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Re: [MiNT] Why not O3


On Mon, Apr 02, 2001 at 02:19:48PM +0200, you wrote:
> Because -O3 is the same as -O2 -funroll-loops, IIRC, which produces larger
> binaries not gaining any significant speedup.

Not quite:

-- snip --
     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.  GCC 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.

     Optimize for size.  `-Os' enables all `-O2' optimizations that do
     not typically increase code size.  It also performs further
     optimizations designed to reduce code size.

     If you use multiple `-O' options, with or without level numbers,
     the last such option is the one that is effective.
-- snip --

Thus, -O3 enables -finline-functions, which - quite similar to
-funroll-loops - produces code that's much larger, but in general not
that much faster. It's IMO much more effective to decide yourself which
functions to inline.



Thomas Binder (Gryf @ IRCNet)  gryf@hrzpub.tu-darmstadt.de
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