[PATCH v2 5/5] libstdc++: Implement zic writezone merge optimization [PR124854]
Tomasz Kamiński
tkaminsk@redhat.com
Tue May 19 07:58:26 GMT 2026
Below including a quick draft of alternative solution where the merging
is integragted into pre-existing loop. We use merge_window (non-zero
only after DST chgange), and skip adding sys_info related to them.
Let me know what you think about these approach. I certainly like it
more.
---
libstdc++-v3/src/c++20/tzdb.cc | 58 ++++++++-----
.../std/time/time_zone/zone_merge.cc | 84 +++++++++++++++++++
2 files changed, 122 insertions(+), 20 deletions(-)
create mode 100644 libstdc++-v3/testsuite/std/time/time_zone/zone_merge.cc
diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 183a0c61a61..e9a348cc3c8 100644
--- a/libstdc++-v3/src/c++20/tzdb.cc
+++ b/libstdc++-v3/src/c++20/tzdb.cc
@@ -28,6 +28,8 @@
// In the usual dual-abi build, std::chrono::tzdb is only defined for cxx11.
#define _GLIBCXX_USE_CXX11_ABI 1
+#define _GLIBCXX_ASSERTIONS
+
#include <chrono>
#include <fstream> // ifstream
#include <sstream> // istringstream
@@ -78,6 +80,8 @@
# endif
#endif
+#include <iostream>
+
namespace __gnu_cxx
{
#ifdef _AIX
@@ -504,6 +508,9 @@ namespace std::chrono
seconds
offset() const noexcept { return m_offset; }
+ minutes
+ save() const noexcept { return m_save; }
+
// RULES: The name of the rules that apply for this period.
string_view
rules() const noexcept
@@ -867,8 +874,15 @@ namespace std::chrono
info.abbrev = ri.format();
string_view letters;
- if (i != infos.begin() && i[-1].expanded())
- letters = i[-1].next_letters();
+ seconds prev_offset = ri.offset();
+ if (i == infos.begin())
+ prev_offset = ri.offset();
+ else if (const ZoneInfo& prev = i[-1]; !prev.expanded())
+ prev_offset = prev.offset();
+ else if (letters = prev.next_letters(); letters.empty())
+ prev_offset = prev.offset() + prev.save();
+ else
+ prev_offset = ri.offset();
if (letters.empty())
{
@@ -940,9 +954,10 @@ namespace std::chrono
}
const Rule* curr_rule = nullptr;
-
+ seconds merge_window = (prev_offset > info.offset) ? (prev_offset - info.offset) : seconds(0);
while (info.begin < info.end && num_after > 0)
{
+
sys_seconds t = info.begin;
const year_month_day date(chrono::floor<days>(t));
const Rule* next_rule = nullptr;
@@ -984,9 +999,6 @@ namespace std::chrono
if (t < rule_start && rule_start < info.end)
{
- if (rule_start - t < days(1)) // XXX shouldn't be needed!
- continue;
-
// Found a closer transition than the previous info.end.
info.end = rule_start;
next_rule = &rule;
@@ -995,7 +1007,6 @@ namespace std::chrono
format_abbrev_str(info, letters);
- bool merged = false;
#if 0
if (!new_infos.empty())
{
@@ -1015,21 +1026,28 @@ namespace std::chrono
else
letters = {};
- if (!merged)
+ // Merge the rule with next one (full description goes here)
+ if (bool merged = (info.end - t <= merge_window))
+ info.begin = t;
+ else
+ {
new_infos.emplace_back(info, letters);
+ if (info.begin <= tp && tp < info.end) // Found the result.
+ result_index = new_infos.size() - 1;
+ else if (result_index >= 0)
+ {
+ // Finish on a DST sys_info if possible, so that if we resume
+ // generating sys_info objects after this time point, save=0
+ // should be correct for the next sys_info.
+ if (num_after > 1 || info.save != 0min)
+ --num_after;
+ }
+ info.begin = info.end;
+ }
- if (info.begin <= tp && tp < info.end) // Found the result.
- result_index = new_infos.size() - 1;
- else if (result_index >= 0 && !merged)
- {
- // Finish on a DST sys_info if possible, so that if we resume
- // generating sys_info objects after this time point, save=0
- // should be correct for the next sys_info.
- if (num_after > 1 || info.save != 0min)
- --num_after;
- }
-
- info.begin = info.end;
+ // Merging applies only when switching between rules with different
+ // offset, for any later applications it should be zero.
+ merge_window = seconds(0);
if (next_rule)
{
info.end = ri.until();
diff --git a/libstdc++-v3/testsuite/std/time/time_zone/zone_merge.cc b/libstdc++-v3/testsuite/std/time/time_zone/zone_merge.cc
new file mode 100644
index 00000000000..75942f976b6
--- /dev/null
+++ b/libstdc++-v3/testsuite/std/time/time_zone/zone_merge.cc
@@ -0,0 +1,84 @@
+// { dg-do run { target c++20 } }
+// { dg-require-effective-target tzdb }
+// { dg-require-effective-target cxx11_abi }
+// { dg-xfail-run-if "no weak override on AIX" { powerpc-ibm-aix* } }
+
+// When two adjacent Zone lines differ in total offset and the new line's
+// rule set has a rule firing within |jump| of the boundary (where jump
+// is a backward local-time jump), zic.c's writezone folds that rule
+// into the boundary, so the new line begins with the post-rule save.
+//
+// Mirrors America/Argentina/Buenos_Aires around 1999-10-03.
+
+#include <chrono>
+#include <fstream>
+#include <testsuite_hooks.h>
+
+static bool override_used = false;
+
+namespace __gnu_cxx
+{
+ const char* zoneinfo_dir_override() {
+ override_used = true;
+ return "./";
+ }
+}
+
+int
+main()
+{
+ using namespace std::chrono;
+
+ // stdoff jumps from -3 to -4 at the same instant a save=1 rule fires.
+ // In the new (-4) frame the rule fires 1 hour after the boundary at
+ // UT 03:00, so the merge folds the rule into the boundary and the
+ // new line begins at offset=-3, save=1 (abbrev "-03").
+ std::ofstream("tzdata.zi") << R"(# version test_zone_merge
+R T 1999 o - O 3 0 1 -
+R T 2000 o - Mar 3 0 0 -
+Z Test/BA -3 - %z 1999 O 3
+ -4 T %z 2000 Mar 3
+ -3 - %z
+)";
+
+ const auto& db = reload_tzdb();
+ VERIFY( override_used );
+ VERIFY( db.version == "test_zone_merge" );
+
+ auto* tz = locate_zone("Test/BA");
+
+ // The boundary is the wall UNTIL "1999 O 3" (default time 00:00)
+ // interpreted in the prior (-3) frame, i.e. UT 03:00 1999-10-03.
+ sys_seconds boundary{sys_days(1999y/October/3) + 3h};
+
+ auto before = tz->get_info(boundary - 1s);
+ VERIFY( before.offset == -3h );
+ VERIFY( before.save == 0min );
+ VERIFY( before.abbrev == "-03" );
+
+ // The new line's first sys_info already has save=1 from the merge,
+ // total offset -3h, abbrev "-03".
+ auto at_boundary = tz->get_info(boundary);
+ VERIFY( at_boundary.offset == -3h );
+ VERIFY( at_boundary.save == 60min );
+ VERIFY( at_boundary.abbrev == "-03" );
+
+ auto plus_30min = tz->get_info(boundary + 30min);
+ VERIFY( plus_30min.offset == -3h );
+ VERIFY( plus_30min.save == 60min );
+ VERIFY( plus_30min.abbrev == "-03" );
+
+ // Sanity: well after the boundary, still in the merged sys_info
+ // until the Mar 3 2000 transition.
+ auto winter = tz->get_info(sys_days(2000y/January/15));
+ VERIFY( winter.offset == -3h );
+ VERIFY( winter.save == 60min );
+ VERIFY( winter.abbrev == "-03" );
+
+ // After Mar 3 2000: line 2 ends, line 3 begins. No DST rule fires
+ // at this boundary, so total offset reverts to -3h with save=0.
+ auto spring = tz->get_info(sys_days(2000y/April/15));
+ VERIFY( spring.offset == -3h );
+ VERIFY( spring.save == 0min );
+ VERIFY( spring.abbrev == "-03" );
+}
--
2.54.0
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