[LLVMdev] Overflows and optimizations

[Vincent Isambart]

Hi,

We have a problem concerning LLVM optimizations and overflows in MacRuby.
I reduced that to the following (architecture x86-64):

$ cat e.c
#include <stdio.h>

long foo(void);

int main() {
    const long a = foo();
    const long b = foo();
    const long res = a * b;
    fprintf(stderr, "0x%lx 0x%lx 0x%lx 0x%lx\n", res, a, b, res / a);

    return 0;
}
$ cat e2.c
long foo(void) {
    return 0x100000000;
}
$ llvm-gcc --emit-llvm e.c -c -o e.bc
$ opt -O3 e.bc -o=eopt.bc
$ llc eopt.bc
$ gcc -o e e2.c eopt.s
$ ./e
0x0 0x100000000 0x100000000 0x100000000

I was expecting the 4th number to be 0, but the CES pass that res / a
= a * res / a = b and so optimizes the division out.

If I use directly a C compiler (same result with gcc, llvm-gcc or
clang) I get the result I was expecting:

$ gcc -O3 -o e e.c e2.c
$ ./e
0x0 0x100000000 0x100000000 0x0

How could I tell opt, and the optimization passes enabled in MacRuby
not to do this optimization when there might be an overflow?

PS: I know if an overflow occurs result is undefined but I guess
there's probably a flag or option for that.

Cheers