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, 13 insertions, 6 deletions

diff --git a/source/slang/slang-emit-spirv.cpp b/source/slang/slang-emit-spirv.cpp
index d5697117a..1b59d3070 100644
--- a/source/slang/slang-emit-spirv.cpp
+++ b/source/slang/slang-emit-spirv.cpp
@@ -6481,12 +6481,19 @@ struct SPIRVEmitContext : public SourceEmitterBase, public SPIRVEmitSharedContex
             SpvStorageClassPhysicalStorageBuffer)
         {
             // If inst has a pointer type with PhysicalStorageBuffer address space,
-            // emit AliasedPointer decoration.
-            emitOpDecorate(
-                getSection(SpvLogicalSectionID::Annotations),
-                nullptr,
-                varInst,
-                (isVar ? SpvDecorationAliasedPointer : SpvDecorationAliased));
+            // emit AliasedPointer or RestrictPointer decoration.
+            SpvDecoration decor;
+            if (ptrType->getAccessQualifier() == AccessQualifier::Immutable)
+            {
+                // We can always safely use RestrictPointer for immutable pointers.
+                // This will allow better optimization.
+                decor = isVar ? SpvDecorationRestrictPointer : SpvDecorationRestrict;
+            }
+            else
+            {
+                decor = isVar ? SpvDecorationAliasedPointer : SpvDecorationAliased;
+            }
+            emitOpDecorate(getSection(SpvLogicalSectionID::Annotations), nullptr, varInst, decor);
         }
         else
         {