[llvm-commits] [llvm] r59756 - /llvm/trunk/include/llvm/Intrinsics.td

[Chris Lattner]

On Nov 21, 2008, at 12:42 AM, Duncan Sands wrote:

> Hi Bill,
>
>> Introduce two new "add" intrinsics. These return the sum plus a bit  
>> indicating
>> that an overflow/carry occured. These are converted into ISD:: 
>> [SU]ADDO nodes,
>> which are lowered in a target-independent way into something sane.  
>> Eventually,
>> each target can implement their own method of checking the overflow/ 
>> carry flags.
>
> are these really needed?  Suppose you want to do 32 bit addition of  
> x and y.
> Zero extend x and y to i33.  Do the addition in i33 and check the  
> top bit.
> If it is non-zero then there was a carry/overflow.  In codegen this  
> gets turned
> into an ADDC node (sounds a lot like your ADDO node...), and on x86  
> at least
> turns by magic into a check on the carry flag.  So at first glance  
> it seems that
> everything is already supported.

This doesn't work for all operations.  To know if a signed add  
overflowed, you have to xor together the new sign bit and the carry  
out.  I doubt the right code would be generated for this.

Lets not even get into multiply etc ;-)

-Chris