[SYCL][FPGA] New spec for controlling load-store units in FPGAs

sycl/doc/extensions/IntelFPGA/FPGALsu.md

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+
+# FPGA lsu
+
+The Intel FPGA `lsu` class is implemented in `CL/sycl/intel/fpga_lsu.hpp` which
+is included in `CL/sycl/intel/fpga_extensions.hpp`.
+
+The class `cl::sycl::intel::lsu` allows users to explicitly request that the
+implementation of a global memory access is configured in a certain way. The
+class has two member functions, `load()` and `store()` which allow loading from
+and storing to a `global_ptr`, respectively, and is templated on the following
+4 optional paremeters:
+
+1.  **`cl::sycl::intel::burst_coalesce<B>`, where `B` is a boolean**: request,
+to the extent possible, that a dynamic burst coalescer be implemented when
+`load` or `store` are called. The default value of this parameter is `false`.
+2. **`cl::sycl::intel::cache<N>`, where `N` is an integer greater or equal to
+0**: request, to the extent possible, that a read-only cache of the specified
+size in bytes be implemented when when `load` is called. It is not allowed to
+use that parameter for `store`. The default value of this parameter is `0`.
+3. **`cl::sycl::intel::statically_coalesce<N>`, where `B` is a boolean**:
+request, to the extent possible, that `load` or `store` accesses, is allowed to
+be statically coalesced with other memory accesses at compile time. The default
+value of this parameter is `true`.
+4. **`cl::sycl::intel::prefetch<B>`, where `N` is a boolean**: request, to the
+extent possible, that a prefetcher be implemented when `load` is called. It is
+not allowed to use that parameter for `store`. The default value of this
+parameter is `false`.
+
+Currently, not every combination of parameters is allowed due to limitations in
+the backend. The following rules apply:
+1. For `store`, `cl::sycl::intel::cache` must be `0` and
+`cl::sycl::intel::prefetch` must be `false`.
+2. For `load`, if `cl::sycl::intel::cache` is set to a value greater than `0`,
+then `cl::sycl::intel::burst_coalesce` must be set to `true`.
+3. For `load`, exactly one of `cl::sycl::intel::prefetch` and
+`cl::sycl::intel::burst_coalesce` is allowed to be `true`.
+4. For `load`, exactly one of `cl::sycl::intel::prefetch` and
+`cl::sycl::intel::cache` is allowed to be `true`.
+
+## Implementation
+
+The implementation relies on the Clang built-in `__builtin_intel_fpga_mem` when
+parsing the SYCL device code. The built-in uses the LLVM `ptr.annotation`
+intrinsic under the hood to annotate the pointer that is being accessed.
+```c++
+template <class... mem_access_params> class lsu final {
+public:
+  lsu() = delete;
+
+  template <typename T> static T &load(sycl::global_ptr<T> Ptr) {
+    check_load();
+#if defined(__SYCL_DEVICE_ONLY__) && __has_builtin(__builtin_intel_fpga_mem)
+    return *__builtin_intel_fpga_mem((T *)Ptr,
+                                     _burst_coalesce | _cache |
+                                         _dont_statically_coalesce | _prefetch,
+                                     _cache_val);
+#else
+    return *Ptr;
+#endif
+  }
+
+  template <typename T> static void store(sycl::global_ptr<T> Ptr, T Val) {
+    check_store();
+#if defined(__SYCL_DEVICE_ONLY__) && __has_builtin(__builtin_intel_fpga_mem)
+    *__builtin_intel_fpga_mem((T *)Ptr,
+                              _burst_coalesce | _cache |
+                                  _dont_statically_coalesce | _prefetch,
+                              _cache_val) = Val;
+#else
+    *Ptr = Val;
+#endif
+  }
+  ...
+}
+```
+
+## Usage
+
+```c++
+#include <CL/sycl/intel/fpga_extensions.hpp>
+...
+cl::sycl::buffer<int, 1> output_buffer(output_data, 1);
+cl::sycl::buffer<int, 1> input_buffer(input_data, 1);
+
+Queue.submit([&](cl::sycl::handler &cgh) {
+  auto output_accessor = output_buffer.get_access<cl::sycl::access::mode::write>(cgh);
+  auto input_accessor = input_buffer.get_access<cl::sycl::access::mode::read>(cgh);
+
+  cgh.single_task<class kernel>([=] {
+    auto input_ptr = input_accessor.get_pointer();
+    auto output_ptr = output_accessor.get_pointer();
+
+    using PrefetchingLSU =
+        cl::sycl::intel::lsu<cl::sycl::intel::prefetch<true>,
+                             cl::sycl::intel::statically_coalesce<false>>;
+
+    using BurstCoalescedLSU =
+        cl::sycl::intel::lsu<cl::sycl::intel::burst_coalesce<false>,
+                             cl::sycl::intel::statically_coalesce<false>>;
+
+    using CachingLSU =
+        cl::sycl::intel::lsu<cl::sycl::intel::burst_coalesce<true>,
+                             cl::sycl::intel::cache<1024>,
+                             cl::sycl::intel::statically_coalesce<true>>;
+
+    using PipelinedLSU = cl::sycl::intel::lsu<>;
+
+    int X = PrefetchingLSU::load(input_ptr); // int X = input_ptr[0]
+    int Y = CachingLSU::load(input_ptr + 1); // int Y = input_ptr[1]
+
+    BurstCoalescedLSU::store(output_ptr, X); // output_ptr[0] = X
+    PipelinedLSU::store(output_ptr + 1, Y);  // output_ptr[1] = Y
+  });
+});
+...
+```