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

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Mon Apr 4 09:49:24 PDT 2016

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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