[SelectionDAG] try harder to convert funnel shift to rotate

Similar to rL337966 - if the DAGCombiner's rotate matching was 
working as expected, I don't think we'd see any test diffs here.

AArch only goes right, and PPC only goes left. 
x86 has both, so no diffs there.

Differential Revision: https://reviews.llvm.org/D50091


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339359 91177308-0d34-0410-b5e6-96231b3b80d8

Author: rotateright

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -5703,14 +5703,21 @@ SelectionDAGBuilder::visitIntrinsicCall(const CallInst &I, unsigned Intrinsic) {
     if (X == Y && isPowerOf2_32(VT.getScalarSizeInBits())) {
       // TODO: This should also be done if the operation is custom, but we have
       // to make sure targets are handling the modulo shift amount as expected.
-      // TODO: If the rotate direction (left or right) corresponding to the
-      // shift is not available, adjust the shift value and invert the
-      // direction.
       auto RotateOpcode = IsFSHL ? ISD::ROTL : ISD::ROTR;
       if (TLI.isOperationLegal(RotateOpcode, VT)) {
         setValue(&I, DAG.getNode(RotateOpcode, sdl, VT, X, Z));
         return nullptr;
       }
+
+      // Some targets only rotate one way. Try the opposite direction.
+      RotateOpcode = IsFSHL ? ISD::ROTR : ISD::ROTL;
+      if (TLI.isOperationLegal(RotateOpcode, VT)) {
+        // Negate the shift amount because it is safe to ignore the high bits.
+        SDValue NegShAmt = DAG.getNode(ISD::SUB, sdl, VT, Zero, Z);
+        setValue(&I, DAG.getNode(RotateOpcode, sdl, VT, X, NegShAmt));
+        return nullptr;
+      }
+
       // fshl (rotl): (X << (Z % BW)) | (X >> ((0 - Z) % BW))
       // fshr (rotr): (X << ((0 - Z) % BW)) | (X >> (Z % BW))
       SDValue NegZ = DAG.getNode(ISD::SUB, sdl, VT, Zero, Z);

test/CodeGen/AArch64/funnel-shift-rot.ll

@@ -65,10 +65,8 @@ define i32 @rotl_i32(i32 %x, i32 %z) {
 define i64 @rotl_i64(i64 %x, i64 %z) {
 ; CHECK-LABEL: rotl_i64:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    neg w9, w1
-; CHECK-NEXT:    lsl x8, x0, x1
-; CHECK-NEXT:    lsr x9, x0, x9
-; CHECK-NEXT:    orr x0, x8, x9
+; CHECK-NEXT:    neg x8, x1
+; CHECK-NEXT:    ror x0, x0, x8
 ; CHECK-NEXT:    ret
   %f = call i64 @llvm.fshl.i64(i64 %x, i64 %x, i64 %z)
   ret i64 %f

test/CodeGen/PowerPC/funnel-shift-rot.ll

@@ -145,7 +145,6 @@ define i32 @rotr_i32(i32 %x, i32 %z) {
 ; CHECK-LABEL: rotr_i32:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    neg 4, 4
-; CHECK-NEXT:    clrlwi 4, 4, 27
 ; CHECK-NEXT:    rlwnm 3, 3, 4, 0, 31
 ; CHECK-NEXT:    blr
   %f = call i32 @llvm.fshr.i32(i32 %x, i32 %x, i32 %z)
@@ -156,8 +155,7 @@ define i64 @rotr_i64(i64 %x, i64 %z) {
 ; CHECK-LABEL: rotr_i64:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    neg 4, 4
-; CHECK-NEXT:    rlwinm 4, 4, 0, 26, 31
-; CHECK-NEXT:    rotld 3, 3, 4
+; CHECK-NEXT:    rldcl 3, 3, 4, 0
 ; CHECK-NEXT:    blr
   %f = call i64 @llvm.fshr.i64(i64 %x, i64 %x, i64 %z)
   ret i64 %f