[SPIR-V] Add proposal for global & local variable address spaces

proposals/0020-SPIRV-variable-address-space.md

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+# SPIRV - Variable address space
+
+ * Proposal: [NNNN](NNNN-SPIRV-variable-address-space.md)
+ * Author(s): [Nathan Gauër](https://github.com/Keenuts)
+ * Status: **Design In Progress**
+
+## Introduction
+
+From the HLSL spec:
+
+> HLSL programs manipulates data stored in four distinct memory spaces: thread, threadgroup, device and constant.
+
+Those four groups represents the user-facing semantic, and the group this
+proposal will focus on is `thread`.
+Following this model, a function local variable and a static global variable
+share the same address space.
+
+On the logical SPIR-V side, variables are attached to a storage class. This
+is a different name to represent the same thing: an address space.
+- A pointer to one storage class is incompatible with a pointer to another.
+
+This proposal will use address space when speaking in HLSL/LLVM-IR terms, and
+storage class when speaking in SPIR-V terms.
+We will not mention C/HLSL style storage classes (static, volatile, etc).
+
+SPIR-V has 2 interesting storage classes:
+ - Function
+ - Private
+A variable declared with the `Function` storage class must be declared in
+the first basic block of a function. It is normaly used to represent function
+local variables.
+
+A variable declared with the `Private` storage class is private to the current
+invocation/thread, but belongs to the global scope.
+This would be the equivalent of a static global variable in HLSL.
+
+Reconciliating the SPIR-V & HLSL side could be done in two ways:
+ - unify the storage classes in SPIR-V.
+ - separate the address spaces in HLSL.
+
+Implementing constant buffers & other resources is done by creating new
+address spaces, making explicit the constraints some allocations have.
+Thus, it seems separating the address spaces for globals & locals would
+allow us to stay consistent with the rest of the language.
+
+## HLSL patterns to look for
+
+This section will explain why some HLSL patterns are hard to lower to SPIR-V.
+
+Note: HLSL does not implement references yet, but we have to make sure our
+design would allow us to implement them. For this reason, we'll assume HLSL
+has references.
+
+### Example 1:
+
+```hlsl
+static int a = 0;
+
+void foo() {
+  int b = 0;
+}
+```
+
+`a` and `b` both share the same address space. But on the SPIR-V side, `a`
+must be a `Private` variable, while `b` must be a `Function` variable.
+This requires the lowering pass to know the context of a variable.
+
+### Example 2:
+
+```hlsl
+static int a = 0;
+
+void foo() {
+  int& ref = a;
+  int b = ref;
+}
+```
+
+`a` is still `Private`, `b` still `Function`. But `ref` points to `a`.
+In SPIR-V, a variable cannot store a pointer pointing to another storage class.
+This means `ref` cannot be stored in a variable in the `Function` class.
+If `a` is `Private`, `ref` could only be declared as `Private`.
+
+### Example 3:
+
+```hlsl
+static int global = 0;
+
+int& foo(int& input, int select) {
+  return select ? input : global;
+}
+
+void main(int select) {
+  int local;
+  int& res1 = foo(local, select);
+  int& res2 = foo(global, select);
+}
+```
+
+`global` is still `Private`.
+`local` is `Function`.
+In SPIR-V, function declarations contains the return and parameters types,
+including the storage classes.
+This means, depending on the call-site, and the value of `select`, the
+return value and parameter would required either the `Function` or the
+`Private` storage class. When this selection depends on a runtime condition,
+this cannot be lowered to SPIR-V as-is.
+
+## Proposed solution: using 2 HLSL address spaces
+
+Thread-local, global variables will be put in the `hlsl_private` address
+space. Thread-local function-local variables will be put in the `default`
+address space.
+
+## Implementing the solution
+
+A new address space will be added to Clang: `hlsl_private`.
+This address space will be mapped to `Spirv::Private` on the SPIR-V backend,
+`PRIVATE_ADDRESS` for AMDGPU, and the address space `0` in DXIL.
+
+Clang codegen will add the new address space annotations, separating
+the `private` from the default.
+
+For the time being, the `private` address space will be marked as a subset of
+the `default` address space, allowing overload resolution for class methods:
+- an object in the `private` address space will be allowed to use a method
+  declared with a `this` in the default address space.
+
+Clang will emit an `addrspacecast` we will have to handle, but that's a known
+issue in address-space overload resolution, and not new to this proposal.
+
+## Alternative design considered
+
+### Force optimizations, and force inlining
+
+One solution is to inline all function call, even those marked as
+noinline. Then replicate all instruction that use pointers so that the
+pointer operand has a single known address space. If those transformations
+were applied, we could avoid address-conflict mismatch for pointers, and all
+we'd have are direct load/stores to global/local variables.
+Functions returning incompatible references wouldn't exist, allowing us to
+generate valid SPIR-V.
+
+Note that those transformations can get arbitrarily complex. The number of
+copies is exponential in regards to the number of pointer operands.
+
+Additionally:
+- HLSL allows using `noinline`: we would have to ignore it.
+- HLSL allows exporting functions to compile to a library: if we need to
+  inline to generate functions, we cannot emit libraries exposing such
+  functions.
+- Runtime conditions causing address-space conflict would require code
+  duplication.
+- It makes reading the generated assembly harder.
+
+### Move all variables to the function scope
+
+HLSL static globals have a known initialization value at compile-time.
+Meaning we could move the global variables to the entrypoint first basic
+block, as local variables.
+If SPIR-V has no global variables, all pointers as `Function`.
+This would require passing references to other functions referencing those
+globals, or inline them, but it would be possible.
+
+But the blocker remains the same: building to a library function.
+If an exported function references a global variable, we cannot change
+the signature of the function.
+
+## Move all variables to the global scope
+
+By moving all local variables to the global scope, we now have a single
+storage class `Private`, and won't have conflict issues.
+This also allows us to compile non-optimized code, and to keep functions if
+required.
+
+HLSL & SPIR-V disallow static recursion. Meaning we know at compile-time
+that each function requires one instance of each local variable.
+This would also work with exported functions: static recursion is still not
+allowed, so cross compile-units recursion is not an issue.
+
+The main issue of this solution can have are:
+- drivers may have a harder time figuring out variable lifetimes.
+- SPIR-V has a hard 65536 global variable limit (vs 500k local variables).
+
+I believe those 2 are not hard blockers, but something we need to be aware of.
+
+## Selectively move variables to the global scope.
+
+If a variable is only loaded/stored from/to, and remains in the function
+scope, there should be no pointer incompatibility.
+This means we could potentially implement the solution 4, but only targeting
+variables for which addresses are moved across their function scope
+boundaries.
+
+This would require additional IR analysis, as we would need to determine
+which address is used in another scope to recreate a global variable.
+
+The motivation we could have for such solution are:
+- if drivers have a hard time optimizing the global variables.
+- if the global variable count limit becomes an issue.
+
+Implementing this solution is more complex, and could be more error prone,
+so until we have a real need, I would recommend against, and moving forward
+with solution 4. If the need comes, moving from solution 4 to solution 5
+would be possible, as it's just an optimization on top.
+