Super weird segfault with gcc 4.7 -- Bug?

Question

Here is a piece of code that I've been trying to compile: ``` #include #define N 3 struct Data { int A[N][N]; int B[N]; }; int foo(int uloc, const int A[N][N], const int B[N]) { for(unsigned int j = 0; j < N; j++) { for( int i = 0; i < N; i++) { for( int r = 0; r < N ; r++) { for( int q = 0; q < N ; q++) { uloc += B[i]*A[r][j] + B[j]; } } } } return uloc; } int apply(const Data *d) { return foo(4,d->A,d->B); } int main(int, char **) { Data d; for(int i = 0; i < N; ++i) { for(int j = 0; j < N; ++j) { d.A[i][j] = 0.0; } d.B[i] = 0.0; } int res = 11 + apply(&d); printf("%d ",res); return 0; } ``` Yes, it looks quite strange, and does not do anything useful at all at the moment, but it is the most concise version of a much larger program which I had the problem with initially. It compiles and runs just fine with GCC(G++) 4.4 and 4.6, but if I use GCC 4.7, and enable third level optimizations: ``` g++-4.7 -g -O3 prog.cpp -o prog ``` I get a segmentation fault when running it. Gdb…

Accepted Answer

It indeed and unfortunately is a bug in gcc. I have not the slightest idea what it is doing there, but the generated assembly for the apply function is ( I compiled it without main btw., and it has foo inlined in it):

```
_Z5applyPK4Data:
        pushq   %r13
        movabsq $17179869184, %r8
        pushq   %r12
        pushq   %rbp
        pushq   %rbx
        movl    36(%rdi), %ecx
        movl    (%rdi,%r8), %ebp
        movl    24(%rdi), %r10d
```

and exactly at the `movl (%rdi,%r8), %ebp` it will crashes, since it adds a nonsensical `0x400000000` to `$rdi` (the first parameter, thus the pointer to `Data`) and dereferences it.

Super weird segfault with gcc 4.7 -- Bug?

Solution

Problem