On Friday, 3 April 2020 at 20:06:50 UTC, Steven Schveighoffer
wrote:
On 4/3/20 3:13 PM, Johan wrote:
On Thursday, 2 April 2020 at 12:41:28 UTC, Steven
Schveighoffer wrote:
Hm... I thought there was precedent for providing fallback
implementations for intrinsics. That is, you define the
function, which is only used if the intrinsic is not
available.
I can't remember where I saw this. But you could try this by
simply implementing the bitops in core.bitop, and see if they
are used outside ctfe.
There are a bunch of functions implemented with `if
(!__ctfe)`. DMD and LDC are smart enough to elide
`if(false/true)` control flow completely even in debug code,
so there is no penalty to using `if (!__ctfe)`.
See for example:
https://github.com/ldc-developers/druntime/blob/ldc/src/core/bitop.d#L85
Nice!
I'm trying to understand that. It looks like you are calling
the intrinsic that llvm recognizes. What can you do when bsf
*is* the intrinsic? Does DMD have to change its intrinsic to
something that's not core.bitop.bsf so we can call it? Or would
that code as written work if ported to druntime mainline?
I think it'd work without any changes needed. (note the version
statements)
I.e. the compiler replaces the bsf with the intrinsic and
ignores the implementation in runtime code, but works at
comiple time.
I think that's correct, yes.
-Johan
