[Doc][RFC][SLP] Add more description for SLP Vectorizer

[eopXD]

Summary

The SLP Vectorizer section have very little coverage. This patch tries to add
more to it. I am not an expert in vectorization, I am just pointing out what I
currently know about the SLP Vectorizer and how a beginner may start for
its first time seeing the SLP Vectorizer pass.

This is an RFC and comments are welcomed!

[eopXD]

You can preview the document here: https://github.com/eopXD/llvm-project/blob/doc/llvm/docs/Vectorizers.rst#the-slp-vectorizer

[lebedev.ri]

This review may be stuck/dead, consider abandoning if no longer relevant.
Removing myself as reviewer in attempt to clean dashboard.

llvm/docs/Vectorizers.rst

.. code-block:: c++
 
   void foo(int a1, int a2, int b1, int b2, int *A) {
     A[0] = a1*(a1 + b1);
     A[1] = a2*(a2 + b2);
     A[2] = a1*(a1 + b1);
     A[3] = a2*(a2 + b2);
   }
 
-The SLP-vectorizer processes the code bottom-up, across basic blocks, in search of scalars to combine.
+The SLP idea is originated from S. Larsen and S. Amarasinghe [1]_. The
+original idea of SLP is to capture chances of parallelism within a basic block,
+that is, without regard to the loop context. It's simple and robust idea is why
+the optimization is applicable to production compilers. The SLP idea is later
+implemented into GCC [2]_ . There is a report about the GCC's implementation
+[3]_ that summarizes the implement methods and possible future work for
+building a SLP Vectorizer.
+
+The LLVM implements SLP vectorization based on the GCC report [3]_, where the
+compiler builds up "computation tree" from bottom-up. The tree starts from
+collecting store and GEP instructions, group them into "bundles", then expand
+when operands of the bundled can be further grouped into more bundles.
+
+By opening the option ``view-slp-tree``, you can get a Graphviz visualization
+of the tree built. The computation tree will look like the picture below.
+"Isomorphic" statements (statements with the same computation) are grouped
+together.
+
+.. image:: slp-tree.png
 
 Usage
 ------
 
 The SLP Vectorizer is enabled by default, but it can be disabled
 through clang using the command line flag:
 
 .. code-block:: console
 
    $ clang -fno-slp-vectorize file.c
+
+You can investigate more on how the SLP Vectorizer works by feeding LLVM IR to
+it. Note that you will need to specify a target with vector register to the
+compiler (e.g. for RISCV, you will need to include the ``V`` or ``Zve``
+extension). Invoke the SLP Vectorizer pass with the following command:
+
+.. code-block:: console
+
+   $ clang --passes="slp-vectorizer" -mtriple=riscv64 -mattr=+v -S file.ll
+
+References
+----------
+.. [1] "Exploiting superword level parallelism with multimedia instruction
+    sets", S. Larsen, S. Amarasinghe, ACM SIGPLAN 2000.
+
+.. [2] "Auto-vectorization in GCC" [`link
+    <https://gcc.gnu.org/projects/tree-ssa/vectorization.html>`]
+
+.. [3] "Loop-aware SLP in GCC", by I. Rosen, D. Nuzman, A.l Zaks,
+    GCC Developer's Summit 2007.