Yes, C provides a way to talk to ABIs, in addition to APIs. It's not just "talking to hardware" it's talking to other software in a reliable way, such that you can upgrade your C compiler and have code written in C89 talk to code written in C11 which is unheard of in most of the other languages that don't support an ABI. (Think Python2 being incompatible with Python3)
Software has gotten much faster. Yes, almost all software benefits from -O3. What do you mean "hardcode"? as far as I know libjpeg can be linked statically...
UB is easy to maintain, lets take integer addition & overflow, you just issue an ADD instruction and however that CPU executes the ADD instruction is how integer overflow works on that platform and then in the C standard you write "integer overflow is undefined behavior".
Software has gotten much faster. Yes, almost all software benefits from -O3. What do you mean "hardcode"? as far as I know libjpeg can be linked statically...
UB is easy to maintain, lets take integer addition & overflow, you just issue an ADD instruction and however that CPU executes the ADD instruction is how integer overflow works on that platform and then in the C standard you write "integer overflow is undefined behavior".