https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100197
Bug ID: 100197
Summary: g++ emits spurious Wstring-compare warnings on
strcmp()
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jaf at meyersound dot com
Target Milestone: ---
Created attachment 50653
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50653&action=edit
preprocessed file from teststring.cpp
Tested with g++ (Ubuntu 10.20-13ubuntu1) 10.2.0 (as installed via "sudp apt-get
install g++") on an Ubuntu 20.10 64-bit VM running inside VMWare Fusion on a
2018 Mac Mini.
To reproduce, compile the attached .cpp file, like this:
$ g++ -O2 -W -c teststring.cpp
In member function ‘bool String::operator==(const char*) const’,
inlined from ‘int main(int, char**)’ at teststring.cpp:52:21:
teststring.cpp:27:21: warning: ‘int strcmp(const char*, const char*)’ of a
string of length 10 and an array of size 8 evaluates to nonzero
[-Wstring-compare]
27 | return (strcmp(myStr, rhs) == 0);
| ~~~~~~^~~~~~~~~~~~
teststring.cpp: In function ‘int main(int, char**)’:
teststring.cpp:52:16: note: in this expression
52 | if ((rand())&&(s == "1234567890")) printf("Mwahey!\n");
| ~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
Note that the emitted warnings are (AFAICT) spurious, because in the example
program the two strings that get passed to strcmp() are both "1234567890". The
8-byte array (_smallBuffer[8]) that the warning mentions is not actually used.
It seems like maybe the compiler is assuming that the
IsArrayDynamicallyAllocated() method in the example will always return false,
which is not a valid assumption.
The fault only happens when I specifed -O2 or -O3, it doesn't happen if I leave
optimization disabled or specify -O1.
