Re: GCC performance regression - up to 20% ?

[Andrew Pinski <pinskia@physics.uc.edu>]

On Monday, April 22, 2002, at 09:20 , Carlo Wood wrote:

> On Mon, Apr 22, 2002 at 08:49:20PM -0400, Andrew Pinski wrote:
>> And it was -O3 that included the inline optimization.
>
> Huh??
> I am using the fact (I hope) that functions marked with 'inline'
> or member functions that are defined inside their class declaration
> are inlined already with -O.
> Talking about real small functions here (accessors etc).
>

Read the manual, under the -O3 option:

Optimize yet more. `-O3' turns on all optimizations specified by `-O2' 
and also turns on the `-finline-functions' and `-frename-registers' 
options.

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. 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, 
function inlining, and register renaming. 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.

Please note the warning under `-fgcse' about invoking `-O2' on programs 
that use computed gotos.

-O3
Optimize yet more. `-O3' turns on all optimizations specified by `-O2' 
and also turns on the `-finline-functions' and `-frename-registers' 
options.

-O0
Do not optimize.

-Os
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.





> --
> Carlo Wood <carlo@alinoe.com>
>
>