On Wednesday, 22 January 2014 at 13:08:53 UTC, Iain Buclaw wrote:
Personally, I would never use -O3 for low level start.o kernel
stuff.
In my simple D program, however, -O2 also doesn't work.
As you are coding on a small board, wouldn't you instead use
-Os ?
I sometimes use -Os and sometimes use -O2/-O3.
If I'm controlling something low-speed like a refrigerator or
other kitchen appliance, I use -Os so I can use the cheapest chip
available.
However, for my current project, I'm making and HMI/Industrial
controller. The HMI will have a software rendered graphics
engine with vector graphics, alpha blending, TrueType fonts,
etc... I've already built this in C++, and the -O2/-O3 was very
significant in my performance benchmarks. I didn't notice any
difference between -O2 and -O3, though. It uses about 700KB of
Flash memory, and most of that is the TrueType font data, so I'm
quite satisfied with my C++ results.
Interestingly, since I started using GCC 4.8 in my C++ project,
-O3 breaks my memset function, but -O2 does not, so I'm sticking
with -O2 at the moment. With GCC 4.7, -O3 worked fine.
If you can't see any error in my D code and the compiler and
optimizer are working properly, shouldn't my program work at
these optimization levels without resorting to special qualifiers
like shared/volatile?
NOTE: I'll post assembly and the optimization tree when I get
home from work today.
