[SLP] Enable 64-bit wide vectorization on AArch64

ARM Neon has native support for half-sized vector registers (64 bits).  This
is beneficial for example for 2D and 3D graphics.  This patch adds the option
to lower MinVecRegSize from 128 via a TTI in the SLP Vectorizer.

*** Performance Analysis

This change was motivated by some internal benchmarks but it is also
beneficial on SPEC and the LLVM testsuite.

The results are with -O3 and PGO.  A negative percentage is an improvement.
The testsuite was run with a sample size of 4.

** SPEC

* CFP2006/482.sphinx3  -3.34%

A pretty hot loop is SLP vectorized resulting in nice instruction reduction.
This used to be a +22% regression before rL299482.

* CFP2000/177.mesa     -3.34%
* CINT2000/256.bzip2   +6.97%

My current plan is to extend the fix in rL299482 to i16 which brings the
regression down to +2.5%.  There are also other problems with the codegen in
this loop so there is further room for improvement.

** LLVM testsuite

* SingleSource/Benchmarks/Misc/ReedSolomon               -10.75%

There are multiple small SLP vectorizations outside the hot code.  It's a bit
surprising that it adds up to 10%.  Some of this may be code-layout noise.

* MultiSource/Benchmarks/VersaBench/beamformer/beamformer -8.40%

The opt-viewer screenshot can be seen at F3218284.  We start at a colder store
but the tree leads us into the hottest loop.

* MultiSource/Applications/lambda-0.1.3/lambda            -2.68%
* MultiSource/Benchmarks/Bullet/bullet                    -2.18%

This is using 3D vectors.

* SingleSource/Benchmarks/Shootout-C++/Shootout-C++-lists +6.67%

Noise, binary is unchanged.

* MultiSource/Benchmarks/Ptrdist/anagram/anagram          +4.90%

There is an additional SLP in the cold code.  The test runs for ~1sec and
prints out over 2000 lines. This is most likely noise.

* MultiSource/Applications/aha/aha                        +1.63%
* MultiSource/Applications/JM/lencod/lencod               +1.41%
* SingleSource/Benchmarks/Misc/richards_benchmark         +1.15%

Differential Revision: https://reviews.llvm.org/D31965

llvm-svn: 303116

1 files changed, 4 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 5f88b415321..64013d6d687 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -316,7 +316,10 @@ public:
     else
       MaxVecRegSize = TTI->getRegisterBitWidth(true);
 
-    MinVecRegSize = MinVectorRegSizeOption;
+    if (MinVectorRegSizeOption.getNumOccurrences())
+      MinVecRegSize = MinVectorRegSizeOption;
+    else
+      MinVecRegSize = TTI->getMinVectorRegisterBitWidth();
   }
 
   /// \brief Vectorize the tree that starts with the elements in \p VL.