Zero-cost version-dependent function call at -O0.
How would you solve this problem: do an optional function call
depending on some version(X). If version(X) is not defined, there
should be no call and no extra code at -O0.
```
{
...
foo(); // either compiles to a function call, or to _nothing_.
...
}
```
In C, you could do something like:
```
#if X
void foo() {..}
#else
#define foo()
#endif
```
How would you do this in D?
I can think of `mixin(foo())` but there is probably a nicer way
that preserves normal function calling syntax.
Cheers,
Johan
Re: Zero-cost version-dependent function call at -O0.
On Sunday, 25 June 2017 at 15:58:48 UTC, Johan Engelen wrote:
How would you solve this problem: do an optional function call
depending on some version(X). If version(X) is not defined,
there should be no call and no extra code at -O0.
```
{
...
foo(); // either compiles to a function call, or to
_nothing_.
...
}
```
In C, you could do something like:
```
#if X
void foo() {..}
#else
#define foo()
#endif
```
How would you do this in D?
I can think of `mixin(foo())` but there is probably a nicer way
that preserves normal function calling syntax.
Cheers,
Johan
Am I missing something, or can't you just version both the
function and the function ćall?
version(X)
void foo() { /* ... */ }
void main()
{
version(X)
{
foo();
}
}
Re: Zero-cost version-dependent function call at -O0.
On Sunday, 25 June 2017 at 15:58:48 UTC, Johan Engelen wrote:
[...]
If version(X) is not defined, there should be no call and no
extra code at -O0.
[...]
In C, you could do something like:
```
#if X
void foo() {..}
#else
#define foo()
#endif
```
How would you do this in D?
By requiring the compiler to inline the empty foo:
```
version (Foo)
{
void foo()
{
import std.stdio;
writeln("foo");
}
} else {
pragma(inline, true)
void foo() {}
}
void main(string[] args)
{
foo();
}
```
See [1] for full assembly, shortened output as follows:
```
void example.foo():
ret
_Dmain:
xor eax, eax
mov qword ptr [rsp - 16], rdi
mov qword ptr [rsp - 8], rsi
ret
```
As you can see, while the code for the function itself will still
be emitted, since it's empty the inlining will result in no
instructions as the result.
[1] https://godbolt.org/g/RLt6vN
Re: Zero-cost version-dependent function call at -O0.
On Sunday, 25 June 2017 at 16:31:52 UTC, Moritz Maxeiner wrote:
On Sunday, 25 June 2017 at 15:58:48 UTC, Johan Engelen wrote:
[...]
If version(X) is not defined, there should be no call and no
extra code at -O0.
[...]
In C, you could do something like:
```
#if X
void foo() {..}
#else
#define foo()
#endif
```
How would you do this in D?
By requiring the compiler to inline the empty foo:
This won't work. Semantically, there is still a call and e.g.
profiling will see it:
https://godbolt.org/g/AUCeuu
-Johan
Re: Zero-cost version-dependent function call at -O0.
On Sunday, 25 June 2017 at 16:29:20 UTC, Anonymouse wrote:
Am I missing something, or can't you just version both the
function and the function ćall?
version(X)
void foo() { /* ... */ }
void main()
{
version(X)
{
foo();
}
}
I am hoping for something where "foo()" would just work.
"version(X) foo();" isn't bad, but it adds a lot of noise.
-Johan
Re: Zero-cost version-dependent function call at -O0.
On Sunday, 25 June 2017 at 21:55:22 UTC, Johan Engelen wrote: On Sunday, 25 June 2017 at 16:31:52 UTC, Moritz Maxeiner wrote: By requiring the compiler to inline the empty foo: This won't work. Yes, it does; comment out the call to `foo` and notice no change in the assembly in my link. Semantically, there is still a call You asked for no call and no extra code at -O0 and that is exactly what this solution provides. and e.g. profiling will see it: https://godbolt.org/g/AUCeuu The solution obviously does *not* work if you change the premise of your question after the fact by artificially injecting instructions into all function bodies (read what `-finstrument-functions` does).
Re: Zero-cost version-dependent function call at -O0.
On Sunday, 25 June 2017 at 22:23:44 UTC, Moritz Maxeiner wrote: The solution obviously does *not* work if you change the premise of your question after the fact by artificially injecting instructions into all function bodies I meant semantically no call. I am asking for a little more imagination, such that I don't have to specify all obvious details. For example, the always inline solution also doesn't work well when `foo` takes parameters. Regardless, perhaps in the meanwhile you've come up with an other solution? I am now thinking about introducing a noop intrinsic... (read what `-finstrument-functions` does). :-)
Re: Zero-cost version-dependent function call at -O0.
On Sunday, 25 June 2017 at 22:53:07 UTC, Johan Engelen wrote: I meant semantically no call. In the existing language, I think version (or static if) at the usage and definition points both is as good as you're going to get. unless you use a preprocessor lol But the good news about version is you could use compiler errors about missing name to know where it needs to be (unless there's another matching overload in scope[!]). So it would be mildly verbose to look at in the code, but not really difficult to get right (and you could write it with autocomplete, like vim's `:abbr foo version(foo) foo`). I am now thinking about introducing a noop intrinsic... That'd be kinda tricky because the arguments would still be liable to be evaluated... so the intrinsic would need to cover and disable that too and I think you'd be in for a fight to get that approved.
Re: Zero-cost version-dependent function call at -O0.
On Sunday, 25 June 2017 at 15:58:48 UTC, Johan Engelen wrote:
In C, you could do something like:
```
#if X
void foo() {..}
#else
#define foo()
#endif
```
Curious no one has mentionned it.
Just use alias.
version(X)
alias myFunc = impl1;
else
alias myFunc = impl2;
I do it a lot for ldc.intrinsics else they don't provide speed
benefits, often not being inlined.
Re: Zero-cost version-dependent function call at -O0.
On Sunday, 25 June 2017 at 23:02:28 UTC, Adam D. Ruppe wrote:
That'd be kinda tricky because the arguments would still be
liable to be evaluated...
Well..
I guess someone might argue that's a mis-feature of my
preprocessor example: "foo(i++)" may not do what you want. (So
the C code would have to do "(void)param" for all params, to
ensure evaluation and avoid compiler warning? =))
So I think I got things to work with inline IR! ^_^
https://godbolt.org/g/HVGTbx
```
version(none) {
void foo(int a, int b, int c) { /*...*/ };
} else {
pragma(LDC_inline_ir) R __ir(string s, R, P...)(P);
alias foo = __ir!(`ret i32 0`, int, int, int, int);
}
void bar()
{
int a;
foo(a++,2,3);
}
```
-Johan
Re: Zero-cost version-dependent function call at -O0.
On Sunday, 25 June 2017 at 23:02:28 UTC, Adam D. Ruppe wrote: On Sunday, 25 June 2017 at 22:53:07 UTC, Johan Engelen wrote: I meant semantically no call. In the existing language, I think version (or static if) at the usage and definition points both is as good as you're going to get. At the usage _and_ at definition point, indeed, that's a very good idea. - Johan
