[mono][interp] Small improvements to get span item (#77331)

* [mono][interp] Improve GETITEM_SPAN intrinsic

Field offsets are known at compile time, they don't need to be included in the code stream.

* [mono][interp] Add version of getitemspan that doesn't use indirection

* [mono][interp] Add general MonoSpanOfVoid struct and use it in interp opcodes

src/mono/mono/metadata/object-internals.h

@@ -176,6 +176,7 @@ struct _MonoArray {
 	} name;
 
 MONO_DEFINE_SPAN_OF_T (MonoSpanOfObjects, MonoObject*)
+MONO_DEFINE_SPAN_OF_T (MonoSpanOfVoid, void)
 
 #define MONO_SIZEOF_MONO_ARRAY (MONO_STRUCT_OFFSET_CONSTANT (MonoArray, vector))
 

src/mono/mono/mini/interp/interp.c

@@ -6061,23 +6061,33 @@ MINT_IN_CASE(MINT_BRTRUE_I8_SP) ZEROP_SP(gint64, !=); MINT_IN_BREAK;
 			MINT_IN_BREAK;
 		}
 		MINT_IN_CASE(MINT_GETITEM_SPAN) {
-			guint8 *span = LOCAL_VAR (ip [2], guint8*);
+			MonoSpanOfVoid *span = LOCAL_VAR (ip [2], MonoSpanOfVoid*);
 			int index = LOCAL_VAR (ip [3], int);
 			NULL_CHECK (span);
 
-			gsize offset_length = (gsize)(gint16)ip [5];
-
-			const gint32 length = *(gint32 *) (span + offset_length);
+			gint32 length = span->_length;
 			if (index < 0 || index >= length)
 				THROW_EX (interp_get_exception_index_out_of_range (frame, ip), ip);
 
 			gsize element_size = (gsize)(gint16)ip [4];
-			gsize offset_pointer = (gsize)(gint16)ip [6];
+			LOCAL_VAR (ip [1], gpointer) = (guint8*)span->_reference + index * element_size;
 
-			const gpointer pointer = *(gpointer *)(span + offset_pointer);
-			LOCAL_VAR (ip [1], gpointer) = (guint8 *) pointer + index * element_size;
+			ip += 5;
+			MINT_IN_BREAK;
+		}
+		MINT_IN_CASE(MINT_GETITEM_LOCALSPAN) {
+			// Same as getitem span but we know the offset of the span structure on the stack
+			MonoSpanOfVoid *span = (MonoSpanOfVoid*)(locals + ip [2]);
+			int index = LOCAL_VAR (ip [3], int);
 
-			ip += 7;
+			gint32 length = span->_length;
+			if (index < 0 || index >= length)
+				THROW_EX (interp_get_exception_index_out_of_range (frame, ip), ip);
+
+			gsize element_size = (gsize)(gint16)ip [4];
+			LOCAL_VAR (ip [1], gpointer) = (guint8*)span->_reference + index * element_size;
+
+			ip += 5;
 			MINT_IN_BREAK;
 		}
 		MINT_IN_CASE(MINT_STRLEN) {

src/mono/mono/mini/interp/mintops.def

@@ -414,7 +414,8 @@ OPDEF(MINT_STELEM_VT, "stelem.vt", 6, 0, 3, MintOpTwoShorts)
 
 OPDEF(MINT_LDLEN, "ldlen", 3, 1, 1, MintOpNoArgs)
 
-OPDEF(MINT_GETITEM_SPAN, "getitem.span", 7, 1, 2, MintOpTwoShorts)
+OPDEF(MINT_GETITEM_SPAN, "getitem.span", 5, 1, 2, MintOpTwoShorts)
+OPDEF(MINT_GETITEM_LOCALSPAN, "getitem.localspan", 5, 1, 2, MintOpTwoShorts)
 
 /* binops */
 OPDEF(MINT_ADD_I4, "add.i4", 4, 1, 2, MintOpNoArgs)

src/mono/mono/mini/interp/transform.c

@@ -2134,16 +2134,16 @@ interp_handle_intrinsics (TransformData *td, MonoMethod *target_method, MonoClas
 				MonoClassField *length_field = mono_class_get_field_from_name_full (target_method->klass, "_length", NULL);
 				g_assert (length_field);
 				int offset_length = m_field_get_offset (length_field) - sizeof (MonoObject);
+				g_assert (offset_length == TARGET_SIZEOF_VOID_P);
 
 				MonoClassField *ptr_field = mono_class_get_field_from_name_full (target_method->klass, "_reference", NULL);
 				g_assert (ptr_field);
 				int offset_pointer = m_field_get_offset (ptr_field) - sizeof (MonoObject);
+				g_assert (offset_pointer == 0);
 
 				int size = mono_class_array_element_size (param_class);
 				interp_add_ins (td, MINT_GETITEM_SPAN);
 				td->last_ins->data [0] = GINT_TO_UINT16 (size);
-				td->last_ins->data [1] = GINT_TO_UINT16 (offset_length);
-				td->last_ins->data [2] = GINT_TO_UINT16 (offset_pointer);
 
 				td->sp -= 2;
 				interp_ins_set_sregs2 (td->last_ins, td->sp [0].local, td->sp [1].local);
@@ -8996,6 +8996,17 @@ interp_cprop (TransformData *td)
 					}
 					needs_retry = TRUE;
 				}
+			} else if (opcode == MINT_GETITEM_SPAN) {
+				InterpInst *ldloca = local_defs [sregs [0]].ins;
+				if (ldloca != NULL && ldloca->opcode == MINT_LDLOCA_S) {
+					int local = ldloca->sregs [0];
+					// Allow ldloca instruction to be killed
+					local_ref_count [sregs [0]]--;
+					// Instead of loading from the indirect pointer pass directly the vt var
+					ins->opcode = MINT_GETITEM_LOCALSPAN;
+					sregs [0] = local;
+					needs_retry = TRUE;
+				}
 			}
 			ins_index++;
 		}