[llvm-commits] cttz optimisation

[Alastair Lynn]

Hi Chris-

Here's an updated version of the patch, with the changes you requested and a similar transform for ctlz.

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On 5 Jan 2010, at 05:18, Chris Lattner wrote:

> 
> On Jan 4, 2010, at 8:35 PM, Alastair Lynn wrote:
> 
>> Hello-
>> 
>> The attached patch catches cases where cttz's value is constant.
> 
> Nice transformation, some nitpicks:
> 
> 
> +    APInt Mask(BitWidth, 0);
> +    Mask.set(TrailingZeros);
> +    Mask = Mask - 1;
> 
> Please use APInt::getLowBitsSet
> 
> +    if ((Mask & KnownZero) == Mask) {
> +      return ReplaceInstUsesWith(CI, ConstantInt::get(IT,
> +                                 APInt(BitWidth, TrailingZeros)));
> +    }
> 
> No need for the braces.
> 
> Please add a comment explaining the intuition, something like "If all bits before the first known one are known to be zero, then we can evaluate this symbolically.
> 
> Please apply with these changes, thanks Alastair!  Want to do a similar patch for ctlz?
> 
> -Chris