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<A NAME="PAGENUM-188"><P>Page 188</P></A>
<P><B>
OPTIMIZATION OPTIONS
</B>
</P>
<P>These options control various sorts of optimizations:
</P>
<TABLE>
<TR><TD>
_O, _O1
</TD><TD>
Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for
a large function.<BR>
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.<BR>
Without _O, only variables declared register are allocated in registers. The resulting
compiled code is a little worse than produced by PCC without
_O.<BR>
With _O, the compiler tries to reduce code size and execution time.<BR>
When you specify _O, the two options
_fthread_jumps and _fdefer_pop are turned on. On machines that have delay slots, the
_fdelayed_branch option is turned on. For those
machines that can support debugging even without a frame pointer, the
_fomit_frame_pointer option is turned on. On some machines other flags may also be turned on.
</TD></TR><TR><TD>
_O2
</TD><TD>
Optimize even more. Nearly all supported optimizations that do not involve a
space-speed tradeoff are performed. Loop unrolling and function inlining are not done, for example.
As compared to _O, this option increases both compilation time and the performance of
the generated code.
</TD></TR><TR><TD>
_O3
</TD><TD>
Optimize yet more. This turns on everything
_O2 does, along with also turning on
_finline_functions.
</TD></TR><TR><TD>
_O0
</TD><TD>
Do not optimize.<BR>
If you use multiple _O options, with or without level numbers, the last such option is the
one that is effective.
</TD></TR></TABLE>
<P>Options of the form _f flag specify machine-independent flags. Most flags have both positive and negative forms;
the negative form of _ffoo would be _fno_foo. The following list shows only one form—the one which is not the default.
You can figure out the other form by either removing
no_ or adding it.
</P>
<TABLE>
<TR><TD>
_ffloat_store
</TD><TD>
Do not store floating-point variables in registers. This 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.<BR>
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.
</TD></TR><TR><TD>
_fmemorize_lookups
</TD><TD>
Use heuristics to compile faster (C++ only). These heuristics are not enabled by default,
</TD></TR><TR><TD>
_fsave_memorized
</TD><TD>
since they are only effective for certain input files. Other input files compile more slowly.<BR>
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"<< \ << " 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.
_fmemorize_ lookups enables the software cache.<BR>
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
_fmemorize_lookups flag, the cache is flushed after every function that is compiled. The
_fsave_memorized flag
</TD></TR></TABLE>
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