Specialize interfaces in DebugFunction (#8617)

E.g. in
[generic-extension-2.slang](https://github.com/shader-slang/slang/blob/master/tests/language-feature/extensions/generic-extension-2.slang),
incorrect DebugFunctions are generated for `getFirstOuter`:

```
let  %33        : Void  = DebugFunction("getFirstOuter", 18 : UInt, 3 : UInt, %26, Func(Int, 0 : Int))
```

This happens because specialization passes are leaving a `%IFoo` in the
function type, instead of replacing with a concrete type:
```
let  %34        : Void  = DebugFunction("getFirstOuter", 18 : UInt, 3 : UInt, %26, Func(Int, %IFoo))
```
and later, `cleanUpInterfaceTypes()` just replaces all interfaces with
the literal zero. So now we have a parameter type which isn't actually a
type at all, but an IntLit instead.

I'm not sure if the approach I picked is good, though. Some other
options that crossed my mind were:
* Make `fixUpFuncType` also update related DebugFunctions
- But is there a reason why DebugFunctions separately carry a function
type in the first place?
* Make `cleanUpInterfaceTypes` less aggressive or at least replace types
with a type instead of a value
    - But this will still make the debug info incorrect :(

4 files changed, 47 insertions, 1 deletions

diff --git a/source/slang/slang-ir-lower-generic-type.cpp b/source/slang/slang-ir-lower-generic-type.cpp
index 9579b0a2b..c4dcd92a9 100644
--- a/source/slang/slang-ir-lower-generic-type.cpp
+++ b/source/slang/slang-ir-lower-generic-type.cpp
@@ -50,6 +50,14 @@ struct GenericTypeLoweringContext
                 structField->setOperand(1, loweredFieldType);
             }
             break;
+        case kIROp_DebugFunction:
+            {
+                auto oldFuncType = as<IRDebugFunction>(inst)->getDebugType();
+                auto newFuncType = sharedContext->lowerType(builder, oldFuncType);
+                if (newFuncType != oldFuncType)
+                    inst = builder->replaceOperand(inst->getOperandUse(4), newFuncType);
+            }
+            break;
         }
         return inst;
     }
diff --git a/source/slang/slang-ir-specialize.cpp b/source/slang/slang-ir-specialize.cpp
index bc7bcab12..0f8672531 100644
--- a/source/slang/slang-ir-specialize.cpp
+++ b/source/slang/slang-ir-specialize.cpp
@@ -1163,7 +1163,8 @@ struct SpecializationContext
                     // top-down through the program, so that we want to process
                     // the children of an instruction in their original order.
                     //
-                    for (auto child = inst->getLastChild(); child; child = child->getPrevInst())
+                    for (auto child = inst->getLastDecorationOrChild(); child;
+                         child = child->getPrevInst())
                     {
                         // Also note that `addToWorkList` has been written
                         // to avoid adding any instruction that is a descendent
@@ -2055,6 +2056,7 @@ struct SpecializationContext
             }
         }
 
+        fixUpDebugFuncType(newFunc);
         return newFunc;
     }
 
diff --git a/source/slang/slang-ir.cpp b/source/slang/slang-ir.cpp
index ba8864684..7b7d5ec17 100644
--- a/source/slang/slang-ir.cpp
+++ b/source/slang/slang-ir.cpp
@@ -815,6 +815,37 @@ IRType* IRFunc::getParamType(UInt index)
     return getDataType()->getParamType(index);
 }
 
+void fixUpDebugFuncType(IRFunc* func)
+{
+    SLANG_ASSERT(func);
+
+    if (auto debugFuncDecor = func->findDecoration<IRDebugFuncDecoration>())
+    {
+        auto funcType = func->getDataType();
+        auto oldDebugFunc = cast<IRDebugFunction>(debugFuncDecor->getDebugFunc());
+
+        // If the existing debug func type is already the same, there's no need
+        // to do anything.
+        if (isTypeEqual(funcType, as<IRType>(oldDebugFunc->getDebugType())))
+            return;
+
+        auto irModule = func->getModule();
+        SLANG_ASSERT(irModule);
+
+        IRBuilder builder(irModule);
+        builder.setInsertInto(irModule->getModuleInst());
+
+        auto newDebugFunc = builder.emitDebugFunction(
+            oldDebugFunc->getName(),
+            oldDebugFunc->getLine(),
+            oldDebugFunc->getCol(),
+            oldDebugFunc->getFile(),
+            funcType);
+        debugFuncDecor->removeAndDeallocate();
+        builder.addDecoration(funcType, kIROp_DebugFuncDecoration, newDebugFunc);
+    }
+}
+
 void fixUpFuncType(IRFunc* func, IRType* resultType)
 {
     SLANG_ASSERT(func);
diff --git a/source/slang/slang-ir.h b/source/slang/slang-ir.h
index aef3d6aeb..54bf23754 100644
--- a/source/slang/slang-ir.h
+++ b/source/slang/slang-ir.h
@@ -2148,6 +2148,11 @@ void fixUpFuncType(IRFunc* func, IRType* resultType);
 ///
 void fixUpFuncType(IRFunc* func);
 
+/// If the function has a DebugFuncDecoration, replaces the function type in
+/// that decoration to match the current type of the function.
+///
+void fixUpDebugFuncType(IRFunc* func);
+
 // A generic is akin to a function, but is conceptually executed
 // before runtime, to specialize the code nested within.
 //