diff --git a/mlir/lib/Conversion/ComplexToStandard/ComplexToStandard.cpp b/mlir/lib/Conversion/ComplexToStandard/ComplexToStandard.cpp
--- a/mlir/lib/Conversion/ComplexToStandard/ComplexToStandard.cpp
+++ b/mlir/lib/Conversion/ComplexToStandard/ComplexToStandard.cpp
@@ -738,6 +738,9 @@
     auto loc = op.getLoc();
     auto type = adaptor.getComplex().getType().cast<ComplexType>();
     auto elementType = type.getElementType().cast<FloatType>();
+    // The hyperbolic tangent for complex number can be calculated as follows.
+    // tanh(x + i * y) = (tanh(x) + i * tan(y)) / (1 + tanh(x) * tan(y))
+    // See: https://proofwiki.org/wiki/Hyperbolic_Tangent_of_Complex_Number
     Value real =
             rewriter.create<complex::ReOp>(loc, elementType, adaptor.getComplex());
     Value imag =