[libc] Add implementation of difftime function.

[rtenneti]

Summary

The difftime function computes the difference between two calendar
times: time1 - time0 as per as per 7.27.2.2 section in
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2478.pdf.

double difftime(time_t time1, time_t time0);

Tested:
Unit tests

Co-authored-by: Jeff Bailey <[e-mail]>

[rtenneti]

Hi Jeff and Siva,

I have abandoned the previous version and reuploaded a newer version with all the changes. PTAL. Thanks much

libc/config/linux/aarch64/entrypoints.txt

list(APPEND TARGET_LIBC_ENTRYPOINTS
     libc.src.threads.tss_create
     libc.src.threads.tss_delete
     libc.src.threads.tss_get
     libc.src.threads.tss_set
 
     # time.h entrypoints
     libc.src.time.asctime
     libc.src.time.asctime_r
+    libc.src.time.difftime
     libc.src.time.gmtime
     libc.src.time.gmtime_r
     libc.src.time.mktime
     libc.src.time.nanosleep
     libc.src.time.clock_gettime
 
     # unistd.h entrypoints
     libc.src.unistd.environ
   )
 endif()
 
 set(TARGET_LLVMLIBC_ENTRYPOINTS
   ${TARGET_LIBC_ENTRYPOINTS}
   ${TARGET_LIBM_ENTRYPOINTS}
 )

libc/config/linux/x86_64/entrypoints.txt

list(APPEND TARGET_LIBC_ENTRYPOINTS
     libc.src.threads.tss_create
     libc.src.threads.tss_delete
     libc.src.threads.tss_get
     libc.src.threads.tss_set
 
     # time.h entrypoints
     libc.src.time.asctime
     libc.src.time.asctime_r
+    libc.src.time.difftime
     libc.src.time.gmtime
     libc.src.time.gmtime_r
     libc.src.time.mktime
     libc.src.time.nanosleep
     libc.src.time.clock_gettime
 
     # unistd.h entrypoints
     libc.src.unistd.environ

libc/docs/date_and_time.rst

 clock
 clock_getcpuclockid
 clock_getres
 clock_gettime         |check|
 clock_nanosleep
 clock_settime
 ctime
 ctime_r
-difftime
+difftime              |check|
 getdate
 gmtime                |check|
 gmtime_r              |check|
 localtime
 localtime_r
 mktime                |check|
 nanosleep             |check|
 strftime

libc/src/time/CMakeLists.txt

add_entrypoint_object(
   DEPENDS
     libc.include.time
     libc.include.sys_syscall
     libc.src.__support.OSUtil.osutil
     libc.src.errno.errno
 )
 
 add_entrypoint_object(
+  difftime
+  SRCS
+    difftime.cpp
+  HDRS
+    difftime.h
+  DEPENDS
+    libc.include.time
+)
+
+add_entrypoint_object(
   gmtime
   SRCS
     gmtime.cpp
   HDRS
     gmtime.h
   DEPENDS
     .time_utils
     libc.include.time

libc/src/time/difftime.h

+//===-- Implementation header of difftime -----------------------*- 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_TIME_DIFFTIME_H
+#define LLVM_LIBC_SRC_TIME_DIFFTIME_H
+
+#include <time.h>
+
+namespace __llvm_libc {
+
+double difftime(time_t end, time_t beginning);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_TIME_DIFFTIME_H
+
+#include "include/time.h"

libc/src/time/difftime.cpp

+//===-- Implementation of difftime function -------------------------------===//
+//
+// 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/time/difftime.h"
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(double, difftime, (time_t end, time_t beginning)) {
+  return end - beginning;
+}
+
+} // namespace __llvm_libc

libc/test/src/time/CMakeLists.txt

SUITE
     libc_time_unittests
   SRCS
   clock_gettime_test.cpp
   DEPENDS
     libc.src.time.clock_gettime
 )
 
 add_libc_unittest(
+  difftime
+  SUITE
+    libc_time_unittests
+  SRCS
+    difftime_test.cpp
+  DEPENDS
+    libc.src.time.difftime
+)
+
+add_libc_unittest(
   gmtime
   SUITE
     libc_time_unittests
   SRCS
     gmtime_test.cpp
   HDRS
     TmMatcher.h
   DEPENDS

libc/test/src/time/difftime_test.cpp

+//===-- Unittests for difftime --------------------------------------------===//
+//
+// 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/__support/FPUtil/FPBits.h"
+#include "src/time/difftime.h"
+#include "src/time/time_utils.h"
+#include "test/ErrnoSetterMatcher.h"
+#include "utils/UnitTest/Test.h"
+
+#include <errno.h>
+
+using __llvm_libc::testing::ErrnoSetterMatcher::Succeeds;
+using __llvm_libc::time_utils::TimeConstants;
+
+TEST(LlvmLibcDifftime, SmokeTest) {
+  time_t t1_seconds = TimeConstants::SECONDS_PER_HOUR;
+  time_t t2_seconds = 0;
+
+  __llvm_libc::fputil::FPBits<long double> expected_fp =
+      __llvm_libc::fputil::FPBits<long double>();
+  expected_fp = __llvm_libc::fputil::FPBits<long double>(
+      static_cast<long double>(t1_seconds));
+
+  double result = __llvm_libc::difftime(t1_seconds, t2_seconds);
+
+  __llvm_libc::fputil::FPBits<long double> actual_fp =
+      __llvm_libc::fputil::FPBits<long double>();
+  actual_fp = __llvm_libc::fputil::FPBits<long double>(
+      static_cast<long double>(result));
+
+  EXPECT_EQ(actual_fp.bits, expected_fp.bits);
+  EXPECT_EQ(actual_fp.get_sign(), expected_fp.get_sign());
+  EXPECT_EQ(actual_fp.get_exponent(), expected_fp.get_exponent());
+  EXPECT_EQ(actual_fp.get_mantissa(), expected_fp.get_mantissa());
+}