Please review: Optimize vector multiply on X86

[Andrea_DiBiagio at sn.scee.net]

Hi Elena,

> From: "Demikhovsky, Elena" <elena.demikhovsky at intel.com>
> I did the MUL optimization common for all targets. Please review.

Your function 'isSplatVector' returns true also when elements are not all 
the same.
Example: a vector of four i64 defined as <i64 4, i64 0, i64 0, i64 0> is 
still considered to be a valid splat
with a SplatBitSize equale to the vector size.

With your patch, if I run
llc -mtriple=x86_64-apple-darwin -mcpu=core-avx2 -mattr=+avx2 
on the following function:

define <8 x i32> @mul_const1(<8 x i32> %x) {
  %y = mul <8 x i32> %x, <i32 2, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, 
i32 0>
  ret <8 x i32> %y
}

I get that the multiply is wrongly combined into
vpaddd %ymm0, %ymm0, %ymm0

Another example is:
define <4 x i64> @mul_const2(<4 x i64> %x) {
  %y = mul <4 x i64> %x, <i64 4, i64 0, i64 0, i64 0>
  ret <4 x i64> %y
}

here the multiply is wrongly combined into
vpsllq $2, %ymm0, %ymm0

If you want to make sure that isSplatVector returns 'true' _only_ in the 
case where the elements of a BUILD_VECTOR are exactly the same (or Undef), 
then you should check both SplatBitSize and HasAnyUndefs. Alternatively in 
`isSplatVector' you could implement a loop that checks if each operand of 
the BUILD_VECTOR is either Undef or the same ConstantSDNode.

I hope this helps!
Andrea Di Biagio
SN Systems - Sony Computer Entertainment Group

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