I have just been hit by something:
lano1106@hpmini ~/dev/gcc-test $ g++ --version
g++ (GCC) 4.8.0 20130502 (prerelease)
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
lano1106@hpmini ~/dev/gcc-test $ g++ -O2 -o test1 test1.cpp test1_init.cpp
lano1106@hpmini ~/dev/gcc-test $ ./test1
item 0
a: 1
lano1106@hpmini ~/dev/gcc-test $ g++ -O1 -o test1 test1.cpp test1_init.cpp
lano1106@hpmini ~/dev/gcc-test $ ./test1
item 0
a: 1
lano1106@hpmini ~/dev/gcc-test $ g++ -O0 -o test1 test1.cpp test1_init.cpp
lano1106@hpmini ~/dev/gcc-test $ ./test1
item 0
a: 1
item 1
a: 2
lano1106@hpmini ~/dev/gcc-test $ cat test1.h
struct A
{
int a;
int b;
int c;
};
struct B
{
int numelem;
/*
* Old C trick to define a dynamically sizable array just by allocating
* sizeof(B) + (numelem-1)*sizeof(A) memory.
*/
A item[1];
};
void initArr(B *p);
lano1106@hpmini ~/dev/gcc-test $ cat test1_init.cpp
#include "test1.h"
void initArr(B *p)
{
p->numelem = 2;
p->item[0].a = 1;
p->item[1].a = 2;
}
lano1106@hpmini ~/dev/gcc-test $ cat test1.cpp
/*
* Author: Olivier Langlois <oliv...@trillion01.com>
*
* Purpose: Small test to highlight gcc optimization bug
*/
#include <stdio.h>
#include <string.h>
#include "test1.h"
/*
* Create a B array with the intent of only using the first item.
* The 19 other items sole purpose is to create a buffer large enough
* to accomodate A array needs.
*/
#define MAXBLEN 20
int main(int argc, char *argv[])
{
B arr[MAXBLEN];
memset(arr,0,sizeof(arr));
initArr(arr);
for( int i = 0; i < arr[0].numelem; ++i )
{
printf( "item %d\n"
" a: %d\n",
i,
arr[0].item[i].a);
}
return 0;
}
>From gcc website, this is not a bug:
Loops do not terminate
This is often caused by out-of-bound array accesses or by signed integer
overflow which both result in undefined behavior according to the ISO C
standard. For example
int
SATD (int* diff, int use_hadamard)
{
int k, satd = 0, m[16], dd, d[16];
...
for (dd=d[k=0]; k<16; dd=d[++k])
satd += (dd < 0 ? -dd : dd);
accesses d[16] before the loop is exited with the k<16 check. This causes
the compiler to optimize away the exit test because the new value of k must be
in the range [0, 15] according to ISO C.
GCC starting with version 4.8 has a new option
-fno-aggressive-loop-optimizations that may help here. If it does, then this is
a clear sign that your code is not conforming to ISO C and it is not a GCC bug.
I am surprised that I didn't hit the problem before but I am seriously
considering using '-fno-aggressive-loop-optimizations' in my own makepkg.conf.
I just want to test others feeling on this discovery to see if it wouldn't be a
good idea to make the switch standard in Arch...
________________________________
CONFIDENTIALITY : This e-mail and any attachments are confidential and may be
privileged. If you are not a named recipient, please notify the sender
immediately and do not disclose the contents to another person, use it for any
purpose or store or copy the information in any medium.
