I have written an std::vectort<T> performance test (T in int, bool, char, int_fast8_t, uint_fast8_t). It has two phases: writing and reading. I’m not afraid of writing being optimized away as I later read the container. However when I force the read to be not optimized away I suspect that I destruct other optimizations as well. The kind that could happen in a “normal” program. I base my suspicion on the fact that writing is many times faster than reading.

I prevent reading being optimized away with this function that I copied from somewhere:

template <typename T>
inline void doNotOptimizeAway(T&& datum) {
    asm volatile ("" : "+r" (datum));
}

I use it as:

for(T val: vec)
  doNotOptimizeAway(val);

Test results are (cut, formatted):

         int: write  84 ms; read 145 ms.
        bool: write 414 ms; read 458 ms.
        char: write  21 ms; read 141 ms.
 int_fast8_t: write  21 ms; read 141 ms.
uint_fast8_t: write  21 ms; read 140 ms.

Good signs:

• char, int_fast8_t, uint_fast8_t reads and writes are of the same speed.
• The write for these types is quite precisely 4 times faster than for int.
• bool is very slow as expected, because of storage optimized specialization.

Bad signs:

• Reading is slower than writing for all non-bool cases.

Complete source: https://pastebin.com/weN6wtpq

The question: How can I prevent the reads being optimized away without destroying other optimizations?