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From: "arthur.j.odwyer at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/36043] gcc reads 8 bytes for a struct of size 6 which leads to sigsegv
Date: Tue, 25 Feb 2014 19:45:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36043

Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |arthur.j.odwyer at gmail dot com

--- Comment #22 from Arthur O'Dwyer <arthur.j.odwyer at gmail dot com> ---
Here's another segfaulting test case, for x86-64. This fails with GCC 4.6.2 and
GCC 4.8.1.
This GCC bug is really hard to work around, since it can strike anywhere
without warning. Is it possible that one of the patches above (e.g., Richard
Biener's comment #9) could be adopted into 4.8.x or 4.9.x?


#include <cstdio>
#include <stdint.h>
#include <sys/mman.h>

struct int24_t { uint8_t m_Internal[3]; };

void foo(int24_t a) { puts("2"); }

int main() {
    const int OS_PAGE_SIZE = 0x1000;
    const int COUNT = (OS_PAGE_SIZE) / sizeof(int24_t);
    printf("sizeof int24_t = %d\n", (int)sizeof(int24_t));

    void* arena1 = mmap(0, OS_PAGE_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE |
MAP_ANONYMOUS, -1, 0);
    int24_t* ptr = (int24_t*)mmap(0, OS_PAGE_SIZE, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    munmap(arena1, OS_PAGE_SIZE);

    puts("1");
    foo(ptr[COUNT-1]);
    puts("3");
}