[lldb][formatters] Add support for printing NSConstantDate and fix distantPast value

[teemperor]

Summary

Certain NSDate constructors return a special NSConstantDate class which currently ends
up being unformatted as it's not in the list of supported classes for the NSDate formatter.
This patch adds that class to the supported class list so LLDB produces a summary for it.

One of these special constructors is [NSDate distantPast] which returns the date for
0001-01-01 00:00:00 UTC. LLDB has a special case for formatting this date but for some
reason we did hardcode the wrong summary string in that special case. Maybe the summary
string was correct back when the code was written but it isn't correct anymore (distantPast
isn't actually defined to be a special date but just some 'a guaranteed temporal boundary.' so
maybe someone changed the value in the last 10 years).

If someone else is wondering why we even have this special case for distantPast but not for
the future. The reason seems to be that our date formatting for really old dates is off by 24
hours. So for example, adding one second to distantPast will cause LLDB to print
0000-12-30 00:00:01 UTC (which is 24 hours behind the expected result). So to make
our code appear to be correct it seems we just hardcoded the most common NSDate result
from that time span. I'll replace that logic with a generic solution in a probably more invasive follow up patch.

I also took the freedom to replace the magic value -63114076800 with some constant
+ documentation. I heard there are some people that don't know from the top of their head that there
are 63114076800 seconds between 1. Jan 0001 and 1. January 2001 in whatever calendar system
NSDate is using.

[teemperor]

• Added a FIXME to the hack.

[mib]

lldb/source/Plugins/Language/ObjC/Cocoa.cpp

858

Can't we get that value programmatically ?

[teemperor]

lldb/source/Plugins/Language/ObjC/Cocoa.cpp

858

I think we could inspect the NSDate _NSDateDistantPast values. But I'm not sure if it's worth it even making a patch for that as this special case should anyway just disappear once we properly format ancient NSDate values (and it's anyway not really in the scope of this patch).

[mib]

lldb/source/Plugins/Language/ObjC/Cocoa.cpp

858

Sounds good to me then.

[teemperor]

• Rebase before commit.

[davide]

lldb/source/Plugins/Language/ObjC/Cocoa.cpp

858

I agree with Raph.

lldb/source/Plugins/Language/ObjC/Cocoa.cpp

bool lldb_private::formatters::NSDateSummaryProvider(
   double date_value = 0.0;
 
   ConstString class_name = descriptor->GetClassName();
 
   static const ConstString g_NSDate("NSDate");
   static const ConstString g___NSDate("__NSDate");
   static const ConstString g___NSTaggedDate("__NSTaggedDate");
   static const ConstString g_NSCalendarDate("NSCalendarDate");
+  static const ConstString g_NSConstantDate("NSConstantDate");
 
   if (class_name.IsEmpty())
     return false;
 
   uint64_t info_bits = 0, value_bits = 0;
   if ((class_name == g_NSDate) || (class_name == g___NSDate) ||
-      (class_name == g___NSTaggedDate)) {
+      (class_name == g___NSTaggedDate) || (class_name == g_NSConstantDate)) {
     if (descriptor->GetTaggedPointerInfo(&info_bits, &value_bits)) {
       date_value_bits = ((value_bits << 8) | (info_bits << 4));
       memcpy(&date_value, &date_value_bits, sizeof(date_value_bits));
     } else {
       llvm::Triple triple(
           process_sp->GetTarget().GetArchitecture().GetTriple());
       uint32_t delta =
           (triple.isWatchOS() && triple.isWatchABI()) ? 8 : ptr_size;
if ((class_name == g_NSDate) || (class_name == g___NSDate) ||
     date_value_bits = process_sp->ReadUnsignedIntegerFromMemory(
         valobj_addr + 2 * ptr_size, 8, 0, error);
     memcpy(&date_value, &date_value_bits, sizeof(date_value_bits));
     if (error.Fail())
       return false;
   } else
     return false;
 
-  if (date_value == -63114076800) {
-    stream.Printf("0001-12-30 00:00:00 +0000");
+  // FIXME: It seems old dates are not formatted according to NSDate's calendar
+  // so we hardcode distantPast's value so that it looks like LLDB is doing
+  // the right thing.
+
+  // The relative time in seconds from Cocoa Epoch to [NSDate distantPast].
+  const double RelSecondsFromCocoaEpochToNSDateDistantPast = -63114076800;
+  if (date_value == RelSecondsFromCocoaEpochToNSDateDistantPast) {
+    stream.Printf("0001-01-01 00:00:00 UTC");
     return true;
   }
 
   // Accomodate for the __NSTaggedDate format introduced in Foundation 1600.
   if (class_name == g___NSTaggedDate) {
     auto *runtime = llvm::dyn_cast_or_null<AppleObjCRuntime>(
         ObjCLanguageRuntime::Get(*process_sp));
     if (runtime && runtime->GetFoundationVersion() >= 1600)

lldb/test/API/functionalities/data-formatter/data-formatter-objc/TestDataFormatterObjCNSDate.py

def nsdate_data_formatter_commands(self):
                              '@"Europe/Rome"',
                              '@"Europe/Paris"'])
 
         self.expect(
             'frame variable mut_bv',
             substrs=[
                 '(CFMutableBitVectorRef) mut_bv = ',
                 '1110 0110 1011 0000 1101 1010 1000 1111 0011 0101 1101 0001 00'])
+
+        self.expect_expr("distant_past", result_summary="0001-01-01 00:00:00 UTC")
+        self.expect_expr("distant_future", result_summary="4001-01-01 00:00:00 UTC")

lldb/test/API/functionalities/data-formatter/data-formatter-objc/main.m

int main(int argc, const char *argv[]) {
   NSDate *date_1970_minus_06 = [NSDate dateWithTimeIntervalSince1970:-0.6];
   NSDate *date_1970_minus_05 = [NSDate dateWithTimeIntervalSince1970:-0.5];
   NSDate *date_1970_minus_04 = [NSDate dateWithTimeIntervalSince1970:-0.4];
 
   NSDate *date_1970_plus_06 = [NSDate dateWithTimeIntervalSince1970:0.6];
   NSDate *date_1970_plus_05 = [NSDate dateWithTimeIntervalSince1970:0.5];
   NSDate *date_1970_plus_04 = [NSDate dateWithTimeIntervalSince1970:0.4];
 
+  NSDate *distant_past = [NSDate distantPast];
+  NSDate *distant_future = [NSDate distantFuture];
+
   CFAbsoluteTime date1_abs = CFDateGetAbsoluteTime(date1);
   CFAbsoluteTime date2_abs = CFDateGetAbsoluteTime(date2);
   CFAbsoluteTime date3_abs = CFDateGetAbsoluteTime(date3);
   CFAbsoluteTime date4_abs = CFDateGetAbsoluteTime(date4);
   CFAbsoluteTime date5_abs = CFDateGetAbsoluteTime(date5);
 
   NSIndexSet *iset1 =
       [[NSIndexSet alloc] initWithIndexesInRange:NSMakeRange(1, 4)];