diff --git a/libc/shared/builtins.h b/libc/shared/builtins.h
index fc4c5aee9a8e4..dca0171c02d18 100644
--- a/libc/shared/builtins.h
+++ b/libc/shared/builtins.h
@@ -17,6 +17,7 @@
 
 #include "libc_common.h"
 
+#include "builtins/adddf3.h"
 #include "builtins/addtf3.h"
 #include "builtins/divtf3.h"
 #include "builtins/multf3.h"
diff --git a/libc/shared/builtins/adddf3.h b/libc/shared/builtins/adddf3.h
new file mode 100644
index 0000000000000..33936cb8b2486
--- /dev/null
+++ b/libc/shared/builtins/adddf3.h
@@ -0,0 +1,29 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This header exposes LLVM-libc's __adddf3 implementation as shared::adddf3 so
+/// that it can be reused by compiler-rt's builtins.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SHARED_BUILTINS_ADDDF3_H
+#define LLVM_LIBC_SHARED_BUILTINS_ADDDF3_H
+
+#include "shared/libc_common.h"
+#include "src/__support/builtins/adddf3.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace shared {
+
+using builtins::adddf3;
+
+} // namespace shared
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SHARED_BUILTINS_ADDDF3_H
diff --git a/libc/src/__support/builtins/CMakeLists.txt b/libc/src/__support/builtins/CMakeLists.txt
index 1b4f9ddab2a90..dee5a830b8857 100644
--- a/libc/src/__support/builtins/CMakeLists.txt
+++ b/libc/src/__support/builtins/CMakeLists.txt
@@ -37,3 +37,12 @@ add_header_library(
     libc.src.__support.FPUtil.generic.div
     libc.src.__support.macros.config
 )
+
+add_header_library(
+  adddf3
+  HDRS
+    adddf3.h
+  DEPENDS
+    libc.src.__support.FPUtil.generic.add_sub
+    libc.src.__support.macros.config
+)
diff --git a/libc/src/__support/builtins/adddf3.h b/libc/src/__support/builtins/adddf3.h
new file mode 100644
index 0000000000000..9fa60ca8d1c69
--- /dev/null
+++ b/libc/src/__support/builtins/adddf3.h
@@ -0,0 +1,32 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This header exposes LLVM-libc's __adddf3 implementation as builtins::adddf3
+/// so that it can be reused by compiler-rt's builtins.
+///
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_BUILTINS_ADDDF3_H
+#define LLVM_LIBC_SRC___SUPPORT_BUILTINS_ADDDF3_H
+
+#include "src/__support/FPUtil/generic/add_sub.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace builtins {
+
+// Addition at double precision; mirrors compiler-rt's __adddf3.
+LIBC_INLINE double adddf3(double x, double y) {
+  return fputil::generic::add<double>(x, y);
+}
+
+} // namespace builtins
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_BUILTINS_ADDDF3_H
diff --git a/libc/test/shared/CMakeLists.txt b/libc/test/shared/CMakeLists.txt
index 8449d18dd6e10..3b6bb6c2cf507 100644
--- a/libc/test/shared/CMakeLists.txt
+++ b/libc/test/shared/CMakeLists.txt
@@ -826,6 +826,7 @@ add_fp_unittest(
   SRCS
     shared_builtins_test.cpp
   DEPENDS
+    libc.src.__support.builtins.adddf3
     libc.src.__support.builtins.addtf3
     libc.src.__support.builtins.divtf3
     libc.src.__support.builtins.multf3
diff --git a/libc/test/shared/shared_builtins_test.cpp b/libc/test/shared/shared_builtins_test.cpp
index b8350829f4f71..4960c7abe6b04 100644
--- a/libc/test/shared/shared_builtins_test.cpp
+++ b/libc/test/shared/shared_builtins_test.cpp
@@ -15,7 +15,7 @@ TEST(LlvmLibcSharedBuiltinsTest, AllFloat) {
 }
 
 TEST(LlvmLibcSharedBuiltinsTest, AllDouble) {
-  // TODO: assertions for shared::*df3 builtins.
+  EXPECT_FP_EQ(3.0, LIBC_NAMESPACE::shared::adddf3(1.0, 2.0));
 }
 
 #ifdef LIBC_TYPES_HAS_FLOAT128