GCC11 issue: -Werror=free-nonheap-object causes build failure

Linux
1

Initially reported at: More GCC11 issues? -Werror=free-nonheap-object causes surge-fx build to fail · Issue #4251 · surge-synthesizer/surge · GitHub

Building Surge-FX with GCC11 (-Werror) & JUCE-6.0.8 on Fedora 34 fails. Example:

/builddir/build/BUILD/surge/libs/juce-6.0.8/juce/modules/juce_core/text/juce_String.cpp:164:52: error: 'void operator delete ’ called on unallocated object ‘juce::emptyString’ [-Werror=free-nonheap-object]
164 | delete reinterpret_cast<char*> (b);

The build log contains more lines than I’m allowed to post here but it’s attached in the github issue referenced above.

2

The solution (credit baconpaul) to prevent the build failure in surge was adding -Wno-free-nonheap-object
See Add -Wno-free-nonheap-object to prevent build failure with GCC11 by plaimbock · Pull Request #4254 · surge-synthesizer/surge · GitHub

3

I’ve tried building JUCE with g++ 11.0.1 20210416 (experimental) and -Wfree-nonheap-object but I can’t reproduce the warning. Perhaps it has been fixed in the compiler.

4

Thanks that’s good to know. The GCC11 version I had in Fedora 34 beta seems from March. But I do see a version 11.0.1 that will become available as an update when Fedora 34 is released. So let’s hope you are right and that this issue has been fixed in 11.0.1.

5

you need to build with -flto to get this warning message. I still see this warning with gcc 11.2 and 5.4.7

7

It needed -flto and -01 or higher.

Unfortunately I don’t think we can work around this in the source code:

8

you can:

so i suggest:

static void no_delete_string(char* a) {}
static void delete_string(char* a) { delete[] a; }

static inline void release (StringHolder* const b) noexcept
{
    void (*deleter)(char*) = isEmptyString (b) ? &no_delete_string : &delete_string;
    if (! isEmptyString (b))
        if (--(b->refCount) == -1)
            deleter(reinterpret_cast<char*> (b));
}
9

Nice find.

I think we’ll fix this by changing the underlying empty string check to use the address of a constexpr static instead, but this will need a bit more testing.

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