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This file documents the use and the internals of the GNU compiler.
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File:, Node: Warning Options, Next: Debugging Options, Prev: C++ 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
These options control the amount and kinds of warnings produced by
Check the code for syntax errors, but don't do anything beyond
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.
Like `-pedantic', except that errors are produced rather than
Inhibit all warning messages.
Inhibit warning messages about the use of `#import'.
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.
Warn whenever a comment-start sequence `/*' appears in a `/*'
comment, or whenever a Backslash-Newline appears in a `//' comment.
Check calls to `printf' and `scanf', etc., to make sure that the
arguments supplied have types appropriate to the format string
Warn when a declaration does not specify a type.
Warn whenever a function is used before being declared.
Same as `-Wimplicit-int' `-Wimplicit-function-declaration'.
Warn if the type of `main' is suspicious. `main' should be a
function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types.
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people
often get confused about.
Also warn about constructions where there may be confusion to which
`if' statement an `else' branch belongs. Here is an example of
such a case:
if (a)
if (b)
foo ();
bar ();
In C, every `else' branch belongs to the innermost possible `if'
statement, which in this example is `if (b)'. This is often not
what the programmer expected, as illustrated in the above example
by indentation the programmer chose. When there is the potential
for this confusion, GNU C will issue a warning when this flag is
specified. To eliminate the warning, add explicit braces around
the innermost `if' statement so there is no way the `else' could
belong to the enclosing `if'. The resulting code would look like
if (a)
if (b)
foo ();
bar ();
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'.
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.
Warn if any trigraphs are encountered (assuming they are enabled).
Warn whenever a variable is unused aside from its declaration,
whenever a function is declared static but never defined, whenever
a label is declared but not used, and whenever a statement
computes a result that is explicitly not used.
In order to get a warning about an unused function parameter, you
must specify both `-W' and `-Wunused'.
To suppress this warning for an expression, simply cast it to
void. For unused variables and parameters, use the `unused'
attribute (*note Variable Attributes::.).
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;
case 2: x = 4;
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
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 all the
functions you use that never return as `noreturn'. *Note Function
`-Wreorder (C++ only)'
Warn when the order of member initializers given in the code does
not match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
Here the compiler will warn that the member initializers for `i'
and `j' will be rearranged to match the declaration order of the
When using templates in a C++ program, warn if debugging is not yet
fully available (C++ only).
All of the above `-W' options combined. This enables all the
warnings about constructions that some users consider
questionable, and that are easy to avoid (or modify to prevent the
warning), even in conjunction with macros.
The following `-W...' options are not implied by `-Wall'. Some of
them warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid in
some cases, and there is no simple way to modify the code to suppress
the warning.
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 or the left-hand side of a comma
expression contains no side effects. To suppress the
warning, cast the unused expression to void. For example, an
expression such as `x[i,j]' will cause a warning, but
`x[(void)i,j]' will not.
* 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.
* If `-Wall' or `-Wunused' is also specified, warn about unused
* A comparison between signed and unsigned values could produce
an incorrect result when the signed value is converted to
unsigned. (But do not warn if `-Wno-sign-compare' is also
* 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 };
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'.
Warn if an undefined identifier is evaluated in an `#if' directive.
Warn whenever a local variable shadows another local variable.
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.
Warn whenever an object of larger than LEN bytes is defined.
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
Warn whenever a function call is cast to a non-matching type. For
example, warn if `int malloc()' is cast to `anything *'.
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 *'.
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.
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.
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
Also, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the
assignment `x = -1' if `x' is unsigned. But do not warn about
explicit casts like `(unsigned) -1'.
Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. This warning is also enabled by `-W'; to get the other
warnings of `-W' without this warning, use `-W -Wno-sign-compare'.
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.)
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.)
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.
Warn if a global function is defined without a previous
declaration. Do so even if the definition itself provides a
prototype. Use this option to detect global functions that are
not declared in header files.
Warn if anything is declared more than once in the same scope,
even in cases where multiple declaration is valid and changes
Warn if an `extern' declaration is encountered within an function.
Warn if a function can not be inlined, and either it was declared
as inline, or else the `-finline-functions' option was given.
Warn if an old-style (C-style) cast is used within a program.
Warn when a derived class function declaration may be an error in
defining a virtual function (C++ only). In a derived class, the
definitions of virtual functions must match the type signature of a
virtual function declared in the base class. With this option, the
compiler warns when you define a function with the same name as a
virtual function, but with a type signature that does not match any
declarations from the base class.
`-Wsynth (C++ only)'
Warn when g++'s synthesis behavior does not match that of cfront.
For instance:
struct A {
operator int ();
A& operator = (int);
main ()
A a,b;
a = b;
In this example, g++ will synthesize a default `A& operator =
(const A&);', while cfront will use the user-defined `operator ='.
Make all warnings into errors.

File:, 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:
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-1+', or
`-gdwarf-1' (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.
Produce debugging information for use by GDB. This means to use
the most expressive format available (DWARF 2, stabs, or the
native format if neither of those are supported), including GDB
extensions if at all possible.
Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by
DBX on most BSD systems. On MIPS, Alpha and System V Release 4
systems this option produces stabs debugging output which is not
understood by DBX or SDB. On System V Release 4 systems this
option requires the GNU assembler.
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.
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.
Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
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, and may cause
assemblers other than the GNU assembler (GAS) to fail with an
Produce debugging information in DWARF version 1 format (if that is
supported). This is the format used by SDB on most System V
Release 4 systems.
Produce debugging information in DWARF version 1 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.
Produce debugging information in DWARF version 2 format (if that is
supported). This is the format used by DBX on IRIX 6.
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'.
Generate extra code to write profile information suitable for the
analysis program `prof'. You must use this option when compiling
the source files you want data about, and you must also use it when
Generate extra code to write profile information suitable for the
analysis program `gprof'. You must use this option when compiling
the source files you want data about, and you must also use it when
Generate extra code to write profile information for basic blocks,
which will record the number of times each basic block is
executed, the basic block start address, and the function name
containing the basic block. If `-g' is used, the line number and
filename of the start of the basic block will also be recorded.
If not overridden by the machine description, the default action is
to append to the text file `bb.out'.
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.
Generate extra code to profile basic blocks. Your executable will
produce output that is a superset of that produced when `-a' is
used. Additional output is the source and target address of the
basic blocks where a jump takes place, the number of times a jump
is executed, and (optionally) the complete sequence of basic
blocks being executed. The output is appended to file `bb.out'.
You can examine different profiling aspects without recompilation.
Your executable will read a list of function names from file
`'. Profiling starts when a function on the list is entered
and stops when that invocation is exited. To exclude a function
from profiling, prefix its name with `-'. If a function name is
not unique, you can disambiguate it by writing it in the form
`/path/filename.d:functionname'. Your executable will write the
available paths and filenames in file `bb.out'.
Several function names have a special meaning:
Write source, target and frequency of jumps to file `bb.out'.
Exclude function calls from frequency count.
Include function returns in frequency count.
Write the sequence of basic blocks executed to file
`bbtrace.gz'. The file will be compressed using the program
`gzip', which must exist in your `PATH'. On systems without
the `popen' function, the file will be named `bbtrace' and
will not be compressed. *Profiling for even a few seconds on
these systems will produce a very large file.* Note:
`__bb_hidecall__' and `__bb_showret__' will not affect the
sequence written to `bbtrace.gz'.
Here's a short example using different profiling parameters in
file `'. Assume function `foo' consists of basic blocks 1
and 2 and is called twice from block 3 of function `main'. After
the calls, block 3 transfers control to block 4 of `main'.
With `__bb_trace__' and `main' contained in file `', the
following sequence of blocks is written to file `bbtrace.gz': 0 3
1 2 1 2 4. The return from block 2 to block 3 is not shown,
because the return is to a point inside the block and not to the
top. The block address 0 always indicates, that control is
transferred to the trace from somewhere outside the observed
functions. With `-foo' added to `', the blocks of function
`foo' are removed from the trace, so only 0 3 4 remains.
With `__bb_jumps__' and `main' contained in file `', jump
frequencies will be written to file `bb.out'. The frequencies are
obtained by constructing a trace of blocks and incrementing a
counter for every neighbouring pair of blocks in the trace. The
trace 0 3 1 2 1 2 4 displays the following frequencies:
Jump from block 0x0 to block 0x3 executed 1 time(s)
Jump from block 0x3 to block 0x1 executed 1 time(s)
Jump from block 0x1 to block 0x2 executed 2 time(s)
Jump from block 0x2 to block 0x1 executed 1 time(s)
Jump from block 0x2 to block 0x4 executed 1 time(s)
With `__bb_hidecall__', control transfer due to call instructions
is removed from the trace, that is the trace is cut into three
parts: 0 3 4, 0 1 2 and 0 1 2. With `__bb_showret__', control
transfer due to return instructions is added to the trace. The
trace becomes: 0 3 1 2 3 1 2 3 4. Note, that this trace is not
the same, as the sequence written to `bbtrace.gz'. It is solely
used for counting jump frequencies.
Instrument "arcs" during compilation. For each function of your
program, GNU CC creates a program flow graph, then finds a
spanning tree for the graph. Only arcs that are not on the
spanning tree have to be instrumented: the compiler adds code to
count the number of times that these arcs are executed. When an
arc is the only exit or only entrance to a block, the
instrumentation code can be added to the block; otherwise, a new
basic block must be created to hold the instrumentation code.
Since not every arc in the program must be instrumented, programs
compiled with this option run faster than programs compiled with
`-a', which adds instrumentation code to every basic block in the
program. The tradeoff: since `gcov' does not have execution
counts for all branches, it must start with the execution counts
for the instrumented branches, and then iterate over the program
flow graph until the entire graph has been solved. Hence, `gcov'
runs a little more slowly than a program which uses information
from `-a'.
`-fprofile-arcs' also makes it possible to estimate branch
probabilities, and to calculate basic block execution counts. In
general, basic block execution counts do not give enough
information to estimate all branch probabilities. When the
compiled program exits, it saves the arc execution counts to a
file called `SOURCENAME.da'. Use the compiler option
`-fbranch-probabilities' (*note Options that Control Optimization:
Optimize Options.) when recompiling, to optimize using estimated
branch probabilities.
Create data files for the `gcov' code-coverage utility (*note
`gcov': a GNU CC Test Coverage Program: Gcov.). The data file
names begin with the name of your source file:
A mapping from basic blocks to line numbers, which `gcov'
uses to associate basic block execution counts with line
A list of all arcs in the program flow graph. This allows
`gcov' to reconstruct the program flow graph, so that it can
compute all basic block and arc execution counts from the
information in the `SOURCENAME.da' file (this last file is
the output from `-fprofile-arcs').
Makes the compiler print out each function name as it is compiled,
and print some statistics about each pass when it finishes.
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:
Dump all macro definitions, at the end of preprocessing, and
write no output.
Dump all macro names, at the end of preprocessing.
Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
Dump debugging information during parsing, to standard error.
Dump after RTL generation, to `FILE.rtl'.
Just generate RTL for a function instead of compiling it.
Usually used with `r'.
Dump after first jump optimization, to `FILE.jump'.
Dump after CSE (including the jump optimization that sometimes
follows CSE), to `FILE.cse'.
Dump after purging ADDRESSOF, to `FILE.addressof'.
Dump after loop optimization, to `FILE.loop'.
Dump after the second CSE pass (including the jump
optimization that sometimes follows CSE), to `FILE.cse2'.
Dump after computing branch probabilities, to `FILE.bp'.
Dump after flow analysis, to `FILE.flow'.
Dump after instruction combination, to the file
Dump after the first instruction scheduling pass, to
Dump after local register allocation, to `FILE.lreg'.
Dump after global register allocation, to `FILE.greg'.
Dump after the second instruction scheduling pass, to
Dump after last jump optimization, to `FILE.jump2'.
Dump after delayed branch scheduling, to `FILE.dbr'.
Dump after conversion from registers to stack, to
Produce all the dumps listed above.
Print statistics on memory usage, at the end of the run, to
standard error.
Annotate the assembler output with a comment indicating which
pattern and alternative was used.
Annotate the assembler output with miscellaneous debugging
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.
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'.
Print the full absolute name of the library file LIBRARY that
would be used when linking--and don't do anything else. With this
option, GNU CC does not compile or link anything; it just prints
the file name.
Like `-print-file-name', but searches for a program such as `cpp'.
Same as `-print-file-name=libgcc.a'.
This is useful when you use `-nostdlib' or `-nodefaultlibs' but
you do want to link with `libgcc.a'. You can do
gcc -nostdlib FILES... `gcc -print-libgcc-file-name`
Print the name of the configured installation directory and a list
of program and library directories gcc will search--and don't do
anything else.
This is useful when gcc prints the error message `installation
problem, cannot exec cpp: No such file or directory'. To resolve
this you either need to put `cpp' and the other compiler
components where gcc expects to find them, or you can set the
environment variable `GCC_EXEC_PREFIX' to the directory where you
installed them. Don't forget the trailing '/'. *Note Environment

File:, Node: Optimize Options, Next: Preprocessor Options, Prev: Debugging Options, Up: Invoking GCC
Options That Control Optimization
These options control various sorts of optimizations:
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', the compiler only allocates variables declared
`register' 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
When you specify `-O', the compiler turns on `-fthread-jumps' and
`-fdefer-pop' on all machines. The compiler turns on
`-fdelayed-branch' on machines that have delay slots, and
`-fomit-frame-pointer' on machines that can support debugging even
without a frame pointer. On some machines the compiler also turns
on other flags.
Optimize even more. GNU CC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. The
compiler does not perform loop unrolling or function inlining when
you specify `-O2'. As compared to `-O', this option increases
both compilation time and the performance of the generated code.
`-O2' turns on all optional optimizations except for loop unrolling
and function inlining. It also turns on the `-fforce-mem' option
on all machines and frame pointer elimination on machines where
doing so does not interfere with debugging.
Optimize yet more. `-O3' turns on all optimizations specified by
`-O2' and also turns on the `inline-functions' option.
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.
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. Similarly for the
x86 architecture. 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.
Do not make member functions inline by default merely because they
are defined inside the class scope (C++ only). Otherwise, when
you specify `-O', member functions defined inside class scope are
compiled inline by default; i.e., you don't need to add `inline'
in front of the member function name.
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.
Force memory operands to be copied into registers before doing
arithmetic on them. This produces better code by making all memory
references potential common subexpressions. When they are not
common subexpressions, instruction combination should eliminate
the separate register-load. The `-O2' option turns on this option.
Force memory address constants to be copied into registers before
doing arithmetic on them. This may produce better code just as
`-fforce-mem' may.
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
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::.
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
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.
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. This switch does not
affect `extern inline' functions.
Emit variables declared `static const' when optimization isn't
turned on, even if the variables aren't referenced.
GNU CC enables this option by default. If you want to force the
compiler to check if the variable was referenced, regardless of
whether or not optimization is turned on, use the
`-fno-keep-static-consts' option.
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
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.
This option allows GCC to violate some ANSI or IEEE rules and/or
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 and that no floating-point values
are NaNs.
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
The following options control specific optimizations. The `-O2'
option turns on all of these optimizations except `-funroll-loops' and
`-funroll-all-loops'. On most machines, the `-O' option turns on the
`-fthread-jumps' and `-fdelayed-branch' options, but specific machines
may handle it differently.
You can use the following flags in the rare cases when "fine-tuning"
of optimizations to be performed is desired.
Perform the optimizations of loop strength reduction and
elimination of iteration variables.
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.
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
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'.
Re-run common subexpression elimination after loop optimizations
has been performed.
Perform a number of minor optimizations that are relatively
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed
branch instructions.
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.
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.
Place each function into its own section in the output file if the
target supports arbitrary sections. The function's name determines
the section's name in the output file.
Use this option on systems where the linker can perform
optimizations to improve locality of reference in the instruction
space. HPPA processors running HP-UX and Sparc processors running
Solaris 2 have linkers with such optimizations. Other systems
using the ELF object format as well as AIX may have these
optimizations in the future.
Only use this option when there are significant benefits from doing
so. When you specify this option, the assembler and linker will
create larger object and executable files and will also be slower.
You will not be able to use `gprof' on all systems if you specify
this option and you may have problems with debugging if you
specify both this option and `-g'.
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.
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 both `-fstrength-reduce' and
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' as well as
Disable any machine-specific peephole optimizations.
After running a program compiled with `-fprofile-arcs' (*note
Options for Debugging Your Program or `gcc': Debugging Options.),
you can compile it a second time using `-fbranch-probabilities',
to improve optimizations based on guessing the path a branch might
With `-fbranch-probabilities', GNU CC puts a `REG_EXEC_COUNT' note
on the first instruction of each basic block, and a `REG_BR_PROB'
note on each `JUMP_INSN' and `CALL_INSN'. These can be used to
improve optimization. Currently, they are only used in one place:
in `reorg.c', instead of guessing which path a branch is mostly to
take, the `REG_BR_PROB' values are used to exactly determine which
path is taken more often.