[mono][wasm][aot] Prevent llvm from optimizing away stack stores of valuetypes containing object references when they are returned from a method as a scalar. (#85152)

Fixes #85012.

Author: vargaz

src/libraries/System.Collections.Immutable/tests/ImmutableListTestBase.cs

@@ -419,7 +419,6 @@ public void BinarySearch()
         }
 
         [Fact]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/85012", typeof(PlatformDetection), nameof(PlatformDetection.IsBrowser), nameof(PlatformDetection.IsMonoAOT))]
         public void BinarySearchPartialSortedList()
         {
             ImmutableArray<int> reverseSorted = ImmutableArray.CreateRange(Enumerable.Range(1, 150).Select(n => n * 2).Reverse());

src/mono/mono/mini/mini-llvm.c

@@ -4756,7 +4756,8 @@ process_call (EmitContext *ctx, MonoBasicBlock *bb, LLVMBuilderRef *builder_ref,
 			mono_llvm_add_instr_byval_attr (lcall, 1 + ainfo->pindex, LLVMGetElementType (param_types [ainfo->pindex]));
 	}
 
-	gboolean is_simd = MONO_CLASS_IS_SIMD (ctx->cfg, mono_class_from_mono_type_internal (sig->ret));
+	MonoClass *retclass = mono_class_from_mono_type_internal (sig->ret);
+	gboolean is_simd = MONO_CLASS_IS_SIMD (ctx->cfg, retclass);
 	gboolean should_promote_to_value = FALSE;
 	const char *load_name = NULL;
 	/*
@@ -4821,11 +4822,17 @@ process_call (EmitContext *ctx, MonoBasicBlock *bb, LLVMBuilderRef *builder_ref,
 	case LLVMArgGsharedvtFixedVtype:
 		values [ins->dreg] = LLVMBuildLoad2 (builder, rtype, convert_full (ctx, addresses [call->inst.dreg]->value, pointer_type (rtype), FALSE), "");
 		break;
-	case LLVMArgWasmVtypeAsScalar:
+	case LLVMArgWasmVtypeAsScalar: {
+		/*
+		 * If the vtype contains references, making the store volatile ensures there is a reference
+		 * on the stack.
+		 */
+		gboolean is_volatile = m_class_has_references (retclass);
 		if (!addresses [call->inst.dreg])
 			addresses [call->inst.dreg] = build_alloca_address (ctx, sig->ret);
-		LLVMBuildStore (builder, lcall, convert_full (ctx, addresses [call->inst.dreg]->value, pointer_type (LLVMTypeOf (lcall)), FALSE));
+		emit_store (builder, lcall, convert_full (ctx, addresses [call->inst.dreg]->value, pointer_type (LLVMTypeOf (lcall)), FALSE), is_volatile);
 		break;
+	}
 	default:
 		if (sig->ret->type != MONO_TYPE_VOID)
 			/* If the method returns an unsigned value, need to zext it */