Comment #0 by bearophile_hugs — 2010-04-02T14:25:39Z
import std.intrinsic: bt;
int foo() {
uint x = uint.max;
return bt(&x, 5);
}
int _ = foo();
void main() {}
dmd 2.042 gives:
test.d(4): Error: cannot evaluate bt((& x),5u) at compile time
test.d(6): Error: cannot evaluate foo() at compile time
test.d(6): Error: cannot evaluate foo() at compile time
In CTFE the the compiler can replace the intrinsics with little functions with the same semantics.
Comment #1 by clugdbug — 2011-09-22T06:27:33Z
I'm not sure why the btXX functions (bt, btc, btr, etc) exist at all.
Although they are a single instruction, they are MUCH slower than the equivalent code using shifts or AND/OR/XOR.
For example, on Core i7 (Sandy Bridge), with a memory operand, they take 6 clock cycles!!!! You can execute 24 integer instructions in that time. On AMD K10, they're even slower. On Pentium 4 they have a latency of EIGHTEEN clock cycles. They're even slow on VIA processors as well -- they're not good anywhere.
I think they should be completely removed. There's a case for the intrinsics mentioned in bug 5703, but I think this should be a WONTFIX. To support them would just encourage slow, non-portable code.
Comment #2 by dmitry.olsh — 2011-09-22T10:42:25Z
(In reply to comment #1)
> I'm not sure why the btXX functions (bt, btc, btr, etc) exist at all.
> Although they are a single instruction, they are MUCH slower than the
> equivalent code using shifts or AND/OR/XOR.
> For example, on Core i7 (Sandy Bridge), with a memory operand, they take 6
> clock cycles!!!! You can execute 24 integer instructions in that time. On AMD
> K10, they're even slower. On Pentium 4 they have a latency of EIGHTEEN clock
> cycles. They're even slow on VIA processors as well -- they're not good
> anywhere.
Damn, and I used them at heart of important loops in FReD ....
Thanks, I'm getting rid of them ASAP %)
This should be probably mentioned somewhere, and then there are these problematic bsr/bsf you mentioned before.
>
> I think they should be completely removed. There's a case for the intrinsics
> mentioned in bug 5703, but I think this should be a WONTFIX. To support them
> would just encourage slow, non-portable code.
Comment #3 by bearophile_hugs — 2011-09-24T18:29:33Z
(In reply to comment #1)
> I think they should be completely removed. There's a case for the intrinsics
> mentioned in bug 5703, but I think this should be a WONTFIX. To support them
> would just encourage slow, non-portable code.
OK, then I close this
But I suggest you to open an "enhancement" request that asks to deprecate the useless/bad intrinsics :-)