[X86] SimplifyDemandedBitsForTargetNode - generalize X86ISD::VSRAI handling when only demanding 'known signbits' (#161523)

If we only demand bits that already match the signbit then we don't need to shift.

Generalizes an existing pattern that just handled signbit-only demanded bits to match what we do for ISD::SRA.

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -44615,8 +44615,11 @@ bool X86TargetLowering::SimplifyDemandedBitsForTargetNode(
 
     APInt DemandedMask = OriginalDemandedBits << ShAmt;
 
-    // If we just want the sign bit then we don't need to shift it.
-    if (OriginalDemandedBits.isSignMask())
+    // If we only want bits that already match the signbit then we don't need
+    // to shift.
+    unsigned NumHiDemandedBits = BitWidth - OriginalDemandedBits.countr_zero();
+    if (TLO.DAG.ComputeNumSignBits(Op0, OriginalDemandedElts, Depth + 1) >=
+        NumHiDemandedBits)
       return TLO.CombineTo(Op, Op0);
 
     // fold (VSRAI (VSHLI X, C1), C1) --> X iff NumSignBits(X) > C1

llvm/test/CodeGen/X86/combine-pack.ll

@@ -5,22 +5,19 @@
 
 declare <8 x i16> @llvm.x86.sse2.packssdw.128(<4 x i32>, <4 x i32>)
 
-; TODO: Failure to remove unnecessary signsplat
 define <8 x i16> @combine_packss_v4i32_signsplat(<4 x i32> %a0, <4 x i32> %a1) {
 ; SSE-LABEL: combine_packss_v4i32_signsplat:
 ; SSE:       # %bb.0:
 ; SSE-NEXT:    pcmpgtd %xmm1, %xmm0
 ; SSE-NEXT:    pcmpeqd %xmm1, %xmm1
 ; SSE-NEXT:    packssdw %xmm1, %xmm0
-; SSE-NEXT:    psraw $15, %xmm0
 ; SSE-NEXT:    retq
 ;
 ; AVX-LABEL: combine_packss_v4i32_signsplat:
 ; AVX:       # %bb.0:
 ; AVX-NEXT:    vpcmpgtd %xmm1, %xmm0, %xmm0
 ; AVX-NEXT:    vpcmpeqd %xmm1, %xmm1, %xmm1
 ; AVX-NEXT:    vpackssdw %xmm1, %xmm0, %xmm0
-; AVX-NEXT:    vpsraw $15, %xmm0, %xmm0
 ; AVX-NEXT:    retq
   %cmp = icmp sgt <4 x i32> %a0, %a1
   %ext = sext <4 x i1> %cmp to <4 x i32>