In fact, does anyone know whether LLVM/GCC can target AVR well? I recently had the displeasure of staring at what sdcc produces from pretty reasonable-looking C... (it doesn't help that some 8-bit microcontrollers are really not C processors.)
I wrote a bunch of stuff for the AVR in c. But it's been a few years since I've done much more than a few fixes. Some of the weird issues with targeting a Harvard architecture is I think fixed.
I'd say: 'works okay'
One thing that annoyed me was calling a variadic function like printf was expensive ~100-120 bytes each call. Eight to Ten printf's --> 1k of flash gone. Far as I could tell most of it was pushing and popping stuff on the stack one byte at a time.
I ported a bunch of AVR code to an ARM Cortex[1]. Code size didn't increase much.
[1] ARM Processors are approximately half the cost of an AVR. Seriously not important if you're building 1000's of something. Important if you're building 100,000s.
GCC at least is just fine. Was involved in developing an embedded system which was controlled by an AVR. There were some cases of unexpectedly big program sizes. But most was due to using large integers unnecessary (higher than 8 bit) and one case of overly aggressive inlining. Don't remember offhand how we solved the last one, but just took a day or two.
