Immutable access qualifier for pointers and use `__ldg` on cuda. (#8710)

This PR implements `Access.Immutable` to allow pointers to immutable
data.

The new type `ImmutablePtr<T>` is defined as an alias of `Ptr<T,
Address.Immutable>`.
By forming a immutable pointer, the programmer is conveying to the
compiler that the data at the pointer address will never change during
the execution of the current program. Therefore loads from immutable
pointers can be deduplicated by the compiler, and will translate to
`__ldg` when generating code for CUDA.

The SPIRV backend is not changed in this PR, since the current SPIRV
spec makes it very difficult to specify loads from immutable address
without generating tons of wrappers and boilerplate type declarations.
We would like to see the spec evolved a bit to around its support of
`NonWritable` physical storage pointers or immutable loads before we
attempt to express such immutability in SPIRV. For now we simply emit
ordinary pointers and loads when generating spirv.

---------

Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/source/slang/slang-type-system-shared.h b/source/slang/slang-type-system-shared.h
index aa9b07d89..b68e44b32 100644
--- a/source/slang/slang-type-system-shared.h
+++ b/source/slang/slang-type-system-shared.h
@@ -129,10 +129,26 @@ enum class MemoryScope : int32_t
     ShaderCall = 6,
 };
 
+// Represents the access qualifier of a pointer type.
 enum class AccessQualifier : uint64_t
 {
     ReadWrite = 0,
+
+    // The data being pointed to by a pointer can only be read through the pointer.
+    // This is to be distinguished from `Immutable`, which means the data being pointed to
+    // won't be changed by any means. In contrast, data pointed to by a `Read` pointer
+    // may still be changed through another pointer that is not read-only.
+    // This means that a pointer with `Read` access is meaningful only to the front-end
+    // type system, and is not expected to provide any optimization opportunities to
+    // the back-end.
     Read = 1,
+
+    // The data being pointed to by a pointer is known to be immutable and won't
+    // be changed by any means during the execution of the program. It is UB if
+    // the data is changed during the program execution. This is a stronger
+    // qualifier than `Read`, and may allow the backend to perform more aggresive
+    // optimizations.
+    Immutable = 2,
 };
 
 } // namespace Slang