Looking at the tangentially linked qrintf project that H2O uses ( https://github.com/h2o/qrintf ), replacing generic sprintf calls with specialised versions for a 10x speed boost - that seems like a brilliant idea, I wonder why it took so long for somebody to think of it?
Also, http://www.ciselant.de/projects/gcc_printf/gcc_printf.html#p... shows gcc did something in this area 9 years ago.
http://www.cygwin.com/ml/libc-hacker/2001-08/msg00003.html indicates gcc had printf optimizations in 2001.
I expect there are older examples.
qrintf seems to be 'just' a more aggressive version of this.
I guess being more aggressive makes sense in applications that do a lot of simple string formatting.