http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48793
Summary: Optimization -O2 Incorrectly Removes Logic
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: ch...@durso.org
Created attachment 24118
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24118
main.ii
When dealing with min_int (0x80000000, a peculiar value as min_int < 0 &&
min_int == -1*min_int) it appears that -O2 and above on on some versions of gcc
will under some circumstances optimize away particular logic for dealing with
this edge case.
Of the versions I've tested:
gcc version 4.1.2 20080704 (Red Hat 4.1.2-48) SUCCESSFUL
gcc version 4.2.1 (Apple Inc. build 5666) (dot 3) FAILS
gcc version 4.4.3 (GCC) Target: i386-pc-solaris2.10 FAILS
FAILURE EXAMPLE:
chdurso@searchdev12:m_index_query_hash_core> uname -a ;
/ebay/vendor/pkg/gcc-4.4.3/bin/g++ -v; /ebay/vendor/pkg/gcc-4.4.3/bin/g++ -Wall
-Wextra -save-temps -O2 -o hello main.cpp ; echo START_OUTPUT; ./hello
SunOS searchdev12.arch.ebay.com 5.10 Generic_120012-14 i86pc i386 i86pc Solaris
Using built-in specs.
Target: i386-pc-solaris2.10
Configured with: /ebay/vendor/Archive/gcc-4.4.3/configure
--prefix=/ebay/vendor/pkg/gcc-4.4.3 --with-gnu-as --with-as=/usr/sfw/bin/gas
--without-gnu-ld --with-ld=/usr/ccs/bin/ld --enable-shared
--enable-languages=c,c++
Thread model: posix
gcc version 4.4.3 (GCC)
START_OUTPUT
The following number is tricky as it is the only negative number unpaired with
a positive analog in 2's compliment arithemetic. It behaves unintuitively and
apparently breaks an optimization.
tricky: -2147483648
abs(tricky): -2147483648
1 * tricky = -2147483648
-1 * tricky = -2147483648
tricky + 1 = -2147483647
tricky - 1 = 2147483647
test 0: tricky: -2147483648
test 1: tricky: -2147483648
Assertion failed: tricky == 0x7fffffff && "on -O2 and above this asserts
apparently drops last if statement", file main.cpp, line 35
Abort (core dumped)
// main.cpp
//
#include <iostream>
#include <stdlib.h>
#include <cassert>
using namespace std;
int main(int , const char**){
cout << "The following number is tricky as it is the only negative number "
"unpaired with a positive analog in 2's compliment arithemetic. "
"It behaves unintuitively and apparently breaks an optimization."<< endl
<< endl;
int tricky = 0x80000000;
cout << "tricky: " << tricky << endl << endl;
cout << "abs(tricky): " << abs(tricky) << endl
<< " 1 * tricky = " << 1* tricky << endl
<< "-1 * tricky = " << -1* tricky << endl
<< "tricky + 1 = " << tricky + 1 << endl
<< "tricky - 1 = " << tricky - 1 << endl;
tricky = 0x1;
for(int i(0); i < 31; ++i) // can't simply assign, but a loop here will
result allow failure
tricky <<= 1;
assert(tricky == int(0x80000000));
// tricky = 0x80000000; this unnecessary assignment will cause it work
correctly
cout << "test 0: tricky: " << tricky << endl;
if (tricky<0) tricky = -tricky; // should always get here
cout << "test 1: tricky: " << tricky << endl;
if (tricky < 0) tricky = 0x7fffffff; // should always get here
assert(tricky == 0x7fffffff && "on -O2 and above this asserts apparently
drops last if statement");
return 0;
}
ATTACHED main.ii
