https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71177
Bug ID: 71177
Summary: [6 Regression] Spurious
-Waggressive-loop-optimizations warning
Product: gcc
Version: 6.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: tavianator at gmail dot com
Target Milestone: ---
Created attachment 38516
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=38516&action=edit
Reduced testcase
The following use of boost::container::string gives a warning since GCC 6:
$ cat stringbug.cpp
#include <boost/container/string.hpp>
using boost::container::string;
string normalize(string token)
{
if (token.length() >= 2) {
token.resize(token.length() - 2);
}
return token;
}
$ g++ -Wall -O3 -c stringbug.cpp
In function ‘boost::container::string normalize(boost::container::string)’:
cc1plus: warning: iteration 9223372036854775807 invokes undefined behavior
[-Waggressive-loop-optimizations]
In file included from stringbug.cpp:1:0:
/usr/include/boost/container/string.hpp:2608:10: note: within this loop
for (; first != last; ++dest, ++first, ++constructed){
^~~
cc1plus: warning: iteration 9223372036854775807 invokes undefined behavior
[-Waggressive-loop-optimizations]
/usr/include/boost/container/string.hpp:2626:7: note: within this loop
for ( ; first != last; ++first, ++result)
^~~
What's happening is that resize() is implemented like this:
void resize(size_type n, CharT c)
{
if (n <= this->size())
this->erase(this->begin() + n, this->end());
else
this->append(n - this->size(), c);
}
After inlining/constant propagation, the else block contains undefined
behaviour for resize(length - 2). But the else block is also unreachable due
to the if (length >= 2) check.
Reduced testcase attached.
