Preventing the Compiler from Optimizing Away Benchmarks

[user1234]

On Monday, 13 March 2023 at 14:17:57 UTC, jmh530 wrote:
> I was looking at [1] for ways to prevent the compiler from 
> optimizing away code when trying to benchmark.
>
> It has the following C++ code as a simpler version:
> ```
> inline BENCHMARK_ALWAYS_INLINE void DoNotOptimize(Tp& value) {
>     asm volatile("" : "+r,m"(value) : : "memory");
> }
> ```
>
> I made an attempt to make a D version of it, but failed. 
> Apparently DMD doesn't like the `""` in the first part of the 
> asm instruction. I'm also not sure the `volatileLoad` command 
> is right, but I didn't know of any other way to have volatile 
> work in D (and I couldn't figure out how it actually worked 
> from looking at the code).
>
> ```
> void DoNotOptimize(T)(T* ptr)
> {
>     import core.volatile: volatileLoad;
>     T value = volatileLoad(ptr);
>     asm {"" : "+r,m"(value) : : "memory";}
> }
> ```
>
> [1] 
> https://theunixzoo.co.uk/blog/2021-10-14-preventing-optimisations.html

that's illegal code. You mix GCC/LLVM syntax with D asm block and 
the front-end wont recognize that.

LDC recognizes a syntax similar to what is described in your 
link, see https://wiki.dlang.org/LDC_inline_assembly_expressions. 
GDC has it too (since that the syntax invented by GCC in first 
place)  but I cant find the documentation ATM.