[llvm-dev] InstructionSimplify: adding a hook for shufflevector instructions

[Sanjay Patel via llvm-dev]

My grasp of LLVM history isn't great, but I think these are missing because
there wasn't much need for vector optimization in IR because there just
weren't that many vector opportunities in IR. Ie, the vectorizers are
relatively new, and hand-written vector code (eg, SSE intrinsics in source)
generally went straight to the backend as target-specific IR intrinsics.

Now that we're vectorizing more aggressively (and plan to do even more) and
we're converting target-specific vector source to generic vector IR
whenever possible, it makes sense to add these kinds of optimizations.

One frequently visible sign of scalar privilege in instcombine is the use
of "m_ConstantInt". In many cases, this can be converted to "m_APInt"
without much effort, and the transform will auto-magically apply to splat
vector constants too.



On Wed, Mar 29, 2017 at 8:37 PM, Rackover, Zvi <zvi.rackover at intel.com>
wrote:

> As Sanjay noted in D31426 <https://reviews.llvm.org/D31426#712701>,
> InstructionSimplify is missing the following simplification:
>
>
>
> This function:
>
> define <4 x i32> @splat_operand(<4 x i32> %x) {
>
>    %splat = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32>
> zeroinitializer
>
>    %shuf = shufflevector <4 x i32> %splat, <4 x i32> undef, <4 x i32> <i32
> 0, i32 3, i32 2, i32 1>
>
>    ret <4 x i32> %shuf
>
> }
>
>
>
> can be simplified to:
>
> define <4 x i32> @splat_operand(<4 x i32> %x) {
>
>   %shuf = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32>
> zeroinitializer
>
>   ret <4 x i32> %shuf
>
> }
>
>
>
> InstCombine covers this case inefficiently.
>
>
>
> I noticed that InstructionSimplify does not do any simplifications for
> shufflevector’s other than constant folding. I just wanted to be sure there
> is no compelling reason for this before I start streaming patches. I assume
> that this is not related to our conservative approach of refraining from
> creation of new shuffle masks that may hurt some target.
>
>
>
> Here are some more opportunities that can be added to InstructionSimplify,
> all of which are covered by InstCombine:
>
>
>
> define <4 x i32> @undef_mask(<4 x i32> %x) {
>
>    %shuf = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> undef
>
>    ret <4 x i32> %shuf
>
> }
>
> à
>
> define <4 x i32> @undef_mask(<4 x i32> %x) {
>
>   ret <4 x i32> undef
>
> }
>
>
>
> define <4 x i32> @identity_mask_0(<4 x i32> %x) {
>
>    %shuf = shufflevector <4 x i32> %x, <4 x i32> undef, <4 x i32> <i32 0,
> i32 1, i32 2, i32 3>
>
>    ret <4 x i32> %shuf
>
> }
>
> à
>
> define <4 x i32> @identity_mask_0(<4 x i32> %x) {
>
>   ret <4 x i32> %x
>
> }
>
>
>
> define <4 x i32> @identity_mask_1(<4 x i32> %x) {
>
>    %shuf = shufflevector <4 x i32> undef, <4 x i32> %x, <4 x i32> <i32 4,
> i32 5, i32 6, i32 7>
>
>    ret <4 x i32> %shuf
>
> }
>
> à
>
> define <4 x i32> @identity_mask_1(<4 x i32> %x) {
>
>   ret <4 x i32> %x
>
> }
>
>
>
> define <4 x i32> @pseudo_identity_mask(<4 x i32> %x) {
>
>    %shuf = shufflevector <4 x i32> %x, <4 x i32> %x, <4 x i32> <i32 0, i32
> 1, i32 2, i32 7>
>
>    ret <4 x i32> %shuf
>
> }
>
> à
>
> define <4 x i32> @pseudo_identity_mask(<4 x i32> %x) {
>
>   ret <4 x i32> %x
>
> }
>
>
>
> define <4 x i32> @const_operand(<4 x i32> %x) {
>
>    %shuf = shufflevector <4 x i32> <i32 42, i32 43, i32 44, i32 45>, <4 x
> i32> %x, <4 x i32> <i32 0, i32 3, i32 2, i32 1>
>
>    ret <4 x i32> %shuf
>
> }
>
> à
>
> define <4 x i32> @const_operand(<4 x i32> %x) {
>
>   ret <4 x i32> <i32 42, i32 45, i32 44, i32 43>
>
> }
>
>
>
> define <4 x i32> @merge(<4 x i32> %x) {
>
>    %lower = shufflevector <4 x i32> %x, <4 x i32> undef, <2 x i32> <i32 1,
> i32 0>
>
>    %upper = shufflevector <4 x i32> %x, <4 x i32> undef, <2 x i32> <i32 2,
> i32 3>
>
>    %merged = shufflevector <2 x i32> %upper, <2 x i32> %lower, <4 x i32>
> <i32 3, i32 2, i32 0, i32 1>
>
>    ret <4 x i32> %merged
>
> }
>
> à
>
> define <4 x i32> @merge(<4 x i32> %x) {
>
>   ret <4 x i32> %x
>
> }
>
>
>
> Would appreciate your comments and feedback.
>
>
>
> Thanks, Zvi
>
>
>
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