QCOMPARE/QVERIFY: fix huge pessimisation in QTestResult

The old code allocated a stack buffer, but asked the runtime to
zero-initialize it. That's 1KiB of writes to the stack on every
QCOMPARE and QVERIFY before any actual work is done. Fixing this
little laissez-faire to just initialize the first character in the
buffer results in nice little speedups of up to or exceeding 2x.

Amends d946507727b363326d05f48da93c2af04bdda76d.

[ChangeLog][QtTestLib] Optimized successful QCOMPARE and QVERIFY for
an up to 2x speedup.

This has the potential to meaningfully reduce the load on the CI, so
picking all the way to 5.15.

Pick-to: 6.3 6.2 5.15
Change-Id: Ib93d69282ec87cbd26a60e4ac14413e8cef8ff78
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/testlib/qtestresult.cpp b/src/testlib/qtestresult.cpp
index beb6291008..40e76146ba 100644
--- a/src/testlib/qtestresult.cpp
+++ b/src/testlib/qtestresult.cpp
@@ -294,7 +294,8 @@ bool QTestResult::verify(bool statement, const char *statementStr,
 {
     QTEST_ASSERT(statementStr);
 
-    char msg[1024] = {'\0'};
+    char msg[1024];
+    msg[0] = '\0';
 
     if (QTestLog::verboseLevel() >= 2) {
         qsnprintf(msg, 1024, "QVERIFY(%s)", statementStr);
@@ -353,7 +354,8 @@ static bool compareHelper(bool success, const char *failureMsg,
                           bool hasValues = true)
 {
     const size_t maxMsgLen = 1024;
-    char msg[maxMsgLen] = {'\0'};
+    char msg[maxMsgLen];
+    msg[0] = '\0';
 
     QTEST_ASSERT(expected);
     QTEST_ASSERT(actual);