Illustrate a redirector using the example of round function from math.h.

Setup demonstrated in this patch is only for ELF-ish platforms.

Also note:

1. Use of redirectors is a temporary scheme. They will be removed once
   LLVM-libc has implementations for the redirected functions.
2. Redirectors are optional. One can choose to not include them in the
   LLVM-libc build for their platform.
3. Even with redirectors used, we want to link to the system libc
   dynamically.

Reviewers: dlj, hfinkel, jakehehrlich, phosek, stanshebs, theraven, alexshap

Subscribers: mgorny, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D69020

4 files changed, 70 insertions, 0 deletions

diff --git a/libc/src/math/round/CMakeLists.txt b/libc/src/math/round/CMakeLists.txt
new file mode 100644
index 00000000000..e7ad9d2a73f
--- /dev/null
+++ b/libc/src/math/round/CMakeLists.txt
@@ -0,0 +1,14 @@
+add_entrypoint_object(
+  round
+  REDIRECTED
+  SRCS
+    round.cpp
+  HDRS
+    round.h
+)
+
+add_redirector_object(
+  round_redirector
+  SRC
+    round_redirector.cpp
+)
diff --git a/libc/src/math/round/round.cpp b/libc/src/math/round/round.cpp
new file mode 100644
index 00000000000..7352b98269f
--- /dev/null
+++ b/libc/src/math/round/round.cpp
@@ -0,0 +1,21 @@
+//===---------------------- Implementation of round -----------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/math/round/round.h"
+
+#include "src/__support/common.h"
+
+namespace llvm_libc {
+
+double __round_redirector(double x);
+
+double LLVM_LIBC_ENTRYPOINT(round)(double x) {
+  return __round_redirector(x);
+}
+
+} // namespace llvm_libc
diff --git a/libc/src/math/round/round.h b/libc/src/math/round/round.h
new file mode 100644
index 00000000000..c13ecccfeca
--- /dev/null
+++ b/libc/src/math/round/round.h
@@ -0,0 +1,18 @@
+//===------------------ Implementation header for round -------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_MATH_ROUND_H
+#define LLVM_LIBC_SRC_MATH_ROUND_H
+
+namespace llvm_libc {
+
+double round(double x);
+
+} // namespace llvm_libc
+
+#endif // LLVM_LIBC_SRC_MATH_ROUND_H
diff --git a/libc/src/math/round/round_redirector.cpp b/libc/src/math/round/round_redirector.cpp
new file mode 100644
index 00000000000..8d37b0895c7
--- /dev/null
+++ b/libc/src/math/round/round_redirector.cpp
@@ -0,0 +1,17 @@
+//===----------------  Implementation of round redirector -----------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include <math.h>
+
+namespace llvm_libc {
+
+double __round_redirector(double x) {
+  return ::round(x);
+}
+
+} // namespace llvm_libc