[SYCL] Add specialization constant feature design doc.

sycl/doc/SpecializationConstants.md

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+# Specialization constants
+
+DPC++ implements this [proposal](https://github.com/codeplaysoftware/standards-proposals/blob/master/spec-constant/index.md)
+with some restrictions. See this [document](https://github.com/intel/llvm/blob/sycl/sycl/doc/extensions/SpecConstants/README.md) for more details.
+
+#### Requirements:
+
+- must work with separate compilation and linking
+- must support AOT compilation
+
+Implementaion is based on SPIR-V specialization constants. But there is one
+important difference between SYCL and SPIR-V: in SYCL speciazation constants are
+identified by a type ID which is mapped to a symbolic name, in SPIR-V - by an
+ordinal number. This complicates the design, as the compiler
+1) needs to propagate symbolic =\> numeric ID correspondence to the runtime
+2) can assign numeric IDs only when linking due to the separate compilation
+
+Simple source code example:
+
+```cpp
+class MyInt32Const;
+...
+  sycl::program p(q.get_context());
+  sycl::ONEAPI::experimental::spec_constant<int32_t, MyInt32Const> i32 =
+      p.set_spec_constant<MyInt32Const>(rt_val);
+  p.build_with_kernel_type<MyKernel>();
+  sycl::buffer<int, 1> buf(vec.data(), vec.size());
+
+  q.submit([&](cl::sycl::handler &cgh) {
+    auto acc = buf.get_access<cl::sycl::access::mode::write>(cgh);
+    cgh.single_task<MyKernel>(
+        p.get_kernel<MyKernel>(),
+        [=]() {
+          acc[0] = i32.get();
+        });
+  });
+...
+```
+
+## Design
+
+This section describes the basic design used to support spec constants of
+primitive numeric types. POD types support is described further in the document.
+
+#### Compiler
+
+Key `spec_constant::get()` function implementation for the device code:
+
+```cpp
+template <typename T, typename ID = T> class spec_constant {
+...
+public:
+  T get() const { // explicit access.
+#ifdef __SYCL_DEVICE_ONLY__
+    const char *TName = __builtin_unique_stable_name(ID);
+    return __sycl_getSpecConstantValue<T>(TName);
+#else
+    return Val;
+#endif // __SYCL_DEVICE_ONLY__
+  }
+```
+
+here `__builtin_unique_stable_name` is a compiler built-in used to translate
+types to unique strings. `__sycl_getSpecConstantValue<T>` is an "intrinsic"
+recognized by a special LLVM pass later.
+
+Compilation and subsequent linkage of device code results in a number of
+`__sycl_getSpecConstantValue` calls whose arguments are symbolic spec constant
+IDs. Before generating the a device binary, each linked device code LLVMIR
+module undergoes processing by the sycl-post-link tool which can run LLVMIR
+passes before passing the module onto the llvm-spirv translator.
+
+There is a `SpecConstants` LLVMIR pass which
+- assigns numeric IDs to the spec constants
+- brings IR to the form expected by the llvm-spirv translator
+- collects and provides \<Symbolic ID\> =\> \<numeric ID\> spec constant information
+  to the sycl-post-link tool
+Particularly, it translates intrinsic calls to the
+`T __sycl_getSpecConstantValue*(const char *symbolic_id)` intrinsic into
+calls to `T __spirv_SpecConstant(int ID, T default_val)` intrinsic known to
+the llvm-spirv translator. Where `ID` is the numeric ID of the corresponding
+spec constant, `default_val` is its default value which will be used if the
+constant is not set at the runtime.
+
+After this pass the sycl-post-link tool will output the
+\<Symbolic ID\> =\> \<numeric ID\> spec constant mapping into a file for later
+attaching this info to the device binary image via the offload wrapper tool as
+a property set:
+
+```cpp
+struct pi_device_binary_struct {
+...
+  // Array of preperty sets; e.g. specialization constants symbol-int ID map is
+  // propagated to runtime with this mechanism.
+  pi_device_binary_property_set PropertySetsBegin;
+  pi_device_binary_property_set PropertySetsEnd;
+};
+```
+
+SYCL runtime can then load and access info about particular spec constant using
+its name as a key into the appropriate property set (named "SYCL/specialization
+constants").
+
+##### Ahead of time compilation
+
+With AOT everything is simplified - the `SpecConstants` pass simply replaces
+the `__sycl_getSpecConstantValue` calls with constants - default values of
+the spec constant's type. No maps are generated, and SYCL program can't change
+the value of a spec constant.
+
+#### LLVMIR-SPIR-V translator
+
+Given the `__spirv_SpecConstant` intrinsic calls produced by the
+`SpecConstants` pass:
+```cpp
+; Function Attrs: alwaysinline
+define dso_local spir_func i32 @get() local_unnamed_addr #0 {
+  ; args are "ID" and "default value":
+  %1 = tail call spir_func i32 @_Z20__spirv_SpecConstantii(i32 42, i32 0)
+  ret i32 %1
+}
+```
+
+the translator will generate `OpSpecConstant` SPIR-V instructions with proper
+`SpecId` decorations:
+
+```cpp
+              OpDecorate %i32 SpecId 42 ; ID
+       %i32 = OpSpecConstant %int 0     ; Default value
+         %1 = OpTypeFunction %int
+
+       %get = OpFunction %int None %1
+         %2 = OpLabel
+              OpReturnValue %i32
+              OpFunctionEnd
+```
+
+#### SYCL runtime
+
+For each device binary compiler generates a map \<Symbolic ID\> =\> \<numeric ID\>
+("ID map"). The SYCL runtime imports that map when loading device binaries.
+It also maintains another map \<Spec const symbolic ID\> =\> \<its value\> 
+("value map") per `sycl::program` object. The value map is updated upon
+`program::set_spec_constant<IDType>(val)` calls from the app.
+
+**_NOTE_**  `IDType` gets translated to the symbolic ID using the integration
+header mechanism, similarly to kernel ID type. The reason why
+`__builtin_unique_stable_name` is not used here is because this code is
+compiled by the host compiler, which can be any C++ 14-compatible compiler
+unaware of the clang-specific built-ins.
+
+Before JIT-ing a program, the runtime "flushes" the spec constants: it iterates
+through the value map and invokes the
+
+```cpp
+pi_result piextProgramSetSpecializationConstant(pi_program prog,
+                                                pi_uint32 spec_id,
+                                                size_t spec_size,
+                                                const void *spec_value);
+```
+
+Plugin Interface function for each entry, taking the `spec_id` from the ID map.
+
+## "Plain Old Data" (POD) types support design
+
+#### Source representation
+
+Say, the POD type is
+
+```cpp
+struct A {
+  int x;
+  float y;
+};
+
+struct POD {
+  A a[2];
+  int b;
+};
+```
+
+and the user says
+
+```cpp
+  POD gold{
+    {
+      { goldi, goldf },
+      { goldi + 1, goldf + 1 },
+    },
+    goldi
+  };
+
+  cl::sycl::ONEAPI::experimental::spec_constant<POD, MyConst> sc =  program4.set_spec_constant<MyConst>(gold);
+```
+
+#### Compiler
+
+##### The SpecConstant pass changes
+
+ - The SpecConstants pass in the post-link will have the following IR as input (`sret` conversion is omitted for clarity):
+
+```cpp
+  %spec_const = call %struct.POD __sycl_getCompositeSpecConstantValue<POD type mangling> ("MyConst_mangled")
+```
+
+where `__sycl_getCompositeSpecConstantValue` is a new "intrinsic"
+ (in addition to `__sycl_getSpecConstantValue`) recognized by SpecConstants pass,
+ which creates a value of a composite (of non-primitive type) specialization constant.
+ It does not need a default value, because its default value consists of default
+ valued of its leaf specialization constants (see below).
+
+ - after spec constant enumeration (symbolic -\> int ID translation), the SpecConstants pass
+ will handle the `__sycl_getCompositeSpecConstantValue`. Given the knowledge of the composite
+ specialization constant's type (`%struct.POD`), the pass will traverse its leaf
+ fields and generate 5 "primitive" spec constants using already existing SPIR-V intrinsic:
+
+```cpp
+%gold_POD_a0x = call i32 __spirv_SpecConstant(i32 10, i32 0)
+%gold_POD_a0y = call float __spirv_SpecConstant(i32 11, float 0)
+%gold_POD_a1x = call i32 __spirv_SpecConstant(i32 12, i32 0)
+%gold_POD_a1y = call float __spirv_SpecConstant(i32 13, float 0)
+%gold_POD_b = call i32 __spirv_SpecConstant(i32 14, i32 0)
+```
+
+And 1 "composite"
+
+```cpp
+  %gold_POD = call %struct.POD __spirvCompositeSpecConstant<POD type mangling>(i32 10, i32 11, i32 12, i32 13, i32 14)
+```
+
+where `__spirvCompositeSpecConstant<POD type mangling>` is a new SPIR-V intrinsic which
+ represents creation of a composite specialization constant. Its arguments are spec
+ constant IDs corresponding to the leaf fields of the POD type of the constant.
+Spec ID for the composite spec constant is not needed, as runtime will never use it - it will use IDs of the leaves instead.
+ Yet, the SPIR-V specification does not allow `SpecID` decoration for composite spec constants.
+
+##### The post-link tool changes
+
+For composite specialization constants the post link tool will additionally
+generate linearized list of \<leaf spec ID,type,offset,size\> tuples (descriptors),
+where each tuple describes a leaf field, and store it together with the
+existing meta-information associated with the specialization constants and
+passed to the runtime. Also, for a composite specialization constant there is
+no ID map entry within the meta information, and the composite constant is
+referenced by its symbolic ID. For example:
+
+```cpp
+MyConst_mangled [10,int,0,4],[11,float,4,4],[12,int,8,4],[13,float,12,4],[14,int,16,4]
+```
+
+#### LLVMIR-\>SPIR-V translator
+
+The translator aims to create the following code (pseudo-code)
+
+```cpp
+%gold_POD_a0x = OpSpecConstant(0)    [SpecId = 10]
+%gold_POD_a0y = OpSpecConstant(0.0f) [SpecId = 11]
+%gold_POD_a1x = OpSpecConstant(0)    [SpecId = 12]
+%gold_POD_a1y = OpSpecConstant(0.0f) [SpecId = 13]
+%gold_POD_b   = OpSpecConstant(0)    [SpecId = 14]
+
+%gold_POD_a0 = OpSpecConstantComposite(
+  %gold_POD_a0x // gold.a[0].x
+  %gold_POD_a0y // gold.a[0].y
+)
+
+%gold_POD_a1 = OpSpecConstantComposite(
+  %gold_POD_a1x // gold.a[1].x
+  %gold_POD_a1y // gold.a[1].y
+)
+
+%gold_POD = OpSpecConstantComposite(
+  %gold_POD_a0,
+  %gold_POD_a1,
+  %gold_POD_b   // gold.b
+}
+```
+
+- First, `OpSpecConstant` instructions are created using already existing mechanism for
+primitive spec constants.
+- Then the translator will handle `__spirvCompositeSpecConstant*` intrinsic.
+It will recursively traverse the spec constant type structure in parallel with
+the argument list - which is a list of primitive spec constant SpecIds.
+When traversing, it will create all the intermediate OpSpecConstantComposite
+instructions as well as the root one (`%gold_POD`) using simple depth-first tree
+traversal with stack. This requires mapping from SpecId decoration number to
+\<id\> of the corresponding OpSpecConstant instruction, but this should be pretty
+straightforward.
+
+#### SYCL runtime
+
+First, when the runtime loads a binary it gets access to specialization
+constant information. So the mapping from a composite spec constant name to
+its constituents (descriptors of leaf fields) generated by the post-link tool
+will be available.
+
+Now, when the program invokes `program4.set_spec_constant<MyConst>(gold)`,
+ SYCL runtime converts the call arguments (template and actual) to the following
+ pair of datums:
+ - the constant name - "MyConst_mangled"
+ - the byte array representing the value of the constant (`gold` value)
+
+Then the runtime fetches the sequence of leaf field descriptors (primitive
+constituents) of the composite constant and iterates through each pair invoking
+`piextProgramSetSpecializationConstant` for each. The ID of the constant is
+taken from the sequence, the value - from the byte array obtained for the
+`gold`.