[PATCH 4/4] Use SOURCE_DATE_EPOCH in place of __TIMESTAMP__ if the latter is newer.

[Ximin Luo <infinity0 at pwned dot gg>]

This brings the behaviour in line with the __DATE__ and __TIME__ macros. Note
though the minor difference: __TIMESTAMP__ is defined as the file-modification
date and not the "current" date or time like the other two macros. Therefore,
we do a clamping behaviour (similar to tar --clamp-mtime).

Acknowledgements
----------------

Reiner Herrmann suggested the clamping behaviour.

ChangeLogs
----------

libcpp/ChangeLog:

2016-11-01  Ximin Luo  <infinity0@pwned.gg>

	* macro.c (_cpp_builtin_macro_text): Use SOURCE_DATE_EPOCH in place of
	__TIMESTAMP__ if the latter is newer than the former.

gcc/ChangeLog:

2016-11-01  Ximin Luo  <infinity0@pwned.gg>

	* doc/cppenv.texi (Environment Variables): Update SOURCE_DATE_EPOCH to
	describe the new effect on __TIMESTAMP__.

gcc/testsuite/ChangeLog:

2016-11-01  Ximin Luo  <infinity0@pwned.gg>

	* gcc.dg/cpp/source_date_epoch-4.c: New test.
	* gcc.dg/cpp/source_date_epoch-5.c: New test.

Index: gcc-7-20161030/libcpp/macro.c
===================================================================
--- gcc-7-20161030.orig/libcpp/macro.c
+++ gcc-7-20161030/libcpp/macro.c
@@ -264,7 +264,30 @@ _cpp_builtin_macro_text (cpp_reader *pfi
 		struct tm *tb = NULL;
 		struct stat *st = _cpp_get_file_stat (file);
 		if (st)
-		  tb = localtime (&st->st_mtime);
+		  {
+		    /* Set a reproducible timestamp for __DATE__ and __TIME__ macro
+		       if SOURCE_DATE_EPOCH is defined.  */
+		    if (pfile->source_date_epoch == (time_t) -2
+			&& pfile->cb.get_source_date_epoch != NULL)
+		      pfile->source_date_epoch = pfile->cb.get_source_date_epoch (pfile);
+
+		    if (pfile->source_date_epoch >= (time_t) 0)
+		      {
+			/* If SOURCE_DATE_EPOCH is set, we want reproducible
+			   timestamps so use gmtime not localtime.  */
+			if (st->st_mtime >= pfile->source_date_epoch)
+			  tb = gmtime (&pfile->source_date_epoch);
+			else
+			  /* Don't use SOURCE_DATE_EPOCH if the timestamp is
+			     older, since that means it was probably already
+			     set in a reproducible way (e.g. via source tarball
+			     extraction or some other method). */
+			  tb = gmtime (&st->st_mtime);
+		      }
+		    else
+		      tb = localtime (&st->st_mtime);
+		  }
+
 		if (tb)
 		  {
 		    char *str = asctime (tb);
Index: gcc-7-20161030/gcc/doc/cppenv.texi
===================================================================
--- gcc-7-20161030.orig/gcc/doc/cppenv.texi
+++ gcc-7-20161030/gcc/doc/cppenv.texi
@@ -83,8 +83,9 @@ main input file is omitted.
 @item SOURCE_DATE_EPOCH
 If this variable is set, its value specifies a UNIX timestamp to be
 used in replacement of the current date and time in the @code{__DATE__}
-and @code{__TIME__} macros, so that the embedded timestamps become
-reproducible.
+and @code{__TIME__} macros, and in replacement of the file modification
+date in the @code{__TIMESTAMP__} macro if the latter is newer than the
+former, so that the embedded timestamps become reproducible.
 
 The value of @env{SOURCE_DATE_EPOCH} must be a UNIX timestamp,
 defined as the number of seconds (excluding leap seconds) since
Index: gcc-7-20161030/gcc/testsuite/gcc.dg/cpp/source_date_epoch-4.c
===================================================================
--- /dev/null
+++ gcc-7-20161030/gcc/testsuite/gcc.dg/cpp/source_date_epoch-4.c
@@ -0,0 +1,13 @@
+/* __TIMESTAMP__ should use SOURCE_DATE_EPOCH if the latter is older. */
+/* { dg-do run } */
+/* { dg-set-compiler-env-var TZ "UTC" } */
+/* { dg-set-compiler-env-var LC_ALL "C" } */
+/* { dg-set-compiler-env-var SOURCE_DATE_EPOCH "0" } */
+
+int
+main ()
+{
+  if (__builtin_strcmp (__TIMESTAMP__, "Thu Jan  1 00:00:00 1970") != 0)
+    __builtin_abort ();
+  return 0;
+}
Index: gcc-7-20161030/gcc/testsuite/gcc.dg/cpp/source_date_epoch-5.c
===================================================================
--- /dev/null
+++ gcc-7-20161030/gcc/testsuite/gcc.dg/cpp/source_date_epoch-5.c
@@ -0,0 +1,13 @@
+/* __TIMESTAMP__ should not use SOURCE_DATE_EPOCH if the latter is newer. */
+/* { dg-do run } */
+/* { dg-set-compiler-env-var TZ "UTC" } */
+/* { dg-set-compiler-env-var LC_ALL "C" } */
+/* { dg-set-compiler-env-var SOURCE_DATE_EPOCH "253402300799" } */
+
+int
+main ()
+{
+  if (__builtin_strcmp (__TIMESTAMP__, "Fri Dec 31 23:59:59 UTC 9999") == 0)
+    __builtin_abort ();
+  return 0;
+}