[ir] Represent fields more direclty

Previously, a `StructType` was an ordinary instruction that took a variable number of types are operands, representing the types of fields.
This ends up being inconvenient for a few reasons:

- To add decorations to the fields, you'd end up having to decorate the struct type instead (SPIR-V has this problem)

- You need to compute field indices during lowering, when you might prefer to defer that until later

- The get/set field operations now need an index, which needs to be an explicit operand, which means a magic numeric literal floating around to represent the index

The new approach fixes for the first two of these, and at least makes the last one a bit nicer.
A `StructDecl` is now a parent instruction, and its sub-instructions represent the fields of the type - each field is an explicit instruction of type `StructField`.
The operation to extract a field takes a direct reference the struct field, so everything is quite explicit.

1 files changed, 17 insertions, 15 deletions

diff --git a/source/slang/ir.cpp b/source/slang/ir.cpp
index d2c3f5ba5..b6af6aabb 100644
--- a/source/slang/ir.cpp
+++ b/source/slang/ir.cpp
@@ -622,20 +622,23 @@ namespace Slang
             getTypeType());
     }
 
-    IRType* IRBuilder::getStructType(
-        UInt            fieldCount,
-        IRType* const*  fieldTypes)
+    IRStructDecl* IRBuilder::createStructType()
     {
-        auto inst = createInstWithTrailingArgs<IRStructType>(
+        return createInst<IRStructDecl>(
             this,
             kIROp_StructType,
-            getTypeType(),
-            fieldCount,
-            (IRValue* const*)fieldTypes);
-        addInst(inst);
-        return inst;
+            getTypeType());
     }
 
+    IRStructField* IRBuilder::createStructField(IRType* fieldType)
+    {
+        return createInst<IRStructField>(
+            this,
+            kIROp_StructField,
+            fieldType);
+    }
+
+
     IRType* IRBuilder::getFuncType(
         UInt            paramCount,
         IRType* const*  paramTypes,
@@ -753,17 +756,16 @@ namespace Slang
     }
 
     IRInst* IRBuilder::emitFieldExtract(
-        IRType*     type,
-        IRValue*    base,
-        UInt        fieldIndex)
+        IRType*         type,
+        IRValue*        base,
+        IRStructField*  field)
     {
         auto inst = createInst<IRFieldExtract>(
             this,
             kIROp_FieldExtract,
             type,
-            base);
-
-        inst->fieldIndex = fieldIndex;
+            base,
+            field);
 
         addInst(inst);
         return inst;