[PATCH v2 0/1] MR10348: ntdll: Avoid expensive inverted DST check on get_timezone_info() cache hit path

[Zhou Qiankang (＠wszqkzqk)]

inverted DST detection (two mktime() calls) out of the cache hit fast path in
get_timezone_info().

Keep the cache key based on the cheap raw bias value and only compute
inverted_dst/adjusted_bias on cache miss. Preserve the inverse DST correctness
fix by still applying adjusted_bias to tzi->Bias in the slow path.

This restores fast-path behavior for frequent GetTimeZoneInformation callers without regressing inverse DST handling.

Fixes regression: bfae6eb8b4f6 ("ntdll: Fix get_timezone_info() for timezones defined with inverse DST on Unix.")

Fixes: https://bugs.winehq.org/show_bug.cgi?id=59514

## Test results

Using a simple test demo: [time_bench.c](/uploads/700a5b087b09a0d93388dbcf437ad42d/time_bench.c)

On AMD Ryzen 5800H with power connected it's **24 times** faster.

Wine 11.4 without this fix:

```
iterations: 10000

time()      : 1.093640 s  (109364 ns/call)
localtime() : 1.109118 s  (110912 ns/call)

sink=17736685140379
```

With this fix:

```
iterations: 10000

time()      : 0.046216 s  (4622 ns/call)
localtime() : 0.046636 s  (4664 ns/call)

sink=17736685140000
```

--
  v2: ntdll: Avoid expensive inverted DST check on get_timezone_info() cache hit path
 
https://gitlab.winehq.org/wine/wine/-/merge_requests/10348

[PATCH v2 1/1] ntdll: Avoid expensive inverted DST check on get_timezone_info() cache hit path

[Zhou Qiankang]

From: Zhou Qiankang <[e-mail]>

inverted DST detection (two mktime() calls) out of the cache hit fast path in
get_timezone_info().

Keep the cache key based on the cheap raw bias value and only compute
inverted_dst/adjusted_bias on cache miss. Preserve the inverse DST correctness
fix by still applying adjusted_bias to tzi->Bias in the slow path.

This restores fast-path behavior for frequent GetTimeZoneInformation callers without regressing inverse DST handling.

Fixes regression: bfae6eb8b4f6 ("ntdll: Fix get_timezone_info() for timezones defined with inverse DST on Unix.")

Fixes: https://bugs.winehq.org/show_bug.cgi?id=59514

Signed-off-by: Zhou Qiankang <[e-mail]>
---
 dlls/ntdll/unix/system.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/dlls/ntdll/unix/system.c b/dlls/ntdll/unix/system.c
index 22f8b496220..d2cd1d1b980 100644
--- a/dlls/ntdll/unix/system.c
+++ b/dlls/ntdll/unix/system.c
@@ -3012,12 +3012,6 @@ static void get_timezone_info( RTL_DYNAMIC_TIME_ZONE_INFORMATION *tzi )
     bias = (LONG)(mktime(tm) - year_start) / 60;
 
     tm = localtime(&year_start);
-    tm1 = tm2 = *tm;
-    tm1.tm_isdst = 0;
-    tm2.tm_isdst = 1;
-    inverted_dst = mktime(&tm1) < mktime(&tm2);
-    if (inverted_dst) bias += 60;
-
     if (current_year == tm->tm_year && current_bias == bias)
     {
         *tzi = cached_tzi;
@@ -3025,10 +3019,16 @@ static void get_timezone_info( RTL_DYNAMIC_TIME_ZONE_INFORMATION *tzi )
         return;
     }
 
+    tm1 = tm2 = *tm;
+    tm1.tm_isdst = 0;
+    tm2.tm_isdst = 1;
+    inverted_dst = mktime(&tm1) < mktime(&tm2);
+    current_bias = bias;
+    if (inverted_dst) bias += 60;
+
     memset(tzi, 0, sizeof(*tzi));
     TRACE("tz data will be valid through year %d, bias %d, inverted_dst %d\n", tm->tm_year + 1900, bias, inverted_dst);
     current_year = tm->tm_year;
-    current_bias = bias;
 
     tzi->Bias = bias;
 
-- 
GitLab

https://gitlab.winehq.org/wine/wine/-/merge_requests/10348

[Paul Gofman (＠gofman)]

Paul Gofman (@gofman) commented about dlls/ntdll/unix/system.c:
> return;
>      }
>
> +    tm1 = tm2 = *tm;
> +    tm1.tm_isdst = 0;
> +    tm2.tm_isdst = 1;
> +    inverted_dst = mktime(&tm1) < mktime(&tm2);
> +    current_bias = bias;

that doesn't affect functionality, but for better readability I'd suggest to keep assigning current_year and current_bias together and not split those across the code. Also probably move those assignments right after the 'if' checking those, before 
'tm1 = tm2= *tm;'.

-- 
https://gitlab.winehq.org/wine/wine/-/merge_requests/10348#note_132386