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
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.