Hi,

How does this mesh with LTO? I would throw in [[gnu::noipa]] there for gcc, and at least [[gnu::noinline]] for clang (although noinline is probably not enough). But we are back at attributes. I certainly see the utility of fine grained optimisation hint attributes. I'm not sure if it needs standardisation, but it's not without precedent (likely/unlikely).

Cheers,
Lénárd


On 10 June 2022 16:06:01 BST, Arthur O'Dwyer via Std-Proposals <std-proposals_at_[hidden]> wrote:
>On Fri, Jun 10, 2022 at 5:34 AM Marcin Jaczewski via Std-Proposals <
>std-proposals_at_[hidden]> wrote:
>
>> > >> >
>> > >> > void foo(int *p) {
>> > >> > if (!p) { printf("null pointer\n"); }
>> > >> > volatile bool b = false;
>> > >> > if (b) { abort(); }
>> > >> > *p = 0;
>> > >> > }
>> [...]
>> Why not simply name this attribute `[[do_not_optimize]]`?
>
>
>Google Benchmark already has this, and I wouldn't be surprised if more than
>one corporate codebase also has it. It's a pure library
>feature, implementable in plain old standard C++.
>
> // In another .cpp file:
> void escape(void*) {}
>
> // In the header file:
> extern void escape(void*);
> template<class T> void DoNotOptimize(T&& t) { escape(&t); }
>
> void foo(int *p) {
> if (!p) { printf("null pointer\n"); }
> DoNotOptimize(p);
> *p = 0;
> }
>
>The compiler can see that `escape(&p)` is called, but it doesn't know at
>compile time whether `escape` might throw or terminate, so it can't assume
>that the line `*p = 0` will necessarily be reached (so it can't hoist that
>line upward and it can't remove the null-check).
>
>–Arthur