How do I do a 64 bits mulhi in D?

[kinke]

On Sunday, 27 November 2022 at 01:55:45 UTC, deadalnix wrote:
> That's a clever trick, but that only work on x86, but also will 
> blind the optimizer as to what's going on.

A solution with LDC, using inline-(LLVM-)IR:

```
ulong mulhi(ulong a, ulong b)
{
     import ldc.llvmasm;
     return __ir!(`
         %a = zext i64 %0 to i128
         %b = zext i64 %1 to i128
         %r = mul i128 %a, %b
         %r2 = lshr i128 %r, 64
         %r3 = trunc i128 %r2 to i64
         ret i64 %r3`, ulong)(a, b);
}
```