-Bsymbolic-functions breaks PMF comparison under ARM

-Bsymbolic-functions breaks PMF comparison under ARM

[Giuseppe D'Angelo]

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Hello,

the attached program changes the output from "true" to "false" when the 
-Bsymbolic / -Bsymbolic-functions options are passed to GCC. This 
happens on ARM -- on x86-64 output is always "true".

The program involves a comparison, within a shared library, of a PMF 
defined inside the shared library itself with the same PMF passed by the 
application.

At this stage I still don't know if it's a GCC problem, a ld.so problem, 
ABI constraints, undefined behaviour, or whatnot; but any help is 
appreciated.

Compile with:

> g++ -fPIC -shared -Wall -o libshared.so -Wl,-Bsymbolic shared.cpp
> g++ -fPIE -Wall -o main main.cpp -L. -lshared

(The long story is that Qt 5 is taking PMFs in its public API, and the 
comparison failing inside of Qt shared libraries is breaking code on 
ARM, as -Bsymbolic is set by default there.)

Thanks,
-- 
Giuseppe D'Angelo | giuseppe.dangelo@kdab.com | Software Engineer
KDAB (UK) Ltd., a KDAB Group company
Tel. UK +44-1738-450410, Sweden (HQ) +46-563-540090
KDAB - Qt Experts - Platform-independent software solutions

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[-- Attachment #1.2: main.cpp --]
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#include <iostream>
#include "shared.h"
 
struct T : S
{
    T() {
        foo();
    }
};
 
bool magic(Spmf f) { return test(reinterpret_cast<void **>(&f)); }
 
int main()
{
    using namespace std;
    T t;
    cout << boolalpha << magic(&T::foo) << endl;
}

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[-- Attachment #1.3: shared.cpp --]
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#include "shared.h"
#include <cstring>
 
S::~S() {}
void S::foo() {}
void S::bar() {}
 
static const Spmf spmfArray[] = { 0, &S::foo, 0 };
static const size_t spmfArraySize = sizeof(spmfArray) / sizeof(spmfArray[0]);
 
bool test(void **f)
{
    for (size_t i = 0; i < spmfArraySize; ++i) {
        if (spmfArray[i] == *reinterpret_cast<Spmf *>(f))
            return true;
    }
   
    return false;
}
 

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[-- Attachment #1.4: shared.h --]
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#ifndef SHARED_H
#define SHARED_H
 
struct S {
    virtual ~S();
    void foo();
    void bar();
};
 
typedef void (S::*Spmf)();
 
bool test(void **f);
 
#endif // SHARED_H

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