[PATCH][X86] Add tablegen patterns to improve the selection of LZCNT and TZCNT instructions in a few cases.

[Benjamin Kramer]

On 16.05.2014, at 17:38, Andrea Di Biagio <andrea.dibiagio at gmail.com> wrote:

> Hi,
> 
> This patch adds new ISel patters to improve the selection of TZCNT and LZCNT.
> 
> Instructions TZCNT (requires BMI1) and LZCNT (requires LZCNT), always
> provides the operand size as output if the input operand is zero. We
> can take advantage of this knowledge during instruction selection
> stage in order to simplify a few corner case.
> 
> Scenario:
> * we want to count the leading/trailing zeros of a value V;
> * then we check if V is equal to zero; in case (if V is zero), we
> propagate a constant equal to the sizeof (in bits) of V.
> 
> Instead of selecting a
>  tzcnt/lzcnt(V) + compare V with zero + conditional move
> we only select
>  tzcnt/lzcnt.
> 
> The patch adds some test cases to verify that the new patterns do
> their job and we don't introduce redundant compare+conditional move
> instructions.
> 
> Please let me know if ok to submit.

LGTM.

- Ben