Old skool book recommendation. Not necessarily C, more ASM but still a good book for the early nineties optimization that still has a mentality that's still valid. I believe that people today would be more concerned about how fast their code runs if it was visible, that is, if they profiled it with good tools!

https://www.amazon.com/Inner-Loops-Sourcebook-Software-Devel...

People interested in this might also enjoy 'Inner Loops' (1997), an excellent book that covers optimizing x86 at that time but is instructive in general and the concepts are still applicable today.

https://www.amazon.com/Inner-Loops-Sourcebook-Software-Devel...

If you love this kinda stuff, Inner Loops [0] is an excellent book that covers low level performance optimization. And the author Rick Booth handles it in a way that allows the reader to transfer the techniques to new platforms.

I like it so much I buy used copies and give them as gifts.

[0] http://www.amazon.com/Inner-Loops-Sourcebook-Software-Develo...