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

James Molloy via llvm-dev llvm-dev at lists.llvm.org
Mon Apr 4 06:59:19 PDT 2016


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

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
  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?


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