On Tue, 9 Jul 2013, Richard Earnshaw wrote:
> On ARM, CLZ has a defined result at zero (32). Furthermore, the ACLE
> specification defines (in the header arm_acle.h) __clz(n) as an intrinsic
> aimed at the CLZ instruction; __clz() has a defined result at 0. We want to
> use __builtin_clz as the implementation for __clz rather than inventing
> another one; but that would require the compiler to handle zero correctly.
Assuming the header will come with GCC, it can assume semantics we don't
document for user code - such as that __builtin_clz is defined at 0, if
that is the case with a given GCC version and target architecture. If the
semantics change, the header can then be changed at the same time.
(I'd still encourage user code wanting a defined value at 0 to do e.g.
static inline int clz (int n) { return n == 0 ? 32 : __builtin_clz (n); }
and hope GCC will optimize away the test for 0 when the instruction
semantics make it unnecessary - if it doesn't, it should be fixed to do
so. And it's certainly fine to put such code in an intrinsic header if
useful.)
--
Joseph S. Myers
jos...@codesourcery.com
