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digitalmars.D.ldc - Can we choose branch prediction like GCC?

reply rempas <rempas tutanota.com> writes:
In GCC, there is a 
[builtin](https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html) 
called "__builtin_expect" that allows us to tell if we expect a 
branch to hit or not so the compiler can choose the branch 
prediction we want (unless I don't understand something 
correctly). Is there a way to do that in LDC?
May 10 2022
parent reply kinke <noone nowhere.com> writes:
On Tuesday, 10 May 2022 at 10:02:21 UTC, rempas wrote:

 Is there a way to do that in LDC?


Yes: 
https://github.com/ldc-developers/druntime/blob/bf27af6ef6dab9e45f005f67cc42e3a3bb393623/src/ldc/intrinsics.di#L616-L624
May 10 2022
parent reply rempas <rempas tutanota.com> writes:
On Tuesday, 10 May 2022 at 13:19:33 UTC, kinke wrote:

 Yes: 
 https://github.com/ldc-developers/druntime/blob/bf27af6ef6dab9e45f005f67cc42e3a3bb393623/src/ldc/intrinsics.di#L616-L624


Thank you for your answer! However, I'm trying to understand how 
it works but I cannot seem to get it. I'm trying to do something 
like the following but it seems that it doesn't work (as it makes 
sense):

```
import ldc.intrinsics;
import core.stdc.stdio;

extern (C) void main() {
   int x = 0;
   llvm_expect(x < 10, 0) {
     printf("x is less than 10!\n");
   }
}
```

I did tried for about half an hour but couldn't find anything... 
Any ideas how it works?
May 10 2022
parent reply kinke <noone nowhere.com> writes:
On Tuesday, 10 May 2022 at 20:31:41 UTC, rempas wrote:

 I did tried for about half an hour but couldn't find 
 anything... Any ideas how it works?


As with gcc's __builtin_expect, one normally uses it in an if 
condition: `if (llvm_expect(x < 10, false)) …`.
May 10 2022
parent rempas <rempas tutanota.com> writes:
On Tuesday, 10 May 2022 at 22:34:23 UTC, kinke wrote:

 As with gcc's __builtin_expect, one normally uses it in an if 
 condition: `if (llvm_expect(x < 10, false)) …`.


Oh, how I didn't thought about trying this out....

Thank you! This will do the trick! Have an amazing day my friend!
May 10 2022