In the 80s the hand written assembly was more reliable and performant than the C, at least on many of the compilers.

Even in 1990, I tried to launch a serious project in C++ on the IBM-PC, and the best available compiler was too buggy to use. It did fine for little demo apps, but by the time you wrote code for 2 weeks, you started hitting bugs - not in your code but in the compiler output... we had to fall back to C for the project. Even later, around 1994, we had two C compilers for 6811 work and one of them was garbage, I could hand write the assembly better and faster without even trying hard. The other one was pretty good, and by the late 1990s I stopped looking at C/C++ compilers' assembly output because it was consistently better than I would write by hand.