0X80000000 - 10 < 0 and 10 - 0X80000000 < 0
gcc version 3.3.4 gets a right result of comparing between 10 and 0X80000000, but gcc version 4.1.0 fails.
the test code followed:
int b = 0X80000000;
if ((10 - b) < 0)
printf("(10 - b) = %d < 0 is True\n", 10 - b);
printf("(10 - b) = %d < 0 is False\n", 10 - b);
if ((b - 10) < 0)
printf("(b - 10) = %d < 0 is True\n", b - 10);
printf("(b - 10) = %d < 0 is False\n", b - 10);
I had tested the code and got the result:
bingo@sttest:~/tmp> ../bin/g++ t.cpp
(10 - b) = -2147483638 < 0 is True
(b - 10) = 2147483638 < 0 is True
(In reply to comment #0)
> int b = 0X80000000;
> if ((b - 10) < 0)
b - 10 is 0xff(...)7ffffff6 and this is an integer overflow.
[ cite: c++ standard / $5.5 ]
if during the evaluation of an expression the result isn't mathematically
defined nor in the range of representable values for its type the behaviuor
is undefined, unless such an expression is a constant expression, in which
case the program is ill-formed. (...)
[ /cite ]
so, use -fwrapv (man g++) to get not quite correct behaviour.
This is signed int overflow being undefined, in fact I should I know because I was the one who write the patch to optimize this.
Ok, you are right. it would be nice if g++ 4.1.0 acts as what g++ 3.3.4 does.
(In reply to comment #3)
> Ok, you are right. it would be nice if g++ 4.1.0 acts as what g++ 3.3.4 does.
Use -fwrapv if you want defined wrapping.
*** Bug 30475 has been marked as a duplicate of this bug. ***