Initial copy elision pass (#8042)

Fixes #7574

Changes:
* Add an initial (fairly simple) optimization pass which is able to
eliminate redundant copies.
* Our current existing optimizer passes remove redundant load/store very
robustly, this pass will focus on other cases of copy elimination
* Primary approach is to make all functions which are `in T` and `T` is
trivial to copy into a `__constref T`. We then (depending on scenario)
manually insert a variable+load if a pass-by-reference is not possible;
otherwise we pass by `constref`.
* Added optimizations to eliminate redundant code which causes
`constref` to fail to compile

---------

Co-authored-by: Harsh Aggarwal <haaggarwal@nvidia.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: slangbot <ellieh+slangbot@nvidia.com>
Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/source/slang/slang-emit-cpp.cpp b/source/slang/slang-emit-cpp.cpp
index ffc4b97ef..66829308d 100644
--- a/source/slang/slang-emit-cpp.cpp
+++ b/source/slang/slang-emit-cpp.cpp
@@ -307,6 +307,25 @@ SlangResult CPPSourceEmitter::calcTypeName(IRType* type, CodeGenTarget target, S
     case kIROp_PtrType:
     case kIROp_ConstRefType:
         {
+            // Special note on `constref` types and why they are not emitted
+            // as a `const` pointer:
+            //
+            // We currently do not propegate/manage "constness" for locals.
+            // This is important since it means that we rely on opimization
+            // passes to remove all temporary pointer-variables created from
+            // our constref, otherwise we will generate invalid code like
+            // `T* var = const_ptr` or `T* var = &const_ptr->member`.
+            //
+            // If emitting `constref` fails due to this error, it is likely
+            // a missing compiler-optimization.
+            //
+            // Additionally, for C++/CUDA, downstream methods are required
+            // to be `const` if we want to use const pointers. This is currently
+            // not handled robustly.
+            //
+            // Due to these cascading issues, we do not emit const and instead
+            // emit as a regular pointer for the time being.
+
             auto elementType = (IRType*)type->getOperand(0);
             SLANG_RETURN_ON_FAIL(calcTypeName(elementType, target, out));
             out << "*";
@@ -599,6 +618,7 @@ CPPSourceEmitter::CPPSourceEmitter(const Desc& desc)
 
 void CPPSourceEmitter::emitParamTypeImpl(IRType* type, String const& name)
 {
+    // For use the CPP-specific emitType implementation
     emitType(type, name);
 }