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

[Evan Cheng]

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.

Can codegen really handle i33 arithmetic? Even if it does,  
transforming it the ADDC / ADDO + check probably requires quite a bit  
of work in dag combine. We'd also like to support it in fast-isel. I  
think the intrinsics are needed, at least in the short term.

As for ADDC vs ADDO. I think we have come to the same conclusion. I  
like to explicitly modeled the carry bit. In the end one of them  
probably should be eliminated. But that may take a bit of effort.

Evan

>
>
> Ciao,
>
> Duncan.
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