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<A NAME="PAGENUM-189"><P>Page 189</P></A>
<TABLE>
<TR><TD>
</TD><TD>
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.
</TD></TR><TR><TD>
_fno_default_inline
</TD><TD>
Don't make member functions inline by default merely because they are defined inside the
class scope (C++ only).
</TD></TR><TR><TD>
_fno_defer_pop
</TD><TD>
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.
</TD></TR><TR><TD>
_fforce_mem
</TD><TD>
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.
</TD></TR><TR><TD>
_fforce_addr
</TD><TD>
Force memory address constants to be copied into registers before doing arithmetic on
them.<BR>
This may produce better code just as
_fforce_mem may. I am interested in hearing about
the difference this makes.
</TD></TR><TR><TD>
_fomit_frame_pointer
</TD><TD>
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 most machines.<BR>
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.
</TD></TR><TR><TD>
_finline_functions
</TD><TD>
Integrate all simple functions into their callers. The compiler heuristically decides
which functions are simple enough to be worth integrating in this way.<BR>
If all calls to a given function are integrated, and the function is declared
static, then gcc normally does not output the function as assembler code in its own right.
</TD></TR><TR><TD>
_fcaller_saves
</TD><TD>
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.<BR>
This option is enabled by default on certain machines, usually those which have no
call-preserved registers to use instead.
</TD></TR><TR><TD>
_fkeep_inline_functions
</TD><TD>
Even if all calls to a given function are integrated, and the function is declared
static, nevertheless output a separate runtime callable version of the function.
</TD></TR><TR><TD>
_fno_function_cse
</TD><TD>
Do not put function addresses in registers; make each instruction that calls a constant
function contain the function's address explicitly.<BR>
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.
</TD></TR><TR><TD>
_fno_peephole
</TD><TD>
Disable any machine-specific peephole optimizations.
</TD></TR><TR><TD>
_ffast-math
</TD><TD>
This option allows gcc to violate some ANSI or IEEE specifications in the interest of
optimizing code for speed. For example, it allows the compiler to assume arguments to the
sqrt function are nonnegative numbers.<BR>
This option should never be turned on by any
_O option because it can result in incorrect output for programs which depend on an exact implementation of IEEE or ANSI
rules/specifications for math functions.
</TD></TR></TABLE>
<P>The following options control specific optimizations. The
_O2 option turns on all of these optimizations except
_funroll_loops and _funroll_all_loops.
</P>
<P>The _O option usually turns on the
_fthread_jumps and _fdelayed_branch options, but specific machines may change
the default optimizations.
</P>
<A NAME="PAGENUM-190"><P>Page 190</P></A>
<P>You can use the following flags in the rare cases when fine-tuning of optimizations to be performed is desired:
</P>
<TABLE>
<TR><TD>
_fstrength_reduce
</TD><TD>
Perform the optimizations of loop strength reduction and elimination of iteration variables.
</TD></TR><TR><TD>
_fthread_jumps
</TD><TD>
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.
</TD></TR><TR><TD>
_funroll_loops
</TD><TD>
Perform the optimization of loop unrolling. This is only done for loops whose number
of iterations can be determined at compile time or runtime.
</TD></TR><TR><TD>
_funroll_all_loops
</TD><TD>
Perform the optimization of loop unrolling. This is done for all loops. This usually
makes programs run more slowly.
</TD></TR><TR><TD>
_fcse_follow_jumps
</TD><TD>
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.
</TD></TR><TR><TD>
_fcse_skip_blocks
</TD><TD>
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.
</TD></TR><TR><TD>
_frerun_cse_after_loop
</TD><TD>
Rerun common subexpression elimination after loop optimizations has been performed.
</TD></TR><TR><TD>
_felide_constructors
</TD><TD>
Elide constructors when this seems plausible (C++ only). With this flag, GNU C++ initializes
y directly from the call to foo without going through a temporary in the following code:<BR>
A foo (); A y = foo ();<BR>
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.<BR>
The default behavior
(_fno_elide_constructors) is specified by the draft ANSI C++ standard.
If your program's constructors have side effects, using
_felide-constructors can make your program act differently, since some constructor calls may be omitted.
</TD></TR><TR><TD>
_fexpensive_optimizations
</TD><TD>
Perform a number of minor optimizations that are relatively expensive.
</TD></TR><TR><TD>
_fdelayed_branch
</TD><TD>
If supported for the target machine, attempt to reorder instructions to exploit instruction
slots available after delayed branch instructions.
</TD></TR><TR><TD>
_fschedule_insns
</TD><TD>
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.
</TD></TR><TR><TD>
_fschedule_insns2
</TD><TD>
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.
</TD></TR></TABLE>
<P><B>
TARGET OPTIONS
</B>
</P>
<P>By default, GNU CC compiles code for the same type of machine that you are using.
</P>
<P>However, it can also be installed as a cross-compiler, to compile for some other type of machine. In fact, several
different configurations of GNU CC, for different target machines, can be installed side by side. Then you specify which one to
use with the _b option.
</P>
<P>In addition, older and newer versions of GNU CC can be installed side by side. One of them (probably the newest) will
be the default, but you may sometimes want to use another.
</P>
<TABLE>
<TR><TD>
_b machine
</TD><TD>
The argument machine specifies the target machine for compilation. This is useful when
you have installed GNU CC as a cross-compiler.
The value to use for machine is the same as was specified as the machine type when
configuring GNU CC as a cross-compiler. For example, if a cross-compiler was configured with
configure
</TD></TR></TABLE>
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