[cfe-dev] Why no NSW/NUW with SHL instruction?

Ryan Taylor ryta1203 at gmail.com
Fri Nov 21 10:00:52 PST 2014

So why in some cases does it still produce the nsw/nuw flag? Just curious.

We are going with intrinsics, but I'm still curious. Thanks.

On Fri, Nov 21, 2014 at 12:54 PM, David Chisnall <
David.Chisnall at cl.cam.ac.uk> wrote:

> On 21 Nov 2014, at 17:39, Ryan Taylor <ryta1203 at gmail.com> wrote:
> > I don't care if clang only produces shl, I'm more curious how to get the
> signed info to the backend for shl when it's not producing a nsw/nuw flag?
> I'm currently not sure how this would be done, is there another arch that
> manages this somehow?
> I think the point is that the only difference between the two is the
> overflow flag, which is not something that C exposes, and LLVM IR tends to
> be driven by the needs of the front ends.
> If you wanted an IR instruction that represented a left shift that set a
> flag, then you'd need something that returned both the result and the
> overflow flag (look at the existing overflow checked addition intrinsics
> for inspiration).  This sort of thing tends only to be added when someone
> needs it.  If the first someone to need it is you, then... patches
> welcome.  I think that you'd probably want to implement it as a
> shift-with-overflow intrinsic rather than a variation on the shl
> instruction and you'd probably need to provide some expansion for it for
> processors that don't have a direct mapping.
> David
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