
memory references potential common subexpressions. When they are
not common subexpressions, instruction combination should
eliminate the separate register-load. I am interested in hearing
about the difference this makes.
`-fforce-addr'
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
`-fforce-mem' may. I am interested in hearing about the
difference this makes.
`-fomit-frame-pointer'
Don't keep the frame pointer in a register for functions that
don't need one. This avoids the instructions to save, set up and
restore frame pointers; it also makes an extra register available
in many functions. *It also makes debugging impossible on some
machines.*
On some machines, such as the Vax, this flag has no effect, because
the standard calling sequence automatically handles the frame
pointer and nothing is saved by pretending it doesn't exist. The
machine-description macro `FRAME_POINTER_REQUIRED' controls
whether a target machine supports this flag. *Note Registers::.
`-fno-inline'
Don't pay attention to the `inline' keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded
inline.
`-finline-functions'
Integrate all simple functions into their callers. The compiler
heuristically decides which functions are simple enough to be worth
integrating in this way.
If all calls to a given function are integrated, and the function
is declared `static', then the function is normally not output as
assembler code in its own right.
`-fkeep-inline-functions'
Even if all calls to a given function are integrated, and the
function is declared `static', nevertheless output a separate
run-time callable version of the function.
`-fno-default-inline'
Don't make member functions inline by default merely because they
are defined inside the class scope (C++ only).
`-fno-function-cse'
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the
optimizations performed when this option is not used.
`-ffast-math'
This option allows GCC to violate some ANSI or IEEE
rules/specifications in the interest of optimizing code for speed.
For example, it allows the compiler to assume arguments to the
`sqrt' function are non-negative numbers.
This option should never be turned on by any `-O' option since it
can result in incorrect output for programs which depend on an
exact implementation of IEEE or ANSI rules/specifications for math
functions.
`-felide-constructors'
Elide constructors when this seems plausible (C++ only). With this
option, GNU C++ initializes `y' directly from the call to `foo'
without going through a temporary in the following code:
A foo ();
A y = foo ();
Without this option, GNU C++ first initializes `y' by calling the
appropriate constructor for type `A'; then assigns the result of
`foo' to a temporary; and, finally, replaces the initial value of
`y' with the temporary.
The default behavior (`-fno-elide-constructors') is specified by
the draft ANSI C++ standard. If your program's constructors have
side effects, `-felide-constructors' can change your program's
behavior, since some constructor calls may be omitted.
`-fmemoize-lookups'
`-fsave-memoized'
Use heuristics to compile faster (C++ only). These heuristics are
not enabled by default, since they are only effective for certain
input files. Other input files compile more slowly.
The first time the compiler must build a call to a member function
(or reference to a data member), it must (1) determine whether the
class implements member functions of that name; (2) resolve which
member function to call (which involves figuring out what sorts of
type conversions need to be made); and (3) check the visibility of
the member function to the caller. All of this adds up to slower
compilation. Normally, the second time a call is made to that
member function (or reference to that data member), it must go
through the same lengthy process again. This means that code like
this
cout << "This " << p << " has " << n << " legs.\n";
makes six passes through all three steps. By using a software
cache, a "hit" significantly reduces this cost. Unfortunately,
using the cache introduces another layer of mechanisms which must
be implemented, and so incurs its own overhead.
`-fmemoize-lookups' enables the software cache.
Because access privileges (visibility) to members and member
functions may differ from one function context to the next, G++
may need to flush the cache. With the `-fmemoize-lookups' flag,
the cache is flushed after every function that is compiled. The
`-fsave-memoized' flag enables the same software cache, but when
the compiler determines that the context of the last function
compiled would yield the same access privileges of the next
function to compile, it preserves the cache. This is most helpful
when defining many member functions for the same class: with the
exception of member functions which are friends of other classes,
each member function has exactly the same access privileges as
every other, and the cache need not be flushed.
The following options control specific optimizations. The `-O2'
option turns on all of these optimizations except `-funroll-loops' and
`-funroll-all-loops'. The `-O' option usually turns on the
`-fthread-jumps' and `-fdelayed-branch' options, but specific machines
may change the default optimizations.
You can use the following flags in the rare cases when "fine-tuning"
of optimizations to be performed is desired.
`-fstrength-reduce'
Perform the optimizations of loop strength reduction and
elimination of iteration variables.
`-fthread-jumps'
Perform optimizations where we check to see if a jump branches to a
location where another comparison subsumed by the first is found.
If so, the first branch is redirected to either the destination of
the second branch or a point immediately following it, depending
on whether the condition is known to be true or false.
`-fcse-follow-jumps'
In common subexpression elimination, scan through jump instructions
when the target of the jump is not reached by any other path. For
example, when CSE encounters an `if' statement with an `else'
clause, CSE will follow the jump when the condition tested is
false.
`-fcse-skip-blocks'
This is similar to `-fcse-follow-jumps', but causes CSE to follow
jumps which conditionally skip over blocks. When CSE encounters a
simple `if' statement with no else clause, `-fcse-skip-blocks'
causes CSE to follow the jump around the body of the `if'.
`-frerun-cse-after-loop'
Re-run common subexpression elimination after loop optimizations
has been performed.
`-fexpensive-optimizations'
Perform a number of minor optimizations that are relatively
expensive.
`-fdelayed-branch'
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed
branch instructions.
`-fschedule-insns'
If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required data
being unavailable. This helps machines that have slow floating
point or memory load instructions by allowing other instructions
to be issued until the result of the load or floating point
instruction is required.
`-fschedule-insns2'
Similar to `-fschedule-insns', but requests an additional pass of
instruction scheduling after register allocation has been done.
This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more
than one cycle.
`-fcaller-saves'
Enable values to be allocated in registers that will be clobbered
by function calls, by emitting extra instructions to save and
restore the registers around such calls. Such allocation is done
only when it seems to result in better code than would otherwise
be produced.
This option is enabled by default on certain machines, usually
those which have no call-preserved registers to use instead.
`-funroll-loops'
Perform the optimization of loop unrolling. This is only done for
loops whose number of iterations can be determined at compile time
or run time. `-funroll-loop' implies `-fstrength-reduce' and
`-frerun-cse-after-loop'.
`-funroll-all-loops'
Perform the optimization of loop unrolling. This is done for all
loops and usually makes programs run more slowly.
`-funroll-all-loops' implies `-fstrength-reduce' and
`-frerun-cse-after-loop'.
`-fno-peephole'
Disable any machine-specific peephole optimizations.
File: gcc.info, Node: Preprocessor Options, Next: Assembler Options, Prev: Optimize Options, Up: Invoking GCC
Options Controlling the Preprocessor
====================================
These options control the C preprocessor, which is run on each C
source file before actual compilation.
If you use the `-E' option, nothing is done except preprocessing.
Some of these options make sense only together with `-E' because they
cause the preprocessor output to be unsuitable for actual compilation.
`-include FILE'
Process FILE as input before processing the regular input file. In
effect, the contents of FILE are compiled first. Any `-D' and
`-U' options on the command line are always processed before
`-include FILE', regardless of the order in which they are
written. All the `-include' and `-imacros' options are processed
in the order in which they are written.
`-imacros FILE'
Process FILE as input, discarding the resulting output, before
processing the regular input file. Because the output generated
from FILE is discarded, the only effect of `-imacros FILE' is to
make the macros defined in FILE available for use in the main
input.
Any `-D' and `-U' options on the command line are always processed
before `-imacros FILE', regardless of the order in which they are
written. All the `-include' and `-imacros' options are processed
in the order in which they are written.
`-nostdinc'
Do not search the standard system directories for header files.
Only the directories you have specified with `-I' options (and the
current directory, if appropriate) are searched. *Note Directory
Options::, for information on `-I'.
By using both `-nostdinc' and `-I-', you can limit the include-file
search path to only those directories you specify explicitly.
`-nostdinc++'
Do not search for header files in the C++-specific standard
directories, but do still search the other standard directories.
(This option is used when building `libg++'.)
`-undef'
Do not predefine any nonstandard macros. (Including architecture
flags).
`-E'
Run only the C preprocessor. Preprocess all the C source files
specified and output the results to standard output or to the
specified output file.
`-C'
Tell the preprocessor not to discard comments. Used with the `-E'
option.
`-P'
Tell the preprocessor not to generate `#line' commands. Used with
the `-E' option.
`-M'
Tell the preprocessor to output a rule suitable for `make'
describing the dependencies of each object file. For each source
file, the preprocessor outputs one `make'-rule whose target is the
object file name for that source file and whose dependencies are
all the files `#include'd in it. This rule may be a single line
or may be continued with `\'-newline if it is long. The list of
rules is printed on standard output instead of the preprocessed C
program.
`-M' implies `-E'.
Another way to specify output of a `make' rule is by setting the
environment variable `DEPENDENCIES_OUTPUT' (*note Environment
Variables::.).
`-MM'
Like `-M' but the output mentions only the user header files
included with `#include "FILE"'. System header files included
with `#include <FILE>' are omitted.
`-MD'
Like `-M' but the dependency information is written to files with
names made by replacing `.o' with `.d' at the end of the output
file names. This is in addition to compiling the input files as
specified--`-MD' does not inhibit ordinary compilation the way
`-M' does.
The Mach utility `md' can be used to merge the `.d' files into a
single dependency file suitable for using with the `make' command.
`-MMD'
Like `-MD' except mention only user header files, not system
header files.
`-H'
Print the name of each header file used, in addition to other
normal activities.
`-AQUESTION(ANSWER)'
Assert the answer ANSWER for QUESTION, in case it is tested with a
preprocessor conditional such as `#if #QUESTION(ANSWER)'. `-A-'
disables the standard assertions that normally describe the target
machine.
`-DMACRO'
Define macro MACRO with the string `1' as its definition.
`-DMACRO=DEFN'
Define macro MACRO as DEFN. All instances of `-D' on the command
line are processed before any `-U' options.
`-UMACRO'
Undefine macro MACRO. `-U' options are evaluated after all `-D'
options, but before any `-include' and `-imacros' options.
`-dM'
Tell the preprocessor to output only a list of the macro
definitions that are in effect at the end of preprocessing. Used
with the `-E' option.
`-dD'
Tell the preprocessing to pass all macro definitions into the
output, in their proper sequence in the rest of the output.
`-dN'
Like `-dD' except that the macro arguments and contents are
omitted. Only `#define NAME' is included in the output.
`-trigraphs'
Support ANSI C trigraphs. You don't want to know about this
brain-damage. The `-ansi' option also has this effect.
File: gcc.info, Node: Assembler Options, Next: Link Options, Prev: Preprocessor Options, Up: Invoking GCC
Passing Options to the Assembler
================================
`-Wa,OPTION'
Pass OPTION as an option to the assembler. If OPTION contains
commas, it is split into multiple options at the commas.