Deprecate PI gates implementation

.github/CHANGELOG.md

@@ -23,6 +23,9 @@
 
 ### Breaking changes
 
+* Deprecate PI gates implementation.
+  [(#923)](https://github.com/PennyLaneAI/pennylane-lightning/pull/923)
+
 * Update MacOS wheel builds to require Monterey (12.0) or greater for x86_64 and ARM.
   [(#901)](https://github.com/PennyLaneAI/pennylane-lightning/pull/901)
 

doc/benchmark.rst

@@ -30,9 +30,6 @@ One can also choose a specific datatype by providing an option:
     $ ./benchmarks/plot_gate_benchmark.py --precision float ./bench_result.json # Results for std::complex<float>
     $ ./benchmarks/plot_gate_benchmark.py --precision double ./bench_result.json # Results for std::complex<double>
 
-
-Currently, we have two different kernels in ``Lightning Qubit`` named ``PI`` and ``LM``. For difference between two kernels, see the documents :cpp:class:`Pennylane::Gates::GateImplementationsPI` and :cpp:class:`Pennylane::Gates::GateImplementationsLM`.
-
 Here are some example plots:
 
 .. image:: ./_static/benchmark/PauliX.png

doc/lightning_qubit/development/add_gate_kernel.rst

@@ -40,7 +40,7 @@ This can be done by modifying two files:
 
     // file: gates/KernelType.hpp
     namespace Pennylane {
-    enum class KernelType { PI, LM, MyKernel /* This is added */, None };
+    enum class KernelType { LM, MyKernel /* This is added */, None };
 
     /* Rest of the file */
 
@@ -53,7 +53,6 @@ and
     // file: gates/AvailableKernels.hpp
     namespace Pennylane {
         using AvailableKernels = Util::TypeList<GateImplementationsLM,
-                                                GateImplementationsPI,
                                                 MyGateImplementation /* This is added*/,
                                                 void>;
     } // namespace Pennylane
@@ -111,7 +110,6 @@ To test your own kernel implementations, you can go to ``tests/TestKernels.hpp``
 .. code-block:: cpp
 
     using TestKernels = Pennylane::Util::TypeList<Pennylane::Gates::GateImplementationsLM,
-                                                  Pennylane::Gates::GateImplementationsPI,
                                                   MyGateImplementation /*This is added */, void>;
 
 It will automatically test your gate implementation.

doc/lightning_qubit/development/avx_kernels/build_system.rst

@@ -8,11 +8,10 @@ when the library is loaded, and it is used at runtime when it is the most suitab
 To support AVX2 and AVX512 kernels, we always compile those kernels if the target system is UNIX on x86-64.
 Specifically, we made separate C++ files for AVX2 and AVX512 kernels and build them as a static library with the corresponding compile options. This is handled by CMake. One can check ``gates/CMakeLists.txt`` file for details.
 
-One caveat is that we want to make sure that default kernels (``KernelType::PI`` and ``KernelType::LM``) are only instantiated once with specific compiler flags during the compile process.
+One caveat is that we want to make sure that default ``KernelType::LM`` kernels are only instantiated once with specific compiler flags during the compile process.
 This is important as the linker sometimes cannot choose the right instantiation when there are multiple instantiations of the same template class.
 This problem does not arise when all instantiations are compiled with the same options, but with the AVX2/512 kernels, we use different compile options for each translation unit. We solve this problem by adding explicit instantiation declarations in the header files for these kernels
-(:ref:`file_pennylane_lightning_core_src_simulators_lightning_qubit_gates_cpu_kernels_GateImplementationsLM.hpp` and
-:ref:`file_pennylane_lightning_core_src_simulators_lightning_qubit_gates_cpu_kernels_GateImplementationsPI.hpp`)
+(:ref:`file_pennylane_lightning_core_src_simulators_lightning_qubit_gates_cpu_kernels_GateImplementationsLM.hpp`)
 and compile them as a separate static library.
 
 With this, the AVX2/512 kernels are always included in the binary when compiled for UNIX-compatible OSs on x86-64 architecture.

pennylane_lightning/core/src/simulators/lightning_qubit/gates/KernelMap.hpp

@@ -242,11 +242,9 @@ class OperationKernelMap {
         : allowed_kernels_{
               // LCOV_EXCL_START
               {CPUMemoryModel::Unaligned, {KernelType::LM}},
-              {CPUMemoryModel::Aligned256,
-               {KernelType::LM, KernelType::AVX2}},
+              {CPUMemoryModel::Aligned256, {KernelType::LM, KernelType::AVX2}},
               {CPUMemoryModel::Aligned512,
-               {KernelType::LM, KernelType::AVX2,
-                KernelType::AVX512}},
+               {KernelType::LM, KernelType::AVX2, KernelType::AVX512}},
               // LCOV_EXCL_STOP
           } {}
 

pennylane_lightning/core/src/simulators/lightning_qubit/gates/tests/Test_Internal.cpp

@@ -29,7 +29,6 @@
 
 /// @cond DEV
 namespace {
-using namespace Pennylane::LightningQubit;
 using namespace Pennylane::Util;
 } // namespace
 /// @endcond

pennylane_lightning/core/src/utils/TestKernels.hpp

@@ -22,9 +22,7 @@
 #include "TypeList.hpp"
 
 #include "cpu_kernels/GateImplementationsLM.hpp"
-#include "cpu_kernels/GateImplementationsPI.hpp"
 
 using TestKernels = Pennylane::Util::TypeList<
-    Pennylane::LightningQubit::Gates::GateImplementationsPI,
     Pennylane::LightningQubit::Gates::GateImplementationsLM, void>;
 #endif