<< shiftedbits

The other day I needed to write a function to give me a formatted string representation of a quantity of bytes. Pretty trivial function, but nontheless, I found myself optimizing it. I ended up with the following:

static uint64_t count = 7;
static NSString *suffixes[7] = 
    { @"B", @"KiB", @"MiB", @"GiB", @"TiB", @"PiB", @"EiB"};
uint64_t i, c;
for (i = 1024, c = 0; i < (count << 60); i <<= 10, c++) {
    if (bytes < i) 
        return [NSString stringWithFormat:  @"%0.2f%@", 
                                            (double)bytes / (double)(i >> 10), 
                                            suffixes[c]];
}
return @"Big";

Now, optimizing away divisions (except for the final conversion to string) really doesn’t save any time at all. The creation and subsequent autorelease of a NSString will take far longer than the division operations, even for large byte counts. Thus, I believe this code sample qualifies as pointless optimization, although it was entertaining to write.

If you’re looking to eek some extra performance out of your drawing code, take a look at how you’re setting your fill and stroke colors. Setting the color using CGContextSetRGBFillColor() or creating a new CGColorRef/NSColor object each time you set the color is quite expensive to do each time you draw. Depending on how many colors you use in your drawing, consider caching the CGColorRef objects and using CGContextSetFillColorWithColor() or -[NSColor setFill] instead.

A CGColorRef is 48 bytes for a RGB color. A NSColor is 32 bytes for a RGB color, plus 48 bytes for a CGColorRef. If you can afford the memory hit, you’ll see quite a bit of speed improvement from caching colors.

Below is the output from a small test app (source available here) that tests various color setting methods: (Each test is 1,000,000 iterations.)

CGContextSetRGBFillColor: 4.368422 seconds
CGContextSetFillColorWithColor, static CGColorRef: 0.599552 seconds
-[NSColor setFillColor], static NSColor: 0.832328 seconds
-[NSColor setFillColor], dynamic NSColor: 5.847799 seconds