The compiler writers are right in the sense that for every one of those optimizations you can have a microbenchmark that will benefit hugely. The opponents are

The compiler writers are right in the sense that for every one of those optimizations you can have a microbenchmark that will benefit hugely. The opponents are right too, in the sense that for most codebases the more niche optimizations don't matter at all. The right answer, as always, is to take a particular case you actually care about and benchmark yourself. There are no benchmarks that would be generally representative, there is no meaningful "average case". Personally, I mostly use C/C++ for fairly high performance numerical code and happen to benefit greatly from all the "unsafe" stuff, including optimizations only enabled by compilers taking advantage of undefined behavior. I'm therefore naturally inclined to strongly oppose any attempts to eliminate undefined behavior from the language standards. At the same time, however, I fully recognize that most people would probably benefit from a safer set of compiler defaults (as well as actually reading the compiler manuals once in a while) or even using languages with more restrictive semantics. Ultimately, there is no free lunch and performance doesn't come without its costs.