Add a new preprocessor option `-fbuild-session-timestamp-date-time-macros`

[appcs]

Summary

Add a new preprocessor option -fbuild-session-timestamp-date-time-macros to
set the predefined macros: DATE and TIME using option
-fbuild-session-timestamp or -fbuild-session-file=<file>

Motivation:
0. Using the SOURCE_DATE_EPOCH environmental variable to specify reproducible

embedded timestamps. See: 

a. https://reproducible-builds.org/specs/source-date-epoch/
b. https://gcc.gnu.org/onlinedocs/cpp/Environment-Variables.html

1. Add a -ffixed-date-time= flag that sets the initial value of DATE, TIME, TIMESTAMP
   
   https://reviews.llvm.org/D23934

[thakis]

Why isn't D23934 sufficient? What do these flags with very long names do that ffixed-date-time doesn't?

[bruno]

Why isn't D23934 sufficient? What do these flags with very long names do that ffixed-date-time doesn't?

+1

Also, how does this work with the testcase added in D23934? Also, along the lines of similar questions in D23934, how does this affect -Wdate-time and -Wpch-date-time?

[appcs]

In D121040#3362536, @thakis wrote:

Why isn't D23934 sufficient? What do these flags with very long names do that ffixed-date-time doesn't?

Given that -fbuild-session-timestamp and -fbuild-session-file=<file> already exist, and that I had implemented the above since asking if https://reviews.llvm.org/D23934 would get committed, I submitted this.
The thought behind the choice of option name was that all the related options would appear together under --help

I will port https://reviews.llvm.org/D23934 to top of trunk, thank you.

[appcs]

In D121040#3365149, @bruno wrote:

...
Also, along the lines of similar questions in D23934, how does this affect -Wdate-time and -Wpch-date-time?

In the rebase of D23934:

a. Disallow the simultaneous use of -Wdate-time/-Wpch-date-time and -ffixed-date-time=<timestamp>?
b. Disallow -ffixed-date-time=<timestamp>:

0. if PCH's are used
1. in PCH compilation

Also, (just as -Wdate-time warns on TIMESTAMP use), do we (in a separate patch), extend the scope of -Wpch-date-time to cover the use of TIMESTAMP is in a PCH?

Thank you.

[appcs]

I will port https://reviews.llvm.org/D23934 to top of trunk, thank you.

Rebased. Please see: D122517

clang/include/clang/Basic/DiagnosticDriverKinds.td

def err_drv_needs_hvx_version : Error<
   "%0 is not supported on HVX %1">;
 
 def err_drv_module_header_wrong_kind : Error<
   "header file '%0' input type '%1' does not match type of prior input "
   "in module compilation; use '-x %2' to override">;
 def err_drv_modules_validate_once_requires_timestamp : Error<
   "option '-fmodules-validate-once-per-build-session' requires "
   "'-fbuild-session-timestamp=<seconds since Epoch>' or '-fbuild-session-file=<file>'">;
+def err_build_session_timestamp_date_time_macros_no_timestamp : Error<
+  "option '-fbuild-session-timestamp-date-time-macros' requires "
+  "'-fbuild-session-timestamp=<seconds since Epoch>' "
+  "or '-fbuild-session-file=<file>'">;
 
 def err_test_module_file_extension_format : Error<
   "-ftest-module-file-extension argument '%0' is not of the required form "
   "'blockname:major:minor:hashed:user info'">;
 
 def warn_slash_u_filename : Warning<"'/U%0' treated as the '/U' option">,
   InGroup<DiagGroup<"slash-u-filename">>;
 def note_use_dashdash : Note<

clang/include/clang/Driver/Options.td

def fmodules_prune_after : Joined<["-"], "fmodules-prune-after=">, Group<i_Group>,
   MarshallingInfoInt<HeaderSearchOpts<"ModuleCachePruneAfter">, "31 * 24 * 60 * 60">;
 def fbuild_session_timestamp : Joined<["-"], "fbuild-session-timestamp=">,
   Group<i_Group>, Flags<[CC1Option]>, MetaVarName<"<time since Epoch in seconds>">,
   HelpText<"Time when the current build session started">,
   MarshallingInfoInt<HeaderSearchOpts<"BuildSessionTimestamp">, "0", "uint64_t">;
 def fbuild_session_file : Joined<["-"], "fbuild-session-file=">,
   Group<i_Group>, MetaVarName<"<file>">,
   HelpText<"Use the last modification time of <file> as the build session timestamp">;
+def fbuild_session_timestamp_date_time_macros : Flag<["-"], "fbuild-session-timestamp-date-time-macros">,
+    Flags<[CC1Option]>, Group<Preprocessor_Group>,
+    HelpText<"Use option -fbuild-session-timestamp or -fbuild-session-file=<file> to set the predefined macros: __DATE__ and __TIME__">,
+    MarshallingInfoFlag<PreprocessorOpts<"BuildSessionTimestampDateTimeMacros">>;
 def fmodules_validate_once_per_build_session : Flag<["-"], "fmodules-validate-once-per-build-session">,
   Group<i_Group>, Flags<[CC1Option]>,
   HelpText<"Don't verify input files for the modules if the module has been "
            "successfully validated or loaded during this build session">,
   MarshallingInfoFlag<HeaderSearchOpts<"ModulesValidateOncePerBuildSession">>;
 def fmodules_disable_diagnostic_validation : Flag<["-"], "fmodules-disable-diagnostic-validation">,
   Group<i_Group>, Flags<[CC1Option]>,
   HelpText<"Disable validation of the diagnostic options when loading the module">,

clang/include/clang/Lex/HeaderSearchOptions.h

public:
   /// accessed in this many seconds will be removed. The default value is
   /// large, e.g., a month, to avoid forcing infrequently-used modules to be
   /// regenerated often.
   unsigned ModuleCachePruneAfter = 31 * 24 * 60 * 60;
 
   /// The time in seconds when the build session started.
   ///
   /// This time is used by other optimizations in header search and module
-  /// loading.
+  /// loading, and to set the predefined macros: __DATE__ & __TIME__ if so
+  /// desired.
   uint64_t BuildSessionTimestamp = 0;
 
   /// The set of macro names that should be ignored for the purposes
   /// of computing the module hash.
   llvm::SmallSetVector<llvm::CachedHashString, 16> ModulesIgnoreMacros;
 
   /// The set of user-provided virtual filesystem overlay files.
   std::vector<std::string> VFSOverlayFiles;

clang/include/clang/Lex/PreprocessorOptions.h

ExcludedPreprocessorDirectiveSkipMapping
       *ExcludedConditionalDirectiveSkipMappings = nullptr;
 
   /// Set up preprocessor for RunAnalysis action.
   bool SetUpStaticAnalyzer = false;
 
   /// Prevents intended crashes when using #pragma clang __debug. For testing.
   bool DisablePragmaDebugCrash = false;
 
+  /// Use option -fbuild-session-timestamp/-fbuild-session-file to set the
+  /// predefined macros: __DATE__ and __TIME__
+  bool BuildSessionTimestampDateTimeMacros = false;
+
 public:
   PreprocessorOptions() : PrecompiledPreambleBytes(0, false) {}
 
   void addMacroDef(StringRef Name) {
     Macros.emplace_back(std::string(Name), false);
   }
   void addMacroUndef(StringRef Name) {
     Macros.emplace_back(std::string(Name), true);

clang/lib/Driver/ToolChains/Clang.cpp

if (types::getPreprocessedType(InputType) != types::TY_INVALID)
     AddPreprocessingOptions(C, JA, D, Args, CmdArgs, Output, Inputs);
 
   // Don't warn about "clang -c -DPIC -fPIC test.i" because libtool.m4 assumes
   // that "The compiler can only warn and ignore the option if not recognized".
   // When building with ccache, it will pass -D options to clang even on
   // preprocessed inputs and configure concludes that -fPIC is not supported.
   Args.ClaimAllArgs(options::OPT_D);
 
+  // Check if the preprocessor needs to set the predefined __DATE__ and __TIME__
+  // macros based on the build session timestamp.
+  if (Args.hasArg(options::OPT_fbuild_session_timestamp_date_time_macros)) {
+    if (!Args.getLastArg(options::OPT_fbuild_session_timestamp,
+                         options::OPT_fbuild_session_file))
+      D.Diag(diag::err_build_session_timestamp_date_time_macros_no_timestamp);
+    CmdArgs.push_back("-fbuild-session-timestamp-date-time-macros");
+  }
+
   // Manually translate -O4 to -O3; let clang reject others.
   if (Arg *A = Args.getLastArg(options::OPT_O_Group)) {
     if (A->getOption().matches(options::OPT_O4)) {
       CmdArgs.push_back("-O3");
       D.Diag(diag::warn_O4_is_O3);
     } else {
       A->render(Args, CmdArgs);
     }

clang/lib/Lex/PPMacroExpansion.cpp

 #include "clang/Basic/LangOptions.h"
 #include "clang/Basic/ObjCRuntime.h"
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Lex/CodeCompletionHandler.h"
 #include "clang/Lex/DirectoryLookup.h"
 #include "clang/Lex/ExternalPreprocessorSource.h"
 #include "clang/Lex/HeaderSearch.h"
+#include "clang/Lex/HeaderSearchOptions.h"
 #include "clang/Lex/LexDiagnostic.h"
 #include "clang/Lex/LiteralSupport.h"
 #include "clang/Lex/MacroArgs.h"
 #include "clang/Lex/MacroInfo.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Lex/PreprocessorLexer.h"
 #include "clang/Lex/PreprocessorOptions.h"
 #include "clang/Lex/Token.h"

 }
 
 /// ComputeDATE_TIME - Compute the current time, enter it into the specified
 /// scratch buffer, then return DATELoc/TIMELoc locations with the position of
 /// the identifier tokens inserted.
 static void ComputeDATE_TIME(SourceLocation &DATELoc, SourceLocation &TIMELoc,
                              Preprocessor &PP) {
   time_t TT = time(nullptr);
-  struct tm *TM = localtime(&TT);
+  struct tm *TM = nullptr;
+  const HeaderSearchOptions &HSOpts =
+      PP.getHeaderSearchInfo().getHeaderSearchOpts();
+  const PreprocessorOptions &PPOpts = PP.getPreprocessorOpts();
+  if (HSOpts.BuildSessionTimestamp &&
+      PPOpts.BuildSessionTimestampDateTimeMacros) {
+    TT = static_cast<time_t>(HSOpts.BuildSessionTimestamp);
+    TM = gmtime(&TT);
+  } else {
+    TT = time(nullptr);
+    TM = localtime(&TT);
+  }
 
   static const char * const Months[] = {
     "Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"
   };
 
   {
     SmallString<32> TmpBuffer;
     llvm::raw_svector_ostream TmpStream(TmpBuffer);

clang/test/Preprocessor/build-session-timestamp-date-time-macros.c

+char build_date[] = __DATE__;
+char build_time[] = __TIME__;
+
+// The predefined macros __DATE__ & __TIME__ are set via the -fbuild-session-timestamp option:
+// NOTE:
+// $ export SOURCE_DATE_EPOCH=`date +%s --date="Apr  2 2022 23:59:18 UTC"`; echo $SOURCE_DATE_EPOCH
+// 1648943958
+// $
+// RUN: export SOURCE_DATE_EPOCH=1648943958; %clang_cc1 -fbuild-session-timestamp-date-time-macros -fbuild-session-timestamp=$SOURCE_DATE_EPOCH -E %s | FileCheck -check-prefix=SET_VIA_BUILD_SESSION_TIMESTAMP %s
+// SET_VIA_BUILD_SESSION_TIMESTAMP: char build_date[] = "Apr  2 2022";
+// SET_VIA_BUILD_SESSION_TIMESTAMP: char build_time[] = "23:59:18";
+
+// The predefined macros __DATE__ & __TIME__ are set via the -fbuild-session-file option:
+// RUN: touch --date="Thu Feb 10 22:40:16 UTC 2020" %t.build-session-file; %clang -fbuild-session-timestamp-date-time-macros -fbuild-session-file=%t.build-session-file %s -E | FileCheck -check-prefix=SET_VIA_BUILD_SESSION_FILE %s
+// SET_VIA_BUILD_SESSION_FILE: char build_date[] = "Feb 10 2020";
+// SET_VIA_BUILD_SESSION_FILE: char build_time[] = "22:40:16";
+
+// An error is issued if neither -fbuild-session-timestamp nor -fbuild-session-file option is specified:
+// RUN: %clang -fbuild-session-timestamp-date-time-macros -E -### %s 2>&1 | FileCheck -check-prefix=ERR_IF_NO_BUILD_SESSION_OPTION %s
+// ERR_IF_NO_BUILD_SESSION_OPTION: option '-fbuild-session-timestamp-date-time-macros' requires '-fbuild-session-timestamp=<seconds since Epoch>' or '-fbuild-session-file=<file>'