[llvm-dev] RFC: Constant folding math functions for long double

[Reid Kleckner via llvm-dev]

My feeling is that we shouldn't be relying on host long double routines.
We're already skating on thin ice by relying on host double and float
routines. This is a great way to make the compilation result vary depending
on the host, which is something we try to avoid.

An optional MPFR dependency would also be pretty painful. I expect it will
frequently be missing and will not be exercised by most buildbots.

On Mon, Apr 4, 2016 at 6:59 AM, James Molloy via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> Clang is currently unable to constant fold calls to math.h functions such
> as logl(), expl() etc.
>
> The problem is that APFloat doesn't have these functions, so Clang is
> forced to rely on the host math library. Because long double isn't
> portable, we only ever query the host math library for double or float
> results.
>
> I can see three methods for allowing constant folding for types that are
> larger than double, some more expensive than others:
>
>   1. Introduce a dependency on libMPFR, as GCC does. The dependency could
> be hard or soft, with a fallback to the current behaviour if it doesn't
> exist.
>   2. Write the trancendental functions ourselves in APFloat (yuck!)
>   3. If the long double format on the compiler host is the same as the
> target, use the host library.
>
> (2) is the hardest. (3) is the easiest, but only works in a subset of
> cases and I really don't like the idea of better output when compiling on
> one platform compared to another (with equivalent targets).
>
> What do people think about (1)? Or is this completely out of the question?
>
> Cheers,
>
> James
>
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