[cfe-dev] Unresolved bug with arm neon instructions

[Tim Northover via cfe-dev]

Hi,

On Sat, 9 Nov 2019 at 19:03, via cfe-dev <cfe-dev at lists.llvm.org> wrote:
> with clang 8.0.7 and other versions there is an error on compilation. With gcc all works without issue.
> Could you fix this bug?

It's an incompatibility, but it's unclear that it's a bug. Clang
intentionally range-checks these intrinsics because they map to
instructions that only have a valid form with the permitted (positive)
shift amounts.

If we relaxed that requirement then -O0 compilation would still be
broken but with a significantly worse error message, which I don't
think many Clang devs would consider acceptable (both because of the
different behaviour depending on -O* and the message) --  I certainly
wouldn't. GCC appears to do some kind of basic optimization that
eliminates the invalid instructions even at -O0, but that's not how
Clang works and is also unlikely to change in the short-medium term.

I have seen someone propose that intrinsics taking immediates should
also accept variables and lower to multiple-instruction equivalents if
necessary, but I wasn't terribly keen on that either personally. It
would add a reciprocal incompatibility with GCC in at least the short
term (possiblty forever if they nope out), and I tend to think most
people writing these would prefer to know if their call was
inefficient anyway.

Someone keen *might* be able to get a patch through cfe-commits that
allows the user to demote these errors to a warning as a compromise,
but I'm afraid otherwise the only suggestion is to rewrite code so
that invalid intrinsics are never called statically.

Cheers.

Tim.