Initial commit of mktime.

[rtenneti]

Summary

This introduces mktime to LLVM libc, based on C99/C2X/Single Unix Spec.

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

This change doesn't handle TIMEZONE, tm_isdst and leap seconds. It returns -1 for invalid dates. I have verified the return results for all the possible dates with glibc's mktime.

TODO:
+ Handle leap seconds.
+ Handle out of range time and date values that don't overflow or underflow.
+ Implement the following suggestion Siva - As we start accumulating the seconds, we should be able to check if the next amount of seconds to be added can lead to an overflow. If it does, return the overflow value. If not keep accumulating. The benefit is that, we don't have to validate every input, and also do not need the special cases for sizeof(time_t) == 4.
+ Handle timezone and update of tm_isdst

[MaskRay]

Drive-by: POSIX.1-2017: "Local timezone information shall be set as though mktime() called tzset()." Is "TZ" handled in this patch?

libc/config/linux/api.td

34 ↗

(On Diff #305544)

Seems that tm_isdst is entirely unhandled.

libc/src/time/mktime.cpp

11

Drive-by: LLVM Coding Standards talk about the use of anonymous namespaces: https://llvm.org/docs/CodingStandards.html#anonymous-namespaces
https://llvm.org/docs/CodingStandards.html#use-namespace-qualifiers-to-implement-previously-declared-functions

15

namespace-scope const/constexpr variables are automatically of internal linkage. You don't need an anonymous namespace (which is also discouraged) in this case.

36

Have two arrays for leap/nonleap wastes space (libc.a space matters a lot). Having one array plus leap adjustment is sufficient.

[rtenneti]

Remove anonymous namespace, fix typos in ILP64 msg.
Removed duplicate arrays for days calculation.

[sivachandra]

Raman, looks like you lost other files when you updated the patch.

[rtenneti]

Fixed the comments from Fangrui.

[sivachandra]

libc/src/time/mktime.cpp

45

The standard says,

"The values in time are not checked for being out of range."

So, is all this checking necessary?

53

Could we perhaps define a OVERFLOW_VALUE instead of doing (time_t)(-1) everywhere? You can define it in the implementation header as a static constexpr which then allows us to use it in the tests as well.

64

Could we do this overflow check in a more general fashion? For example, we can find a maxYears that can fit in TIME_T_MAX (I am making up TIME_T_MAX). After that, assuming valid inputs for other fields, we can progressively check if the result will fit in time_t.

66

Its not clear to me as to what would/should change on a ILP64 platform. Can add a comment explaining this?

libc/test/src/time/mktime_test.cpp

23 ↗

(On Diff #305819)

Except for this line, it doesn't seem like you need this function initialize_tm_data. A pattern like this does not work?

tm temp{...};
...
temp = {...};

33 ↗

(On Diff #305819)

You can inline lines 31 to 33 to:

EXPECT_EQ(__llvm_libc::mktime(&temp), OVERFLOW_VALUE);

And use the same pattern at other places where relevant.

[jeffbailey]

libc/src/time/mktime.cpp

45

The Single Unix Spec says this:

The mktime() function shall return the specified time since the Epoch encoded as a value of type time_t. If the time since the Epoch cannot be represented, the function shall return the value (time_t)-1 [CX] [Option Start] and set errno to indicate the error.

So our bug is that we're not setting errno here to EOVERFLOW.

53

This should be OUT_OF_RANGE since it serves underflow as well.

66

We'll add a comment. In brief: because struct tm is ints, a 32-bit int year can be represented in a 64-bit time_t. However, a 64-bit int year cannot. Without a good way to run tests on it, I'd rather just assume a 32-bit int and make sure it catches fire at build time when that's violated.

libc/test/src/time/mktime_test.cpp

23 ↗

(On Diff #305819)

This is the case where I was hoping we could use a C99/C++20 designated initializer so that it was readable and clean.

[rtenneti]

Fixed comments.

[rtenneti]

Fixed code review comments.

[rtenneti]

Jeff and I have made the changes. Please take another look. Thanks.

libc/config/linux/api.td

34 ↗

(On Diff #305544)

Added a TODO to handle tm_isdst . Also needs to read timezone file.

libc/src/time/mktime.cpp

45

Hi Siva and Jeff,

Added a TODO to handle out of range date and time (it seems mktime in C converts invalid dates to valid dates.  For example  "October 40th is silently converted to November 9th")

64

Add a TODO to investigate the above idea.

libc/test/src/time/mktime_test.cpp

23 ↗

(On Diff #305819)

Done.

[jeffbailey]

libc/src/time/mktime.cpp

64

Is the TODO still necessary with the creation of outOfRange() that sets errno and returns -1?

[rtenneti]

Fixed Jeff's comments.

[rtenneti]

Fixed the todo.

[rtenneti]

Removed the TODO. Thanks Jeff.

[sivachandra]

Mostly LGTM. I have left a few nits along with a not-a-nit comment.

From offline discussions, I understand you are preparing follow ups. It should be OK to land WiP pieces as long as you have clear TODOs about what is missing.

libc/src/time/mktime.cpp

42

Nit: Use static_cast instead of C-style casts, similar to how you do in tests. Even better, define another constant like this:

constexpr time_t OutOfRangeReturnValue = -1;

45

Can you add TODOs for the pieces missing in this implementation? Just rephrase what you have in the commit message as a TODO.

64

Along with the above comment about checking for invalid input, what I meant was this: As we start accumulating the seconds, we should be able to check if the next amount of seconds to be added can lead to an overflow. If it does, return the overflow value. If not keep accumulating. The benefit is that, we don't have to validate every input, and also do need the special cases for sizeof(time_t) == 4.

libc/test/src/time/mktime_test.cpp

30 ↗

(On Diff #306955)

Nit: To avoid confusions, can you rename this function to call_mktime?

44 ↗

(On Diff #306955)

Nit: Define a global perhaps to avoid repeated listings of static_cast. Something like:

static constexpr time_t OutOfRangeReturnValue = -1;

[rtenneti]

Fixed Siva's comments and added a TODO.

[sivachandra]

libc/src/time/mktime.cpp

53

Ah, a typo in my comment got carried over here.... "... also, do NOT need the special cases ..."

[rtenneti]

Added "not" to the comment.

[rtenneti]

libc/src/time/mktime.cpp

64

Added a TODO. The above is my intetntion when implementing handle of invalid data (of some one passing INT_MAX in either seconds, minutes, hours ...).

[MaskRay]

libc/config/linux/api.td

41 ↗

(On Diff #308529)

This may not be a good declaration for 32-bit systems (their support may have lower priority, though, I guess). https://sourceware.org/glibc/wiki/Y2038ProofnessDesign

Then you can avoid all the sizeof(time_t) == 4 special cases.

[jeffbailey]

libc/config/linux/api.td

41 ↗

(On Diff #308529)

The declaration of time_t comes from the following:

The range and precision of times representable in clock_t and time_t are
implementation-defined. (C99 7.23.1(4))

time_t shall be an integer type. (https://pubs.opengroup.org/onlinepubs/9699919799/toc.htm)

IBM has an analysis: https://www.ibm.com/support/knowledgecenter/SSFKSJ_7.5.0/com.ibm.mq.ref.dev.doc/q104610_.htm

In practice, it will be somewhat more nuanced because of backwards compatibility. I expect that *historical* 32-bit systems will need to maintain a 32-bit signed time_t for compat. However, *new* ones (like em32_t) would use a 64-bit time_t. Since this libc doesn't any support for that we've used this. Someone implementing em32_t would have to go over I think many such types, so I'm not worried about this being extra work for them later if/when that happens.

libc/src/time/mktime.cpp

 //===-- Implementation of mktime 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/__support/common.h"
 #include "src/time/mktime.h"
 
 namespace __llvm_libc {
 
-namespace {
 constexpr int SecondsPerMin = 60;
 constexpr int MinutesPerHour = 60;
 constexpr int HoursPerDay = 24;
 constexpr int DaysPerWeek = 7;
 constexpr int MonthsPerYear = 12;
 constexpr int DaysPerNonLeapYear = 365;
 constexpr int TimeYearBase = 1900;
 constexpr int EpochYear = 1970;
 constexpr int EpochWeekDay = 4;
 // The latest time that can be represented in this form is 03:14:07 UTC on
 // Tuesday, 19 January 2038 (corresponding to 2,147,483,647 seconds since the
 // start of the epoch). This means that systems using a 32-bit time_t type are
 // susceptible to the Year 2038 problem.
 constexpr int EndOf32BitEpochYear = 2038;
 
-constexpr int LeapYearDaysInMonth[] = {
-  31 /* Jan */, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
-};
-
 constexpr int NonLeapYearDaysInMonth[] = {
   31 /* Jan */, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
 };
 
-constexpr int LeapYearDaysByMonth[] = {
-  0 /* Jan */, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335
-};
-
-constexpr int NonLeapYearDaysByMonth[] = {
-  0 /* Jan */, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
-};
-
 constexpr bool isLeapYear (const time_t year) {
   return (((year) % 4) == 0 && (((year) % 100) != 0 || ((year) % 400) == 0));
 }
-}
 
 time_t LLVM_LIBC_ENTRYPOINT(mktime)(struct tm *t1) {
   // Unlike most C Library functions, mktime doesn't just die on bad input.
   // TODO(rtenneti); Handle leap seconds.
   if (t1->tm_sec < 0 || t1->tm_sec > (SecondsPerMin - 1))
     return (time_t)(-1);
   if (t1->tm_min < 0 || t1->tm_min > (MinutesPerHour - 1))
     return (time_t)(-1);
   if (t1->tm_hour < 0 || t1->tm_hour > (HoursPerDay - 1))
     return (time_t)(-1);
   time_t tmYearFromBase = t1->tm_year + TimeYearBase;
 
   if (tmYearFromBase < EpochYear)
     return (time_t)(-1);
 
   // 32-bit end-of-the-world is 03:14:07 UTC on 19 January 2038.
   if (sizeof(time_t) == 4 && tmYearFromBase >= EndOf32BitEpochYear) {
     if (tmYearFromBase > EndOf32BitEpochYear)
       return (time_t)(-1);
     if (t1->tm_mon > 0)
       return (time_t)(-1);
     if (t1->tm_mday > 19)
       return (time_t)(-1);
     if (t1->tm_hour > 3)
       return (time_t)(-1);
     if (t1->tm_min > 14)
       return (time_t)(-1);
     if (t1->tm_sec > 7)
       return (time_t)(-1);
   }
 
   static_assert(sizeof(int) == 4,
-                "ILP64 is uimplemented.  Ths implementation requires 32-bit "
-                "integers.");
+                "ILP64 is unimplemented.  This implementation requires "
+                "32-bit integers.");
 
   if (t1->tm_mon < 0 || t1->tm_mon > (MonthsPerYear - 1))
     return (time_t)(-1);
   bool tmYearIsLeap = isLeapYear(tmYearFromBase);
-  time_t daysInMonth = tmYearIsLeap
-                       ? LeapYearDaysInMonth[t1->tm_mon]
-                       : NonLeapYearDaysInMonth[t1->tm_mon];
+  time_t daysInMonth = NonLeapYearDaysInMonth[t1->tm_mon];
+  // Add one day if it is a leap year and the month is February.
+  if (tmYearIsLeap && t1->tm_mon == 1)
+    ++daysInMonth;
   if (t1->tm_mday < 1 || t1->tm_mday > daysInMonth)
     return (time_t)(-1);
 
   time_t totalDays = t1->tm_mday - 1;
-  totalDays += (tmYearIsLeap
-                ? LeapYearDaysByMonth[t1->tm_mon]
-                : NonLeapYearDaysByMonth[t1->tm_mon]);
+  for (int i = 0; i < t1->tm_mon; ++i)
+    totalDays += NonLeapYearDaysInMonth[i];
+  // Add one day if it is a leap year and the month is after February.
+  if (tmYearIsLeap && t1->tm_mon > 1)
+    totalDays++;
   t1->tm_yday = totalDays;
   totalDays += (tmYearFromBase - EpochYear) * DaysPerNonLeapYear;
 
   // Add an extra day for each leap year, starting with 1972
   for (time_t year = EpochYear + 2; year < tmYearFromBase;) {
     if (isLeapYear(year)) {
         totalDays += 1;
         year += 4;
     } else {
       year++;
     }
   }
 
   t1->tm_wday = (EpochWeekDay + totalDays) % DaysPerWeek;
   if (t1->tm_wday < 0)
     t1->tm_wday += DaysPerWeek;
-  // TODO(rtenneti): Need to handle timezone
+  // TODO(rtenneti): Need to handle timezone and update of tm_isdst.
   return t1->tm_sec
       + t1->tm_min * SecondsPerMin
       + t1->tm_hour * MinutesPerHour * SecondsPerMin
       + totalDays * HoursPerDay * MinutesPerHour * SecondsPerMin;
 }
 
 } // namespace __llvm_libc