[ValueTracking] Add support for fshl/fshr in `isKnownToBeAPowerOf…

…Two`

If the funnel shifts are rotates (op0 == op1) then the number of 1s/0s
don't change so we can just look through op0/op1.

Proofs: https://alive2.llvm.org/ce/z/Pja5yu

Differential Revision: https://reviews.llvm.org/D157307

llvm/lib/Analysis/ValueTracking.cpp

@@ -2137,6 +2137,12 @@ bool isKnownToBeAPowerOfTwo(const Value *V, bool OrZero, unsigned Depth,
       case Intrinsic::bitreverse:
       case Intrinsic::bswap:
         return isKnownToBeAPowerOfTwo(II->getArgOperand(0), OrZero, Depth, Q);
+      case Intrinsic::fshr:
+      case Intrinsic::fshl:
+        // If Op0 == Op1, this is a rotate. is_pow2(rotate(x, y)) == is_pow2(x)
+        if (II->getArgOperand(0) == II->getArgOperand(1))
+          return isKnownToBeAPowerOfTwo(II->getArgOperand(0), OrZero, Depth, Q);
+        break;
       default:
         break;
       }

llvm/test/Analysis/ValueTracking/known-power-of-two.ll

@@ -214,7 +214,8 @@ define i16 @fshl_is_pow2_or_zero(i16 %x, i16 %y, i16 %z) {
 ; CHECK-SAME: (i16 [[X:%.*]], i16 [[Y:%.*]], i16 [[Z:%.*]]) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl i16 4, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.fshl.i16(i16 [[XP2]], i16 [[XP2]], i16 [[Z]])
-; CHECK-NEXT:    [[R:%.*]] = urem i16 [[Y]], [[XX]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add i16 [[XX]], -1
+; CHECK-NEXT:    [[R:%.*]] = and i16 [[TMP1]], [[Y]]
 ; CHECK-NEXT:    ret i16 [[R]]
 ;
   %xp2 = shl i16 4, %x
@@ -262,7 +263,7 @@ define i1 @fshl_is_pow2(i16 %x, i16 %y, i16 %z) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl nuw i16 1, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.fshl.i16(i16 [[XP2]], i16 [[XP2]], i16 [[Z]])
 ; CHECK-NEXT:    [[AND:%.*]] = and i16 [[XX]], [[Y]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i16 [[AND]], [[XX]]
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i16 [[AND]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %xp2 = shl i16 1, %x
@@ -295,7 +296,8 @@ define i16 @fshr_is_pow2_or_zero(i16 %x, i16 %y, i16 %z) {
 ; CHECK-SAME: (i16 [[X:%.*]], i16 [[Y:%.*]], i16 [[Z:%.*]]) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl i16 4, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.fshr.i16(i16 [[XP2]], i16 [[XP2]], i16 [[Z]])
-; CHECK-NEXT:    [[R:%.*]] = urem i16 [[Y]], [[XX]]
+; CHECK-NEXT:    [[TMP1:%.*]] = add i16 [[XX]], -1
+; CHECK-NEXT:    [[R:%.*]] = and i16 [[TMP1]], [[Y]]
 ; CHECK-NEXT:    ret i16 [[R]]
 ;
   %xp2 = shl i16 4, %x
@@ -343,7 +345,7 @@ define i1 @fshr_is_pow2(i16 %x, i16 %y, i16 %z) {
 ; CHECK-NEXT:    [[XP2:%.*]] = shl nuw i16 1, [[X]]
 ; CHECK-NEXT:    [[XX:%.*]] = call i16 @llvm.fshr.i16(i16 [[XP2]], i16 [[XP2]], i16 [[Z]])
 ; CHECK-NEXT:    [[AND:%.*]] = and i16 [[XX]], [[Y]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i16 [[AND]], [[XX]]
+; CHECK-NEXT:    [[R:%.*]] = icmp ne i16 [[AND]], 0
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %xp2 = shl i16 1, %x