[LLVMdev] Effective Sign Extension Elimination

[Evan Cheng]

C ABI requires function parameters and return value to be promoted to  
32-bit. Perhaps we can eliminate these extensions at IPO time if both  
caller and callee agrees to break the ABI convention? This can have  
minor performance impact on targets with sub-registers (e.g. x86).

Evan

On Jan 4, 2007, at 2:02 PM, Chris Lattner wrote:

> On Thu, 4 Jan 2007, Bill Wendling wrote:
>> Now that we have sign/zero extensions, have people given thought to
>> the elimination of these? There's a paper I downloaded a few years
>> ago called "Effective Sign Extension Elimination" by M. Kawahito, H.
>> Komatsu, and T. Nakatani.
>
> Sign extension elimination is most useful in the backend, when  
> targeting
> processors like PowerPC that have a single register width (e.g. 32- 
> bits).
> We already do quite a bit of sign extension removal in the dag  
> combiner,
> but this is limited to a single basic block at a time.  In the  
> future, I'd
> like to extend the selectiondag to work on entire functions at a time,
> which would increase the scope of this optimization, but this isn't  
> likely
> to happen in the near future.
>
> -Chris
>
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