test::black_box for big size T?

[kkimdev]

I'm not sure if this is a bug or not, but I noticed that FB Folly has two versions of doNotOptimizeAway

https://github.com/facebook/folly/blob/1c098e2758d1bd55c48ffa26dab389dbbe96a666/folly/Benchmark.h#L308

asm volatile("" ::"r"(datum));
asm volatile("" ::"m"(datum) : "memory");

First one for size <= long, second one for larger sizes. Where as our test::black_box only has the first version. Is the second one needed?

[kkimdev]

@nagisa @huonw

[nagisa]

We do

asm!("" : : "r"(&dummy))

which is pretty much the same as "m"(dummy). We could make it

unsafe { asm!(""::"*m"(&mut x)); }

in order to use a technically correct reference type and avoid a single instruction for putting the address into a register, but it seems like a wasted effort to me.

[kkimdev]

fwiw, this if Folly's commit log facebook/folly@81d9192

[Mark-Simulacrum]

Can we just pass &T instead of T directly to black_box? I'm not familiar with the implementation so I don't really know.

[dtolnay]

I don't know what the right answers are, but tagging feature-accepted because I would be interested in seeing a PR that applies some of the ideas from doNotOptimizeAway to our black_box.

[steveklabnik]

Triage: don't believe black_box has been modified in a very long time.