[PATCH][X86] Teach the backend how to lower vector shift left into multiply rather than scalarizing it.

[Nadav Rotem]

Please remember to update the x86 cost model in  X86TTI::getArithmeticInstrCost.  In this function you should be able to check if the LHS is a constant. 

Thanks,
Nadav

On Feb 6, 2014, at 2:26 PM, Jim Grosbach <grosbach at apple.com> wrote:

> Hi Andrea,
> 
> This is a very nice improvement, but should do a bit more, I believe.
> 
> AVX2 adds 256-bit wide vector versions of these instructions, so if AVX2 is available, the same transformation should be applied to v16i16 and v8i32 shifts. Worth looking to see if AVX512 extends 
> 
> The test cases should check that when compiling for AVX, the VEX prefixed form of the instructions are generated instead of the SSE versions.
> 
> Thanks,
> -Jim
> 
> On Feb 6, 2014, at 11:45 AM, Andrea Di Biagio <andrea.dibiagio at gmail.com> wrote:
> 
>> Hi,
>> 
>> This patch teaches the backend how to efficiently lower a packed
>> vector shift left into a packed vector multiply if the vector of shift
>> counts is known to be constant (i.e. a constant build_vector).
>> 
>> Instead of expanding a packed shift into a sequence of scalar shifts,
>> the backend should try (when possible) to convert the vector shift
>> into a vector multiply.
>> 
>> Before this patch, a shift of a MVT::v8i16 vector by a build_vector of
>> constants was always scalarized into a long sequence of "vector
>> extracts + scalar shifts + vector insert".
>> With this patch, if there is SSE2 support, we emit a single vector multiply.
>> 
>> The new x86 test 'vec_shift6.ll' contains some examples of code that
>> are affected by this patch.
>> 
>> Please let me know if ok to submit.
>> 
>> Thanks,
>> Andrea Di Biagio
>> SN Systems - Sony Computer Entertainment Group
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