[SingleSource/Vectorizer] Add unit tests for conditional scalar assignment pattern #155
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…nment pattern. Dedicated unit tests for loops which contain a conditional assignment inside the loop body. An earlier version of this patch was posted as https://reviews.llvm.org/D144909. Some updates have been made to cover more test cases. I tested this patch with the CSA patch applied (I will add a link once the CSA patch is posted) using the following options: ``` -O3 -mllvm -enable-csa-vectorization -mcpu=sifive-x280 ``` I also ran it with `-mllvm -force-tail-folding-style=data-with-evl` appended to the previous option set as well as `-mllvm -force-tail-folding-style=none`. I also verified that vector code was generated for functions that we are currently able to vectorize using objdump. This patch contains some examples that we are not able to vectorize today but should be able to in the future.
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This patch adds initial support for CSA vectorization LLVM. This new class
can be characterized by vectorization of assignment to a scalar in a loop,
such that the assignment is conditional from the perspective of its use.
An assignment is conditional in a loop if a value may or may not be assigned
in the loop body.
For example:
```
int t = init_val;
for (int i = 0; i < N; i++) {
if (cond[i])
t = a[i];
}
s = t; // use t
```
Using pseudo-LLVM code this can be vectorized as
```
vector.ph:
...
%t = %init_val
%init.mask = <all-false-vec>
%init.data = <poison-vec> ; uninitialized
vector.body:
...
%mask.phi = phi [%init.mask, %vector.ph], [%new.mask, %vector.body]
%data.phi = phi [%data.mask, %vector.ph], [%new.mask, %vector.body]
%cond.vec = <widened-cmp> ...
%a.vec = <widened-load> %a, %i
%b = <any-lane-active> %cond.vec
%new.mask = select %b, %cond.vec, %mask.phi
%new.data = select %b, %a.vec, %data.phi
...
middle.block:
%s = <extract-last-active-lane> %new.mask, %new.data
```
On each iteration, we track whether any lane in the widened condition was active,
and if it was take the current mask and data as the new mask and data vector.
Then at the end of the loop, the scalar can be extracted only once.
This transformation works the same way for integer, pointer, and floating point
conditional assignment, since the transformation does not require inspection
of the data being assigned.
In the vectorization of a CSA, we will be introducing recipes into the vector
preheader, the vector body, and the middle block. Recipes that are introduced
into the preheader and middle block are executed only one time, and recipes
that are in the vector body will be possibly executed multiple times. The more
times that the vector body is executed, the less of an impact the preheader
and middle block cost have on the overall cost of a CSA.
A detailed explanation of the concept can be found [here](https://discourse.llvm.org/t/vectorization-of-conditional-scalar-assignment-csa/80964).
This patch is further tested in llvm/llvm-test-suite#155.
This patch contains only the non-EVL related code. The is based on the larger
patch of llvm#106560, which contained both EVL and non-EVL related parts.
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Dedicated unit tests for loops which contain a conditional assignment inside the loop body.
An earlier version of this patch was posted as https://reviews.llvm.org/D144909. Some updates have been made to cover more test cases.
I tested this patch with the CSA vectorization patch applied (I will add a link once the CSA patch is posted) using the following options:
I also ran it with
-mllvm -force-tail-folding-style=data-with-evlappended to the previous option set as well as-mllvm -force-tail-folding-style=none.I also verified that vector code was generated for functions that we are currently able to vectorize using objdump. This patch contains some examples that we are not able to vectorize today but should be able to in the future.
An RFC that describes the class of problems being tested can be found here.
llvm/llvm-project#106560 is the patch that implements CSA vectorization.