[cfe-dev] constexpr with builtin

[Richard Smith]

On Mon, Sep 9, 2013 at 7:20 PM, fil <fil at pobox.com> wrote:

> Sebastian Redl wrote
> > I believe the simple test that makes the code above not compile does not
> > evaluate potential results of conditional expressions: only one branch
> > needs to be a constant expression, and the test does not reason about
> > "value == value" always taking one branch. So the version with the
> > tautological test will compile by itself, but you'll never be able to use
> > it in a constexpr context, versus the simple function above is recognized
> > as not being constexpr.
> >
> > Making built-in functions with simple arithmetic meanings constexpr is a
> > different issue.
>
> Thanks for the reply. Two questions:
>
> 1. know of a way to use builtin functions (like clz) in a constexpr
> context?
> I am figuring I need to re-implement it, no way to use the CPU instruction
> at compile time (?)
>

__builtin_clz works in constant expressions in recent Clang revisions. You
only need to upgrade =)

2. why does a branch allow it to be compiled? I would have thought that the
> compiler would inspect both branches and insist they both be a const
> friendly.
>

The compiler does inspect both branches, but it only insists that *one* of
them is constexpr-friendly. It's valid for a ?: to select between a
potentially-constant expression and a never-constant expression within a
constexpr function.

So, I should expect the branched version compile to fail if I actually use
> it in a constexpr context?


Yes.
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