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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', 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
time.
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.
-O2
- 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 frame pointer elimination on
machines where doing so does not interfere with debugging.
-O3
- Optimize yet more. `-O3' turns on all optimizations specified by
`-O2' and also turns on the `inline-functions' option.
-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-default-inline
- 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.
-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. See section `Register Usage' in Using and Porting GCC.
-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-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 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 functions.
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.
-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 both `-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' as well as `-frerun-cse-after-loop'.
-fno-peephole
- Disable any machine-specific peephole optimizations.
-fbranch-probabilities
- After running a program compiled with `-fprofile-arcs'
(see section Options for Debugging Your Program or GNU CC), you can compile it a second time using
`-fbranch-probabilities', to improve optimizations based on
guessing the path a branch might take.
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