[llvm-dev] [RFC] Saturating left shift intrinsics

[Nikita Popov via llvm-dev]

On Wed, Jul 8, 2020 at 5:31 PM Bevin Hansson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
>
>
> This is an RFC for adding intrinsics which perform saturating
> signed/unsigned left shift.
>
> There is currently a patch on Phabricator here:
>
> https://reviews.llvm.org/D83216
>
>
>
> The intrinsics are of the form
>
>
>
>     i32 @llvm.sshl.sat.i32(i32, i32)
>
>     i32 @llvm.ushl.sat.i32(i32, i32)
>
>     <4 x i32> @llvm.sshl.sat.v4i32(<4 x i32>, <4 x i32>)
>
>     <4 x i32> @llvm.ushl.sat.v4i32(<4 x i32>, <4 x i32>)
>
>
>
> and are overloaded with a single type which can be either an integer type
> or a vector of integers. The value(s) provided in the first operand are
> shifted left by the amount(s) given by the second operand. As with regular
> left shift instructions, the second operand is an unsigned quantity and
> must be less than the integer bitwidth.
>
>
>
> If the true result of the operation (given infinite precision) lies
> outside of the maximum or minimum representable value of the
> signed/unsigned integer, the result saturates to the maximum or minimum
> depending on the sign of the shifted value.
>
>
>
> These are useful for implementing the Embedded-C fixed-point arithmetic
> support in Clang, as previously detailed and discussed here:
>
> http://lists.llvm.org/pipermail/llvm-dev/2018-August/125433.html
>
> http://lists.llvm.org/pipermail/cfe-dev/2018-May/058019.html
>
>
>
> It is of course possible to emit these shifts as regular IR instructions,
> but targets which support saturating shifts might have difficulties
> efficiently selecting native instructions if the optimizer undoes the
> particular expected instruction pattern that the shifts would be selected
> on.
>

These generally look reasonable to me, and fit in with the existing
saturating arithmetic intrinsics. My only question here would be what kind
of support these intrinsics are targeting.

Are you planning to work on high-quality middle end support for them, with
an eventual goal of canonicality (as is the case for the existing
saturating arithmetic intrinsics), or are these intended to be target
intrinsics in all but name (as is the case for the existing fixed-point
intrinsics)?

Regards,
Nikita
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200716/a12d3b0d/attachment.html>