Eliminate empty struct on metal target (#6603)

* Eliminate empty struct on metal target

Close 6573.

We previously disabled the type legalization for ParameterBlock on
Metal, but Metal doesn't allow empty struct in the argument buffer
which is mapped from ParameterBlock, so we will need legalizeEmptyTypes
on Metal target.

* update test

* update function name

5 files changed, 59 insertions, 1 deletions

diff --git a/source/slang/slang-emit.cpp b/source/slang/slang-emit.cpp
index 841f44a80..8d7577b52 100644
--- a/source/slang/slang-emit.cpp
+++ b/source/slang/slang-emit.cpp
@@ -1278,6 +1278,15 @@ Result linkAndOptimizeIR(
         //
         legalizeResourceTypes(targetProgram, irModule, sink);
 
+        // We also need to legalize empty types for Metal targets.
+        switch (target)
+        {
+        case CodeGenTarget::Metal:
+        case CodeGenTarget::MetalLib:
+        case CodeGenTarget::MetalLibAssembly:
+            legalizeEmptyTypes(targetProgram, irModule, sink);
+            break;
+        }
         //  Debugging output of legalization
 #if 0
         dumpIRIfEnabled(codeGenContext, irModule, "LEGALIZED");
diff --git a/source/slang/slang-ir-legalize-types.cpp b/source/slang/slang-ir-legalize-types.cpp
index 197cd93c5..837193abc 100644
--- a/source/slang/slang-ir-legalize-types.cpp
+++ b/source/slang/slang-ir-legalize-types.cpp
@@ -4123,6 +4123,11 @@ struct IREmptyTypeLegalizationContext : IRTypeLegalizationContext
 
     bool isSimpleType(IRType* type) override
     {
+        if (isMetalTarget(targetProgram->getTargetReq()))
+        {
+            return false;
+        }
+
         // If type is used as public interface, then treat it as simple.
         for (auto decor : type->getDecorations())
         {
@@ -4145,6 +4150,11 @@ struct IREmptyTypeLegalizationContext : IRTypeLegalizationContext
     {
         return LegalType();
     }
+
+    virtual bool shouldLegalizeParameterBlockElementType() override
+    {
+        return isMetalTarget(targetProgram->getTargetReq());
+    }
 };
 
 // The main entry points that are used when transforming IR code
diff --git a/source/slang/slang-legalize-types.cpp b/source/slang/slang-legalize-types.cpp
index e26475522..cc2c12d42 100644
--- a/source/slang/slang-legalize-types.cpp
+++ b/source/slang/slang-legalize-types.cpp
@@ -1211,11 +1211,15 @@ LegalType legalizeTypeImpl(TypeLegalizationContext* context, IRType* type)
         LegalType legalElementType;
 
         if (isMetalTarget(context->targetProgram->getTargetReq()) &&
-            as<IRParameterBlockType>(uniformBufferType))
+            as<IRParameterBlockType>(uniformBufferType) &&
+            !context->shouldLegalizeParameterBlockElementType())
         {
             // On Metal, we do not need to legalize the element type of
             // a parameter block because we can translate it directly into
             // an argument buffer.
+            //
+            // But we do need empty type legalized for Metal, because Metal doesn't
+            // allow empty struct in argument buffer.
             legalElementType = LegalType::simple(originalElementType);
         }
         else
diff --git a/source/slang/slang-legalize-types.h b/source/slang/slang-legalize-types.h
index ae76cbd39..f1b15877d 100644
--- a/source/slang/slang-legalize-types.h
+++ b/source/slang/slang-legalize-types.h
@@ -659,6 +659,8 @@ struct IRTypeLegalizationContext
         IROp op,
         LegalType legalElementType,
         IRInst* layoutOperand) = 0;
+
+    virtual bool shouldLegalizeParameterBlockElementType() { return false; }
 };
 
 // This typedef exists to support pre-existing code from when
diff --git a/tests/metal/empty-struct-remove.slang b/tests/metal/empty-struct-remove.slang
new file mode 100644
index 000000000..2d9ee436b
--- /dev/null
+++ b/tests/metal/empty-struct-remove.slang
@@ -0,0 +1,33 @@
+
+//TEST:SIMPLE(filecheck=LIB):-target metallib -entry computeMain -stage compute -DMETAL
+//TEST:SIMPLE(filecheck=METAL):-target metal -entry computeMain -stage compute -DMETAL
+
+// METAL-NOT: struct emptyStruct
+struct emptyStruct
+{
+    void set(RWStructuredBuffer<int> buffer, int value) {buffer[0] = value;}
+}
+
+
+struct MyStruct
+{
+    RWStructuredBuffer<int> buffer;
+    int value;
+    void set()
+    {
+        e.set(buffer, value);
+    }
+    emptyStruct e;
+}
+
+ParameterBlock<MyStruct> param;
+
+// LIB: @computeMain
+[shader("compute")]
+[numthreads(1, 1, 1)]
+void computeMain(
+    uint tid: SV_DispatchThreadID,
+)
+{
+    param.set();
+}