[MLIR][KernelGen] Add `tf.Atanh` kernels

PiperOrigin-RevId: 352393602

include/mlir-hlo/Dialect/mhlo/IR/chlo_ops.td

@@ -397,6 +397,20 @@ def HLOClient_AtanOp : HLOClient_UnaryElementwiseOp<"atan", [],
   }];
 }
 
+def HLOClient_AtanhOp : HLOClient_UnaryElementwiseOp<"atanh", [],
+    HLO_FpOrComplexTensor> {
+  let summary = "Atanh operator";
+
+  let description = [{
+    Returns `Atanh(operand)` element-wise.
+
+    $$
+    \atanh(x) = 0.5 * log((1 + x) / (1 - x)) if abs(x) <= 1
+              = nan                          otherwise
+    $$
+  }];
+}
+
 def HLOClient_ConjOp : HLOClient_UnaryElementwiseOp<"conj", [],
     HLO_FpOrComplexTensor> {
   let summary = "Conj operator";

lib/Dialect/mhlo/transforms/chlo_legalize_to_hlo_patterns.td

@@ -175,6 +175,29 @@ def : Pat<(HLOClient_AtanOp $input),
     (HLO_ConstantLike<"1"> $input)
   )>;
 
+// Express `atanh` as follows:
+//   atanh(x) = 0.5 * log((1 + x) / (1 - x)) if abs(x) <= 1
+//   atanh(x) = nan                          otherwise
+def : Pat<(HLOClient_AtanhOp NonComplexElementType:$input),
+  (HLO_SelectOp
+    (HLO_CompareOp
+      (HLO_AbsOp $input),
+      (HLO_ConstantLike<"1"> $input),
+      HLO_COMPARISON_DIRECTION_GT,
+      (HLO_DEFAULT_COMPARISON_TYPE)
+    ),
+    (HLO_ConstantLike<"NAN"> $input),
+    (HLO_MulOp
+      (HLO_SubOp
+        (HLO_Log1pOp $input),
+        (HLO_Log1pOp
+          (HLO_NegOp $input)
+        )
+      ),
+      (HLO_ConstantLike<"0.5"> $input)
+    )
+  )>;
+
 // Express `conj` as
 //   conj(x) = (re(x), -im(x)).
 def : Pat<(HLOClient_ConjOp $v),

lib/Dialect/mhlo/transforms/transform_unranked_hlo.cc

@@ -51,8 +51,9 @@ namespace {
 
 // TODO(herhut): Generate these out of op definitions.
 #define MAP_CHLO_OPERATION_CWISE_UNARY(fn, sep)                            \
-  fn(AcosOp) sep fn(AsinOp) sep fn(AsinhOp) sep fn(AtanOp) sep fn(ConjOp) \
-      sep fn(CoshOp) sep fn(ErfOp) sep fn(ErfcOp) sep fn(SinhOp) sep fn(TanOp)
+  fn(AcosOp) sep fn(AsinOp) sep fn(AsinhOp) sep fn(AtanOp) sep fn(AtanhOp) \
+      sep fn(ConjOp) sep fn(CoshOp) sep fn(ErfOp) sep fn(ErfcOp)           \
+          sep fn(SinhOp) sep fn(TanOp)
 
 template <typename OpTy>
 inline void AddLegalOpOnRankedTensor(ConversionTarget *target) {