I have this simple C ++ code:
int testFunction(int* input, long length) {
int sum = 0;
for (long i = 0; i < length; ++i) {
sum += input[i];
}
return sum;
}
#include <stdlib.h>
#include <iostream>
using namespace std;
int main()
{
union{
int* input;
char* cinput;
};
size_t length = 1024;
input = new int[length];
cout<<testFunction(input, length-1);
}
If I compile it with g ++ 4.9.2 with -O3, it works fine. I expected that if I uncomment the penultimate line, it will work more slowly, however it completely crashes with SIGSEGV.
Program received signal SIGSEGV, Segmentation fault.
0x0000000000400754 in main ()
(gdb) disassemble
Dump of assembler code for function main:
0x00000000004006e0 <+0>: sub $0x8,%rsp
0x00000000004006e4 <+4>: movabs $0x100000000,%rdi
0x00000000004006ee <+14>: callq 0x400690 <_Znam@plt>
0x00000000004006f3 <+19>: lea 0x1(%rax),%rdx
0x00000000004006f7 <+23>: and $0xf,%edx
0x00000000004006fa <+26>: shr $0x2,%rdx
0x00000000004006fe <+30>: neg %rdx
0x0000000000400701 <+33>: and $0x3,%edx
0x0000000000400704 <+36>: je 0x4007cc <main+236>
0x000000000040070a <+42>: cmp $0x1,%rdx
0x000000000040070e <+46>: mov 0x1(%rax),%esi
0x0000000000400711 <+49>: je 0x4007f1 <main+273>
0x0000000000400717 <+55>: add 0x5(%rax),%esi
0x000000000040071a <+58>: cmp $0x3,%rdx
0x000000000040071e <+62>: jne 0x4007e1 <main+257>
0x0000000000400724 <+68>: add 0x9(%rax),%esi
0x0000000000400727 <+71>: mov $0x3ffffffc,%r9d
0x000000000040072d <+77>: mov $0x3,%edi
0x0000000000400732 <+82>: mov $0x3fffffff,%r8d
0x0000000000400738 <+88>: sub %rdx,%r8
0x000000000040073b <+91>: pxor %xmm0,%xmm0
0x000000000040073f <+95>: lea 0x1(%rax,%rdx,4),%rcx
0x0000000000400744 <+100>: xor %edx,%edx
0x0000000000400746 <+102>: nopw %cs:0x0(%rax,%rax,1)
0x0000000000400750 <+112>: add $0x1,%rdx
=> 0x0000000000400754 <+116>: paddd (%rcx),%xmm0
0x0000000000400758 <+120>: add $0x10,%rcx
0x000000000040075c <+124>: cmp $0xffffffe,%rdx
0x0000000000400763 <+131>: jbe 0x400750 <main+112>
0x0000000000400765 <+133>: movdqa %xmm0,%xmm1
0x0000000000400769 <+137>: lea -0x3ffffffc(%r9),%rcx
---Type <return> to continue, or q <return> to quit---
Why is he falling? Is this a compiler error? Am I causing some undefined behavior? Does the compiler expect ints to always be aligned by 4 bytes?
I also tested it on clang and there was no failure.
Here's the g ++ build: http://pastebin.com/CJdCDCs4