Please review: Optimize vector multiply on X86

[Andrea_DiBiagio at sn.scee.net]

Hi Elena,

> From: "Demikhovsky, Elena" <elena.demikhovsky at intel.com>
> Hi Andrea,
> 
> You are right that 'isSplatVector' returns true if elements are not 
> the same. But later I check the slat value for 0, -1, pow2
> And in the tests I check everything:

What I am saying is that the size in bit of the splat value returned by 
'isSplatVector' matters. For example, it may be bigger than the vector 
element size in bits.

Example, given the following <4 x i64> constant vectors:
a) <i64 2, i64 0, i64 0, i64 0>
b) <i64 2, i64 0, i64 2, i64 0>
c) <i64 2, i64 2, i64 2, i64 2>

They are all considered to be valid vector splats according to 
'isSplatVector' and the splat value is the same for all of them. What 
changes however is the SplatBitSize:
a) SplatValue = 2 ;; SplatBitSize = 256;
b) SplatValue = 2 ;; SplatBitSize = 128;
c) SplatValue = 2 ;; SplatBitSize = 64;

> ==== Multiple 2 gives add
> +define <8 x i32> @mul_const1(<8 x i32> %x) {
> +  %y = mul <8 x i32> %x, <i32 2, i32 2, i32 2, i32 2, i32 2, i32 2,
> i32 2, i32 2>
> +  ret <8 x i32> %y
> +}

In the example above:
if you substitute the second operand of the multiply instruction with any 
of the following constant vectors:

 - <i32 2, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0, i32 0>
 - <i32 2, i32 0, i32 0, i32 0, i32 2, i32 0, i32 0, i32 0>
 - <i32 2, i32 0, i32 2, i32 0, i32 2, i32 0, i32 2, i32 0>

You will find out that your new combine transformation always optimizes 
the multiply into an add, and that causes incorrect codegen.
That is because the compiler applies the following sequence of rules:
 1) fold (mul x, (1 << c)) -> x << c  (in DAGCombiner::visitMul())
 2) (shl V, 1) -> add V, V (in X86ISelLowering.cpp, function 
PerformSHLCombine())

> === non-spalt int value is not optimized
> +define <8 x i32> @mul_const6(<8 x i32> %x) {
> +  %y = mul <8 x i32> %x, <i32 0, i32 0, i32 0, i32 2, i32 0, i32 2,
> i32 0, i32 0>
> +  ret <8 x i32> %y
> +}

That is true for the example above but it is not always true in general.
If you change the code above into:

define <8 x i32> @mul_const6(<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
}

You will get that the multiply is wrongly converted into an add.

Andrea Di Biagio
SN Systems - Sony Computer Entertainment Group


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