I think part of what they on the Apple developer pages should be taken with a grain of salt. Yes, there can undoubtedly be times when -Os will produce a sequence of instructions that are shorter and quicker than -O2. The second URL even shows an example. That said, I believe the gcc folks would claim that the overall performance of the majority of applications benefit from -O2 over -Os. Not only that, but they would say of performance is critical, do not rely on -O2 or -O3, but to use -fprofile-arcs to instrument your code, run a representative work load on the instrumented code and then re-compile with -fprofile-use.
Ryan On 6/11/07, Daniel J. Luke <[EMAIL PROTECTED]> wrote:
On Jun 11, 2007, at 9:38 AM, Jochen Küpper wrote: > On 11.06.2007, at 11:02, Pierre Queinnec wrote: >> IIRC Apple recommends -Os, not -O2. > > Why? > Where is that documented? I've seen it elsewhere, but in this link, the "Fastest, smallest" Xcode setting corresponds to -Os http://developer.apple.com/documentation/Performance/Conceptual/ CodeFootprint/Articles/CompilerOptions.html There's also discussion here: http://developer.apple.com/tools/gcc_overview.html Which mentions - "When in doubt, it may be better to optimize for size, since smaller code may execute relatively faster. For example, large functions or loops containing data access patterns that do not exhibit a strong locality of reference may not fit into a processor's cache lines, which can lead to cache misses and subsequent fetches from memory. Smaller functions or loops with local data access stand a better chance of fitting in a given cache line and requiring fewer memory accesses." -- Daniel J. Luke +========================================================+ | *---------------- [EMAIL PROTECTED] ----------------* | | *-------------- http://www.geeklair.net -------------* | +========================================================+ | Opinions expressed are mine and do not necessarily | | reflect the opinions of my employer. | +========================================================+ _______________________________________________ macports-users mailing list [email protected] http://lists.macosforge.org/mailman/listinfo/macports-users
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