Re: [arch-general] gcc: loop do not terminate

On 2013-05-13 18:20:05, LANGLOIS Olivier PIS -EXT wrote:

Date: Mon, 13 May 2013 18:20:05 +0000 From: LANGLOIS Olivier PIS -EXT To: "General Discussion about Arch Linux (arch-general@archlinux.org)" Subject: [arch-general] gcc: loop do not terminate

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

I have checked the disassemble of -O1 result, in fact, the for-loop seems optimized out. Not increasment operation(++i), and not jump to the loop begining: for( int i = 0; i < arr[0].numelem; ++i ) 4005df: 8b 14 24 mov (%rsp),%edx 4005e2: 85 d2 test %edx,%edx 4005e4: 7e 1b jle 400601 { printf( "item %d/%d: a = %d\n", i, arr[0].numelem, 122 + arr[0].item[i].a ); 4005e6: 8b 44 24 04 mov 0x4(%rsp),%eax 4005ea: 8d 48 7a lea 0x7a(%rax),%ecx 4005ed: be 00 00 00 00 mov $0x0,%esi 4005f2: bf b4 06 40 00 mov $0x4006b4,%edi 4005f7: b8 00 00 00 00 mov $0x0,%eax 4005fc: e8 9f fe ff ff callq 4004a0 } return 0; } 400601: b8 00 00 00 00 mov $0x0,%eax 400606: 48 81 c4 48 01 00 00 add $0x148,%rsp 40060d: c3 retq 40060e: 66 90 xchg %ax,%ax any one can help to explain it?

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 * * Purpose: Small test to highlight gcc optimization bug */

#include #include #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...

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