[libc] Add swab implementation

Swab implementation is added to libc/src/unistd.

Differential Revision: https://reviews.llvm.org/D147970

libc/config/linux/aarch64/entrypoints.txt

@@ -187,6 +187,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.readlink
     libc.src.unistd.readlinkat
     libc.src.unistd.rmdir
+    libc.src.unistd.swab
     libc.src.unistd.symlink
     libc.src.unistd.symlinkat
     libc.src.unistd.sysconf

libc/config/linux/riscv64/entrypoints.txt

@@ -189,6 +189,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.readlink
     libc.src.unistd.readlinkat
     libc.src.unistd.rmdir
+    libc.src.unistd.swab
     libc.src.unistd.symlink
     libc.src.unistd.symlinkat
     libc.src.unistd.sysconf

libc/config/linux/x86_64/entrypoints.txt

@@ -191,6 +191,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.unistd.readlink
     libc.src.unistd.readlinkat
     libc.src.unistd.rmdir
+    libc.src.unistd.swab
     libc.src.unistd.symlink
     libc.src.unistd.symlinkat
     libc.src.unistd.sysconf

libc/spec/posix.td

@@ -544,6 +544,11 @@ def POSIX : StandardSpec<"POSIX"> {
           RetValSpec<IntType>,
           [ArgSpec<ConstCharPtr>]
         >,
+        FunctionSpec<
+          "swab",
+          RetValSpec<VoidType>,
+          [ArgSpec<ConstVoidRestrictedPtr>, ArgSpec<VoidPtr>, ArgSpec<SSizeTType>]
+        >,
         FunctionSpec<
           "symlink",
           RetValSpec<IntType>,

libc/src/unistd/CMakeLists.txt

@@ -191,6 +191,13 @@ add_entrypoint_object(
     .${LIBC_TARGET_OS}.rmdir
 )
 
+add_entrypoint_object(
+  swab
+  ALIAS
+  DEPENDS
+    .${LIBC_TARGET_OS}.swab
+)
+
 add_entrypoint_object(
   symlink
   ALIAS

libc/src/unistd/linux/CMakeLists.txt

@@ -353,6 +353,14 @@ add_entrypoint_object(
     libc.src.errno.errno
 )
 
+add_entrypoint_object(
+  swab
+  SRCS
+    swab.cpp
+  HDRS
+    ../swab.h
+)
+
 add_entrypoint_object(
   symlink
   SRCS

libc/src/unistd/linux/swab.cpp

@@ -0,0 +1,26 @@
+//===-- Implementation of swab --------------------------------------------===//
+//
+// 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/unistd/swab.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(void, swab,
+                   (const void *__restrict from, void *__restrict to,
+                    ssize_t n)) {
+  const unsigned char *f = static_cast<const unsigned char *>(from);
+  unsigned char *t = static_cast<unsigned char *>(to);
+  for (ssize_t i = 1; i < n; i += 2) {
+    t[i - 1] = f[i];
+    t[i] = f[i - 1];
+  }
+}
+
+} // namespace __llvm_libc

libc/src/unistd/swab.h

@@ -0,0 +1,20 @@
+//===-- Implementation header for swab --------------------------*- C++ -*-===//
+//
+// 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_UNISTD_SWAB_H
+#define LLVM_LIBC_SRC_UNISTD_SWAB_H
+
+#include <unistd.h> // For ssize_t
+
+namespace __llvm_libc {
+
+void swab(const void *__restrict from, void *__restrict to, ssize_t n);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_UNISTD_SWAB_H

libc/test/src/unistd/CMakeLists.txt

@@ -225,6 +225,16 @@ add_libc_unittest(
     libc.src.unistd.rmdir
 )
 
+add_libc_unittest(
+  swab_test
+  SUITE
+    libc_unistd_unittests
+  SRCS
+    swab_test.cpp
+  DEPENDS
+    libc.src.unistd.swab
+)
+
 add_libc_unittest(
   readlink_test
   SUITE

libc/test/src/unistd/swab_test.cpp

@@ -0,0 +1,83 @@
+
+//===-- Unittests for swab ------------------------------------------------===//
+//
+// 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/unistd/swab.h"
+
+#include "src/string/string_utils.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcSwabTest, NegativeSizeIsNoOp) {
+  const char *from = "abc";
+  char to[4] = {'x', 'y', 'z', '\0'};
+  __llvm_libc::swab(from, to, -1);
+  ASSERT_STREQ(to, "xyz");
+}
+
+TEST(LlvmLibcSwabTest, ZeroSizeIsNoOp) {
+  const char *from = "abc";
+  char to[4] = {'x', 'y', 'z', '\0'};
+  __llvm_libc::swab(from, to, 0);
+  ASSERT_STREQ(to, "xyz");
+}
+
+TEST(LlvmLibcSwabTest, SingleByteIsNoOp) {
+  char from[] = {'a'};
+  char to[4] = {'x', 'y', 'z', '\0'};
+  __llvm_libc::swab(from, to, sizeof(from));
+  ASSERT_STREQ(to, "xyz");
+}
+
+TEST(LlvmLibcSwabTest, NullPtrsAreNotDeRefedIfNIsLessThanTwo) {
+  // This test passes if a crash does not happen
+  __llvm_libc::swab(nullptr, nullptr, -1);
+  __llvm_libc::swab(nullptr, nullptr, 0);
+  __llvm_libc::swab(nullptr, nullptr, 1);
+}
+
+TEST(LlvmLibcSwabTest, BytesAreSwappedWithEvenN) {
+  {
+    const char *from = "ab";
+    char to[3] = {};
+    __llvm_libc::swab(from, to, __llvm_libc::internal::string_length(from));
+    ASSERT_STREQ(to, "ba");
+  }
+  {
+    const char *from = "abcd";
+    char to[5] = {};
+    __llvm_libc::swab(from, to, __llvm_libc::internal::string_length(from));
+    ASSERT_STREQ(to, "badc");
+  }
+  {
+    const char *from = "aAaAaA";
+    char to[7] = {};
+    __llvm_libc::swab(from, to, __llvm_libc::internal::string_length(from));
+    ASSERT_STREQ(to, "AaAaAa");
+  }
+}
+
+TEST(LlvmLibcSwabTest, LastByteIgnoredWithOddN) {
+  {
+    const char *from = "aba";
+    char to[3] = {};
+    __llvm_libc::swab(from, to, __llvm_libc::internal::string_length(from));
+    ASSERT_STREQ(to, "ba");
+  }
+  {
+    const char *from = "abcde";
+    char to[5] = {};
+    __llvm_libc::swab(from, to, __llvm_libc::internal::string_length(from));
+    ASSERT_STREQ(to, "badc");
+  }
+  {
+    const char *from = "aAaAaAx";
+    char to[7] = {};
+    __llvm_libc::swab(from, to, __llvm_libc::internal::string_length(from));
+    ASSERT_STREQ(to, "AaAaAa");
+  }
+}