utiliD: A library with absolutely no dependencies for bare-metal programming and bootstrapping other D libraries

[Johan Engelen]

On Friday, 10 May 2019 at 23:58:37 UTC, Mike Franklin wrote:
>
> I don't know how a proper assembly implementation would not be 
> performant.  Perhaps you could elaborate.

Inline assembly prevents a lot of optimizations that give large 
performance gains such as constant propagation. Say you implement 
a memcpy with a different signature than C's mempcy (because of 
slices instead of pointers), then the optimizer does not know 
what the semantics of that function are and will need the 
function to be transparent (not assembly) to do such 
optimizations.

But I'm sure you know all that, so that's not your question. :)

In the case of reimplementing memcpy/mem* in a function with the 
same signature as libc, that is not supposed to be inlined (like 
the current libc functions), then I also think the use of inline 
asm will not give a perf penalty. Be careful to recreate the 
exact same semantics as those libc functions because the 
optimizer is going to _assume_ it knows _exactly_ what those 
functions are doing.

cheers,
   Johan