https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121593
Bug ID: 121593
Summary: Miscompiles small function to produce endless(?) loop
Product: gcc
Version: 15.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: marc.mutz at hotmail dot com
Target Milestone: ---
Created attachment 62143
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=62143&action=edit
Preprocessed source
I'm refactoring a small piece of code into an inline function (template) and
the test suite shows that the function no longer terminates (within the
allocated 5min watchdog time; didn't try to see whether it eventually
terminates or not).
This is reproducible on Ubuntu 20.04 with self-compiled GCC 11.5 (also SLES
with that GCC version on the Qt CI fails the same way), 13.4.1 (20250611),
14.3.1 (20250611) and 15.1.1 (20250611).
Clang is fine, Valgrind is silent, and if I switch on asan/ubsan, the test
passes, too. Ditto if I put some more complex code into the calling function,
like a qDebug() (like printf) or even the increment of a `thread_local int
count`. Another fix is to mark the extracted functions as
__attribute__((optimize("O0"))) (O1 already fails).
The extracted function is QUtf8Functions::nextCharacterFromUtf8(), the caller
is `NextResult operator()(const char *&it, const char *end) const`.
I'm attaching the preprocessed source for the old version (.inline.ii) as well
as the diff to the preprocessed source with the port to the extracted function
(.guarded.ii). Unfortunately, the test code is in a different TU (library,
even), so I hope that looking at the assembly will already help. If you remove
the __attribute__((optimize("O0"))) from nextCharacterFromUtf8(), then the test
fails. With both versions of preprocessed source, the test passes.
Tell me how I can help to narrow it down further (my assembly is very bad,
though).
Compile command for the preprocessed file are:
```
c++ -DBACKTRACE_HEADER=\"execinfo.h\" -DCore_EXPORTS
-DELF_INTERPRETER=\"/lib64/ld-linux-x86-64.so.2\" -DQT_ASCII_CAST_WARNINGS
-DQT_BUILDING_QT -DQT_BUILD_CORE_LIB -DQT_DEPRECATED_WARNINGS
-DQT_EXPLICIT_QFILE_CONSTRUCTION_FROM_PATH -DQT_LEAN_HEADERS=1 -DQT_MOC_COMPAT
-DQT_NO_CAST_TO_ASCII -DQT_NO_CONTEXTLESS_CONNECT -DQT_NO_DEBUG -DQT_NO_FOREACH
-DQT_NO_JAVA_STYLE_ITERATORS -DQT_NO_NARROWING_CONVERSIONS_IN_CONNECT
-DQT_NO_QASCONST -DQT_NO_QEXCHANGE -DQT_NO_QPAIR -DQT_NO_QSNPRINTF
-DQT_NO_STD_FORMAT_SUPPORT -DQT_NO_USING_NAMESPACE -DQT_STRICT_QLIST_ITERATORS
-DQT_TYPESAFE_FLAGS -DQT_USE_NODISCARD_FILE_OPEN -DQT_USE_QSTRINGBUILDER
-D_GLIBCXX_ASSERTIONS -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE
-I/home/marc/Qt/qtbase-submit-build/src/corelib/Core_autogen/include
-I/home/marc/Qt/qtbase-submit-build/include
-I/home/marc/Qt/qtbase-submit-build/include/QtCore
-I/home/marc/Qt/qtbase-submit/src/corelib
-I/home/marc/Qt/qtbase-submit-build/src/corelib
-I/home/marc/Qt/qtbase-submit-build/src/corelib/global
-I/home/marc/Qt/qtbase-submit-build/src/corelib/kernel
-I/home/marc/Qt/qtbase-submit/src/corelib/../3rdparty/tinycbor/src
-I/home/marc/Qt/qtbase-submit-build/include/QtCore/6.11.0
-I/home/marc/Qt/qtbase-submit-build/include/QtCore/6.11.0/QtCore
-I/home/marc/Qt/qtbase-submit/src/corelib/../3rdparty/forkfd
-I/home/marc/Qt/qtbase-submit-build/src/corelib/.rcc
-I/home/marc/Qt/qtbase-submit-build/mkspecs/linux-g++ -isystem
/d/openssl/32-dev/include -isystem /usr/include/glib-2.0 -isystem
/usr/lib/x86_64-linux-gnu/glib-2.0/include -g -DNDEBUG -O3 -std=gnu++17 -fPIC
-fvisibility=hidden -fvisibility-inlines-hidden -Wall -Wextra
-Wno-expansion-to-defined -U_FORTIFY_SOURCE -Wsuggest-override
-fcf-protection=full -D_FORTIFY_SOURCE=2 -ftrivial-auto-var-init=pattern
-fstack-protector-strong -fstack-clash-protection -fexceptions -MD -MT
src/corelib/CMakeFiles/Core.dir/serialization/qxmlstream.cpp.o -MF
src/corelib/CMakeFiles/Core.dir/serialization/qxmlstream.cpp.o.d -o
qxmlstream.cpp.inlined.ii -E
/home/marc/Qt/qtbase-submit/src/corelib/serialization/qxmlstream.cpp
```
