Yes, instead of making assumptions about what the compiler can or can't see, it's better to just tell it to not look.

This [[dont_look]] function attribute is spelled [[gnu::noipa]] in gcc, I'm not aware of an equivalent for other compilers.


On 11 June 2022 07:39:44 BST, Bo Persson via Std-Proposals <std-proposals@lists.isocpp.org> wrote:
On 2022-06-10 at 23:09, Arthur O'Dwyer via Std-Proposals wrote:
On Fri, Jun 10, 2022 at 4:43 PM Lénárd Szolnoki <cpp@lenardszolnoki.com <mailto:cpp@lenardszolnoki.com>> wrote:

[...] The thing is, if I have a library funciton called
"DoNotOptimize", but
it fails to be an optimization barrier in some circumstances, then it's
not a very good abstraction. Wouldn't it be nice if this could be a
similar optimization barrier, LTO enabled or not? [...]
[...] I'm interested in optimization
barriers in general, or maybe even more generally fine grained control
of optimization hints/options within the code.

[...] All I'm saying is, if your goal is simply
to mystify the optimizing compiler as to whether a particular
variable is dead or whether a particular write to it can be hoisted,
literally all you have to do is escape that variable's address into a
different translation unit (which is exactly what
benchmark::DoNotOptimize does).

Again, with LTO disabled.


I think I need to re-post this paragraph.

Here's another way to look at it: *"Optimization barrier" is not a first-class principle.* Instead, the fundamental organizing principle is *"The tool can't optimize what it can't see."* So, if you really really want something not to be optimized, then you simply mentally reduce the problem to "How can I make my optimizing tool /not see/ this function?"

So perhaps what we need is a [[dont_look]] directive?




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