QLocale: make qstrnto(u)ll not have output arguments

That is, return everything in the return argument. On the SysV ABI, that
means everything gets returned in registers, in both 32- and 64-bit
platforms (unlike QtPrivate::ParsedNumber). There's a minor but
perceptible performance improvement in parsing strings and byte arrays.

Before:
Parsed string   "42"    "1234"  "-1548860221"
Clock (ns)      16.673  18.878  25.517
CPU cycles      46.548  52.704  71.243
Instructions    201     233     331

After:
Parsed string   "42"    "1234"  "-1548860221"
Clock (ns)      15.577  17.998  24.198
CPU cycles      43.491  49.942  67.552
Instructions    179     211     308

On my Core i7-1165G7 @ 2.80 GHz, the 22-23 instruction gain per
iteration results in half the expected clock gain in runtime (22 /
2.8 GHz = 7.8 ns) because of a slightly lower instruction per cycle
rate. That's acceptable because we need less speculative execution.

Pick-to: 6.4
Task-number: QTBUG-107788
Change-Id: I07ec23f3cb174fb197c3fffd17220fd64d473cc0
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 4 insertions, 5 deletions

diff --git a/src/corelib/tools/qversionnumber.cpp b/src/corelib/tools/qversionnumber.cpp
index 7d6ddc9bcd..f44d5e4e8c 100644
--- a/src/corelib/tools/qversionnumber.cpp
+++ b/src/corelib/tools/qversionnumber.cpp
@@ -404,19 +404,18 @@ static QVersionNumber from_string(QLatin1StringView string, qsizetype *suffixInd
     QVarLengthArray<int, 32> seg;
 
     const char *start = string.begin();
-    const char *end = start;
     const char *lastGoodEnd = start;
     const char *endOfString = string.end();
 
     do {
-        bool ok = false;
-        const qulonglong value = qstrntoull(start, endOfString - start, &end, 10, &ok);
-        if (!ok || value > qulonglong(std::numeric_limits<int>::max()))
+        // parsing as unsigned so a minus sign is rejected
+        auto [value, end] = qstrntoull(start, endOfString - start, 10);
+        if (!end || value > qulonglong(std::numeric_limits<int>::max()))
             break;
         seg.append(int(value));
         start = end + 1;
         lastGoodEnd = end;
-    } while (start < endOfString && end < endOfString && *end == '.');
+    } while (start < endOfString && *lastGoodEnd == '.');
 
     if (suffixIndex)
         *suffixIndex = lastGoodEnd - string.begin();