[Bug middle-end/50481] builtin to reverse the bit order

[matthijsvanduin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=50481

Matthijs van Duin <matthijsvanduin at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |matthijsvanduin at gmail dot com

--- Comment #3 from Matthijs van Duin <matthijsvanduin at gmail dot com> ---
Bump!  Proper intrinsics for bitreverse would be much appreciated!  A plain C
implementation is ugly and results in equally awful code output, while using
inline asm breaks portability and can't be constant-folded or used in
constexpr.

What makes the continued lack of a __builtin_arm_rbit() in gcc a bit bizarre is
that the (identically named) Neon versions of this instruction on AArch64
actually *did* receive proper intrinsics! [1]

It's worth mentioning that clang does support __builtin_arm_rbit(), and they've
actually generalized this to a full set of target-independent bitreverse
builtins [2].

[1] https://gcc.gnu.org/ml/gcc-patches/2014-08/msg01913.html
[2] http://clang.llvm.org/docs/LanguageExtensions.html#builtin-bitreverse