[Mono-dev] Arm overflow arithmetic

[Zoltan Varga]

Hi,

  I would vote for adding a flag, as it is simpler.

           Zoltan

On Sat, Aug 9, 2008 at 12:25 AM, Rodrigo Kumpera <kumpera at gmail.com> wrote:
> Hey guys,
>
> I just got myself into a roadblock today while fixing overflow arithmetic
> under ARM. The issue is that the emitted IR
> cannot be properly translated to native code.
>
> The ARM ISA has the oddity that carry/borrow works in the opposite way. Add
> with carry do a +1 if the carry bit is set,
> but Sub with borrow do a -1 if the carry bit is not set.
>
> Then we have mono_decompose_opcode that use the same condition for both add
> and sub:
>
>     case OP_IADD_OVF:
>         ins->opcode = OP_IADDCC;
>         MONO_EMIT_NEW_COND_EXC (cfg, IOV, "OverflowException");
>         break;
>     case OP_ISUB_OVF:
>         ins->opcode = OP_ISUBCC;
>         MONO_EMIT_NEW_COND_EXC (cfg, IOV, "OverflowException");
>         break;
>
>
> I cannot fix that in the arm back-end as it should not reason about previous
> instructions.
> My idea is to either move the SUB opcodes to arch specific code or add an
> #if MONO_ARCH_SUB_INVERTED_CARRY_FLAG.
>
> What do you guys think about this?
>
> Cheers,
> Rodrigo
>
>
>