[InstCombine] fold popcount of exactly one bit to shift

This is discussed in https://llvm.org/PR48999 ,
but it does not solve that request.

The difference in the vector test shows that some
other logic transform is limited to scalar types.

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp

@@ -522,18 +522,24 @@ static Instruction *foldCtpop(IntrinsicInst &II, InstCombinerImpl &IC) {
     return CallInst::Create(F, {X, IC.Builder.getFalse()});
   }
 
+  KnownBits Known(BitWidth);
+  IC.computeKnownBits(Op0, Known, 0, &II);
+
+  // If all bits are zero except for exactly one fixed bit, then the result
+  // must be 0 or 1, and we can get that answer by shifting to LSB:
+  // ctpop (X & 32) --> (X & 32) >> 5
+  if ((~Known.Zero).isPowerOf2())
+    return BinaryOperator::CreateLShr(
+        Op0, ConstantInt::get(Ty, (~Known.Zero).exactLogBase2()));
+
   // FIXME: Try to simplify vectors of integers.
   auto *IT = dyn_cast<IntegerType>(Ty);
   if (!IT)
     return nullptr;
 
-  KnownBits Known(BitWidth);
-  IC.computeKnownBits(Op0, Known, 0, &II);
-
+  // Add range metadata since known bits can't completely reflect what we know.
   unsigned MinCount = Known.countMinPopulation();
   unsigned MaxCount = Known.countMaxPopulation();
-
-  // Add range metadata since known bits can't completely reflect what we know.
   if (IT->getBitWidth() != 1 && !II.getMetadata(LLVMContext::MD_range)) {
     Metadata *LowAndHigh[] = {
         ConstantAsMetadata::get(ConstantInt::get(IT, MinCount)),

llvm/test/Transforms/InstCombine/ctpop.ll

@@ -96,8 +96,8 @@ define i1 @test6(i1 %arg) {
 
 define i8 @mask_one_bit(i8 %x) {
 ; CHECK-LABEL: @mask_one_bit(
-; CHECK-NEXT:    [[A:%.*]] = and i8 [[X:%.*]], 16
-; CHECK-NEXT:    [[R:%.*]] = call i8 @llvm.ctpop.i8(i8 [[A]]), !range [[RNG1:![0-9]+]]
+; CHECK-NEXT:    [[A:%.*]] = lshr i8 [[X:%.*]], 4
+; CHECK-NEXT:    [[R:%.*]] = and i8 [[A]], 1
 ; CHECK-NEXT:    ret i8 [[R]]
 ;
   %a = and i8 %x, 16
@@ -109,7 +109,7 @@ define <2 x i32> @mask_one_bit_splat(<2 x i32> %x, <2 x i32>* %p) {
 ; CHECK-LABEL: @mask_one_bit_splat(
 ; CHECK-NEXT:    [[A:%.*]] = and <2 x i32> [[X:%.*]], <i32 2048, i32 2048>
 ; CHECK-NEXT:    store <2 x i32> [[A]], <2 x i32>* [[P:%.*]], align 8
-; CHECK-NEXT:    [[R:%.*]] = call <2 x i32> @llvm.ctpop.v2i32(<2 x i32> [[A]])
+; CHECK-NEXT:    [[R:%.*]] = lshr exact <2 x i32> [[A]], <i32 11, i32 11>
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
   %a = and <2 x i32> %x, <i32 2048, i32 2048>