Hi all,

As a general head-up. I'm trying to fix a couple of these warnings in the AI code right now. Most of it is just boring stuff. Ideal to fill a couple of hours on a rainy monday afternoon.

Cheers,
Durk

On Mon, Sep 27, 2010 at 9:47 AM, James Turner <zakalawe@mac.com> wrote:

On 26 Sep 2010, at 22:09, ThorstenB wrote:

> Hi,
> there is a forum topic discussing compiler optimization to improve
> frame rate ( http://www.flightgear.org/forums/viewtopic.php?f=45&p=96830
> ).
> I have also tried this (and successfully improved mine... :-) ).
>
> However, I also compiled with "-Wall" and this shows several issues
> with the FlightGear sources. Simgear is really clean though (no
> warnings with g++ 4.4, *clapclap*).
>
> I'm seeing 55 warnings with FlightGear: mainly (potential) usage of
> uninitialized variables and violations of strict aliasing. The latter
> easily results in broken code when optimizing with "-O2" or above (oh
> no, "-O2" is the FlightGear default!). Well, and there is another 54
> warnings in the AI module, but only since the member variables aren't
> initialized in the sequence of their declaration (very easily fixed).

In general, fixing warnings and switching on -Wall would get my vote; various people have done work on this over the past few years - Torsten Dryer and myself recently, but several others too. I'm not sure we can achieve the equivalent for all compilers, but requiring -Wall and possibly -Werror (optionally!) for GCC 4.4 would be nice. Patches welcome :)

Regarding the compile flags, I'd prefer to use the default system CXXFLAGS by default, but provide a --aggressive or --optimised configure option that does this.

Apple strongly recommend -Os over -O2 or -O3, as the default for production code, with -O2 or -O3 enabled on a file-by-file or module-by-module basis. This is really Apple saying they think i-cache coherency is a bigger win than massive loop unrolling, inlining and the like. They also recommend using profiling to identify the actual hot functions, since I'd guess the hot functions in the FlightGear frame loop are in OSG, not FlightGear itself. Even the FDMs aren't that hot, comparatively.

If anyone could compare -Os with -O3 (with march=native and SSE enabled), and post the frame-rate difference, I'd be interested. (The comments above do date from the PPC days, some of which were i-cache starved)

The SSE math flags are a no-brainer where supported - the -march-native and -sse flags are all Apple defaults in Xcode.

James


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