Maybe. Maybe you work on GCC (or on Clang) and know the dirty details of the code inside ... I don't.

What I do know is that the performance in past C compilers which emitted every memory access unconditionally was considered "blazing fast".

If this is a blow to performance, how significant is it? Can you compile two benchmarks with and without the 'perform every memory access unconditionally' flag? Is there a flag for this? If no one ever did this, how can we tell there would be an impact on performance, nevermind significant impact?

Because I recall seeing benchmarks for things like eliminating `if (x+100 < x)` showing the performance impact to be not even a rounding error. I also recall seeing a benchmark somewhere for with and without the flag that removes null-checks, and once again that performance impact was, for all practical purposes, zero.

In the era of speculative pipelined execution, in respect of a test/check that would be speculated in the pipeline well before it is needed, how exactly will the code run faster if the test/check is removed from the sources?

Once again, I admit that I am not in the weeds in GCC or LLVM or CLang development, but I fail to see how eliminating an instruction that never slows the processor can have a noticeable impact on performance.

-O0 is close to what you want. For hand optimized code there isn't a huge difference between the various optimization levels, but for higher level code C++ code, the difference between -O0 and -O3 are sometimes one or two orders of magnitude.

And yes, not all, or even most, optimizations apply to all programs. All loop optimizations are really most relevant for numerical, vectorizable code; branchy pointer-chasing code won't benefit from them. Branchy code can benefit from other optimizations though that can bring their own issues.