librustc: Don't use an alloca per return if the function doesn't have…

… nested returns.

src/librustc/middle/trans/base.rs

@@ -1214,19 +1214,117 @@ pub fn arrayalloca(cx: &Block, ty: Type, v: ValueRef) -> ValueRef {
 pub fn make_return_slot_pointer(fcx: &FunctionContext, output_type: ty::t) -> ValueRef {
     let lloutputtype = type_of::type_of(fcx.ccx, output_type);
 
-    // Let's create the stack slot
-    let slot = AllocaFcx(fcx, lloutputtype.ptr_to(), "llretslotptr");
+    // We create an alloca to hold a pointer of type `output_type`
+    // which will hold the pointer to the right alloca which has the
+    // final ret value
+    if fcx.needs_ret_allocas {
+        // Let's create the stack slot
+        let slot = AllocaFcx(fcx, lloutputtype.ptr_to(), "llretslotptr");
 
-    // and if we're using an out pointer, then store that in our newly made slot
-    if type_of::return_uses_outptr(fcx.ccx, output_type) {
-        let outptr = get_param(fcx.llfn, 0);
+        // and if we're using an out pointer, then store that in our newly made slot
+        if type_of::return_uses_outptr(fcx.ccx, output_type) {
+            let outptr = get_param(fcx.llfn, 0);
 
-        let b = fcx.ccx.builder();
-        b.position_before(fcx.alloca_insert_pt.get().unwrap());
-        b.store(outptr, slot);
+            let b = fcx.ccx.builder();
+            b.position_before(fcx.alloca_insert_pt.get().unwrap());
+            b.store(outptr, slot);
+        }
+
+        slot
+
+    // But if there are no nested returns, we skip the indirection and have a single
+    // retslot
+    } else {
+        if type_of::return_uses_outptr(fcx.ccx, output_type) {
+            get_param(fcx.llfn, 0)
+        } else {
+            AllocaFcx(fcx, lloutputtype, "sret_slot")
+        }
     }
+}
 
-    slot
+struct CheckForNestedReturnsVisitor {
+    found: bool
+}
+
+impl Visitor<bool> for CheckForNestedReturnsVisitor {
+    fn visit_expr(&mut self, e: &ast::Expr, in_return: bool) {
+        match e.node {
+            ast::ExprRet(..) if in_return => {
+                self.found = true;
+                return;
+            }
+            ast::ExprRet(..) => visit::walk_expr(self, e, true),
+            _ => visit::walk_expr(self, e, in_return)
+        }
+    }
+}
+
+fn has_nested_returns(tcx: &ty::ctxt, id: ast::NodeId) -> bool {
+    match tcx.map.find(id) {
+        Some(ast_map::NodeItem(i)) => {
+            match i.node {
+                ast::ItemFn(_, _, _, _, blk) => {
+                    let mut explicit = CheckForNestedReturnsVisitor { found: false };
+                    let mut implicit = CheckForNestedReturnsVisitor { found: false };
+                    visit::walk_item(&mut explicit, &*i, false);
+                    visit::walk_expr_opt(&mut implicit, blk.expr, true);
+                    explicit.found || implicit.found
+                }
+                _ => tcx.sess.bug("unexpected item variant in has_nested_returns")
+            }
+        }
+        Some(ast_map::NodeTraitMethod(trait_method)) => {
+            match *trait_method {
+                ast::Provided(m) => {
+                    match m.node {
+                        ast::MethDecl(_, _, _, _, _, _, blk, _) => {
+                            let mut explicit = CheckForNestedReturnsVisitor { found: false };
+                            let mut implicit = CheckForNestedReturnsVisitor { found: false };
+                            visit::walk_method_helper(&mut explicit, &*m, false);
+                            visit::walk_expr_opt(&mut implicit, blk.expr, true);
+                            explicit.found || implicit.found
+                        }
+                        ast::MethMac(_) => tcx.sess.bug("unexpanded macro")
+                    }
+                }
+                ast::Required(_) => tcx.sess.bug("unexpected variant: required trait method in \
+                                                  has_nested_returns")
+            }
+        }
+        Some(ast_map::NodeMethod(m)) => {
+            match m.node {
+                ast::MethDecl(_, _, _, _, _, _, blk, _) => {
+                    let mut explicit = CheckForNestedReturnsVisitor { found: false };
+                    let mut implicit = CheckForNestedReturnsVisitor { found: false };
+                    visit::walk_method_helper(&mut explicit, &*m, false);
+                    visit::walk_expr_opt(&mut implicit, blk.expr, true);
+                    explicit.found || implicit.found
+                }
+                ast::MethMac(_) => tcx.sess.bug("unexpanded macro")
+            }
+        }
+        Some(ast_map::NodeExpr(e)) => {
+            match e.node {
+                ast::ExprFnBlock(_, blk) | ast::ExprProc(_, blk) | ast::ExprUnboxedFn(_, blk) => {
+                    let mut explicit = CheckForNestedReturnsVisitor { found: false };
+                    let mut implicit = CheckForNestedReturnsVisitor { found: false };
+                    visit::walk_expr(&mut explicit, &*e, false);
+                    visit::walk_expr_opt(&mut implicit, blk.expr, true);
+                    explicit.found || implicit.found
+                }
+                _ => tcx.sess.bug("unexpected expr variant in has_nested_returns")
+            }
+        }
+
+        Some(ast_map::NodeVariant(..)) | Some(ast_map::NodeStructCtor(..)) => false,
+
+        // glue, shims, etc
+        None if id == ast::DUMMY_NODE_ID => false,
+
+        _ => tcx.sess.bug(format!("unexpected variant in has_nested_returns: {}",
+                                  tcx.map.path_to_string(id)).as_slice())
+    }
 }
 
 // NB: must keep 4 fns in sync:
@@ -1261,13 +1359,15 @@ pub fn new_fn_ctxt<'a>(ccx: &'a CrateContext,
     let substd_output_type = output_type.substp(ccx.tcx(), param_substs);
     let uses_outptr = type_of::return_uses_outptr(ccx, substd_output_type);
     let debug_context = debuginfo::create_function_debug_context(ccx, id, param_substs, llfndecl);
+    let nested_returns = has_nested_returns(ccx.tcx(), id);
 
     let mut fcx = FunctionContext {
           llfn: llfndecl,
           llenv: None,
           llretslotptr: Cell::new(None),
           alloca_insert_pt: Cell::new(None),
           llreturn: Cell::new(None),
+          needs_ret_allocas: nested_returns,
           personality: Cell::new(None),
           caller_expects_out_pointer: uses_outptr,
           llargs: RefCell::new(NodeMap::new()),
@@ -1540,11 +1640,16 @@ pub fn finish_fn<'a>(fcx: &'a FunctionContext<'a>,
 
 // Builds the return block for a function.
 pub fn build_return_block(fcx: &FunctionContext, ret_cx: &Block, retty: ty::t) {
-    if fcx.llretslotptr.get().is_none() {
+    if fcx.llretslotptr.get().is_none() ||
+       (!fcx.needs_ret_allocas && fcx.caller_expects_out_pointer) {
         return RetVoid(ret_cx);
     }
 
-    let retslot = Load(ret_cx, fcx.llretslotptr.get().unwrap());
+    let retslot = if fcx.needs_ret_allocas {
+        Load(ret_cx, fcx.llretslotptr.get().unwrap())
+    } else {
+        fcx.llretslotptr.get().unwrap()
+    };
     let retptr = Value(retslot);
     match retptr.get_dominating_store(ret_cx) {
         // If there's only a single store to the ret slot, we can directly return
@@ -1678,7 +1783,7 @@ pub fn trans_closure(ccx: &CrateContext,
     debuginfo::start_emitting_source_locations(&fcx);
 
     let dest = match fcx.llretslotptr.get() {
-        Some(_) => expr::SaveIn(alloca(bcx, type_of::type_of(bcx.ccx(), block_ty), "iret_slot")),
+        Some(_) => expr::SaveIn(fcx.get_ret_slot(bcx, block_ty, "iret_slot")),
         None => {
             assert!(type_is_zero_size(bcx.ccx(), block_ty));
             expr::Ignore
@@ -1692,7 +1797,7 @@ pub fn trans_closure(ccx: &CrateContext,
     bcx = controlflow::trans_block(bcx, body, dest);
 
     match dest {
-        expr::SaveIn(slot) => {
+        expr::SaveIn(slot) if fcx.needs_ret_allocas => {
             Store(bcx, slot, fcx.llretslotptr.get().unwrap());
         }
         _ => {}
@@ -1862,12 +1967,14 @@ fn trans_enum_variant_or_tuple_like_struct(ccx: &CrateContext,
                           param_substs, None, &arena, TranslateItems);
     let bcx = init_function(&fcx, false, result_ty);
 
+    assert!(!fcx.needs_ret_allocas);
+
     let arg_tys = ty::ty_fn_args(ctor_ty);
 
     let arg_datums = create_datums_for_fn_args(&fcx, arg_tys.as_slice());
 
     if !type_is_zero_size(fcx.ccx, result_ty) {
-        let dest = alloca(bcx, type_of::type_of(bcx.ccx(), result_ty), "eret_slot");
+        let dest = fcx.get_ret_slot(bcx, result_ty, "eret_slot");
         let repr = adt::represent_type(ccx, result_ty);
         for (i, arg_datum) in arg_datums.move_iter().enumerate() {
             let lldestptr = adt::trans_field_ptr(bcx,
@@ -1878,7 +1985,6 @@ fn trans_enum_variant_or_tuple_like_struct(ccx: &CrateContext,
             arg_datum.store_to(bcx, lldestptr);
         }
         adt::trans_set_discr(bcx, &*repr, dest, disr);
-        Store(bcx, dest, fcx.llretslotptr.get().unwrap());
     }
 
     finish_fn(&fcx, bcx, result_ty);

src/librustc/middle/trans/callee.rs

@@ -334,7 +334,7 @@ pub fn trans_unboxing_shim(bcx: &Block,
     let return_type = ty::ty_fn_ret(boxed_function_type);
     let fcx = new_fn_ctxt(ccx,
                           llfn,
-                          -1,
+                          ast::DUMMY_NODE_ID,
                           false,
                           return_type,
                           &empty_param_substs,
@@ -389,8 +389,9 @@ pub fn trans_unboxing_shim(bcx: &Block,
     for i in range(1, arg_types.len()) {
         llshimmedargs.push(get_param(fcx.llfn, fcx.arg_pos(i) as u32));
     }
+    assert!(!fcx.needs_ret_allocas);
     let dest = match fcx.llretslotptr.get() {
-        Some(_) => Some(expr::SaveIn(alloca(bcx, type_of::type_of(ccx, return_type), "ret_slot"))),
+        Some(_) => Some(expr::SaveIn(fcx.get_ret_slot(bcx, return_type, "ret_slot"))),
         None => None
     };
     bcx = trans_call_inner(bcx,
@@ -404,12 +405,6 @@ pub fn trans_unboxing_shim(bcx: &Block,
                            },
                            ArgVals(llshimmedargs.as_slice()),
                            dest).bcx;
-    match dest {
-        Some(expr::SaveIn(slot)) => {
-            Store(bcx, slot, fcx.llretslotptr.get().unwrap());
-        }
-        _ => {}
-    }
 
     bcx = fcx.pop_and_trans_custom_cleanup_scope(bcx, arg_scope);
     finish_fn(&fcx, bcx, return_type);

src/librustc/middle/trans/closure.rs

@@ -574,7 +574,7 @@ pub fn get_wrapper_for_bare_fn(ccx: &CrateContext,
 
     let arena = TypedArena::new();
     let empty_param_substs = param_substs::empty();
-    let fcx = new_fn_ctxt(ccx, llfn, -1, true, f.sig.output,
+    let fcx = new_fn_ctxt(ccx, llfn, ast::DUMMY_NODE_ID, true, f.sig.output,
                           &empty_param_substs, None, &arena, TranslateItems);
     let bcx = init_function(&fcx, true, f.sig.output);
 
@@ -583,8 +583,9 @@ pub fn get_wrapper_for_bare_fn(ccx: &CrateContext,
                                             .as_slice());
     let mut llargs = Vec::new();
     match fcx.llretslotptr.get() {
-        Some(llretslotptr) => {
-            llargs.push(Load(bcx, llretslotptr));
+        Some(llretptr) => {
+            assert!(!fcx.needs_ret_allocas);
+            llargs.push(llretptr);
         }
         None => {}
     }

src/librustc/middle/trans/common.rs

@@ -21,11 +21,13 @@ use middle::def;
 use middle::lang_items::LangItem;
 use middle::subst;
 use middle::subst::Subst;
+use middle::trans::base;
 use middle::trans::build;
 use middle::trans::cleanup;
 use middle::trans::datum;
 use middle::trans::debuginfo;
 use middle::trans::type_::Type;
+use middle::trans::type_of;
 use middle::ty;
 use middle::typeck;
 use util::ppaux::Repr;
@@ -254,6 +256,11 @@ pub struct FunctionContext<'a> {
     pub alloca_insert_pt: Cell<Option<ValueRef>>,
     pub llreturn: Cell<Option<BasicBlockRef>>,
 
+    // If the function has any nested return's, including something like:
+    // fn foo() -> Option<Foo> { Some(Foo { x: return None }) }, then
+    // we use a separate alloca for each return
+    pub needs_ret_allocas: bool,
+
     // The a value alloca'd for calls to upcalls.rust_personality. Used when
     // outputting the resume instruction.
     pub personality: Cell<Option<ValueRef>>,
@@ -345,6 +352,14 @@ impl<'a> FunctionContext<'a> {
         self.llreturn.get().unwrap()
     }
 
+    pub fn get_ret_slot(&self, bcx: &Block, ty: ty::t, name: &str) -> ValueRef {
+        if self.needs_ret_allocas {
+            base::alloca_no_lifetime(bcx, type_of::type_of(bcx.ccx(), ty), name)
+        } else {
+            self.llretslotptr.get().unwrap()
+        }
+    }
+
     pub fn new_block(&'a self,
                      is_lpad: bool,
                      name: &str,

src/librustc/middle/trans/controlflow.rs

@@ -26,7 +26,6 @@ use middle::trans::debuginfo;
 use middle::trans::expr;
 use middle::trans::meth;
 use middle::trans::type_::Type;
-use middle::trans::type_of;
 use middle::ty;
 use middle::typeck::MethodCall;
 use util::ppaux::Repr;
@@ -466,15 +465,15 @@ pub fn trans_ret<'a>(bcx: &'a Block<'a>,
     let dest = match (fcx.llretslotptr.get(), e) {
         (Some(_), Some(e)) => {
             let ret_ty = expr_ty(bcx, &*e);
-            expr::SaveIn(alloca(bcx, type_of::type_of(bcx.ccx(), ret_ty), "ret_slot"))
+            expr::SaveIn(fcx.get_ret_slot(bcx, ret_ty, "ret_slot"))
         }
         _ => expr::Ignore,
     };
     match e {
         Some(x) => {
             bcx = expr::trans_into(bcx, &*x, dest);
             match dest {
-                expr::SaveIn(slot) => {
+                expr::SaveIn(slot) if fcx.needs_ret_allocas => {
                     Store(bcx, slot, fcx.llretslotptr.get().unwrap());
                 }
                 _ => {}

src/librustc/middle/trans/glue.rs

@@ -467,7 +467,7 @@ fn make_generic_glue(ccx: &CrateContext,
 
     let arena = TypedArena::new();
     let empty_param_substs = param_substs::empty();
-    let fcx = new_fn_ctxt(ccx, llfn, -1, false, ty::mk_nil(),
+    let fcx = new_fn_ctxt(ccx, llfn, ast::DUMMY_NODE_ID, false, ty::mk_nil(),
                           &empty_param_substs, None, &arena, TranslateItems);
 
     let bcx = init_function(&fcx, false, ty::mk_nil());

src/librustc/middle/trans/reflect.rs

@@ -310,7 +310,7 @@ impl<'a, 'b> Reflector<'a, 'b> {
                                                     sym.as_slice());
                 let arena = TypedArena::new();
                 let empty_param_substs = param_substs::empty();
-                let fcx = new_fn_ctxt(ccx, llfdecl, -1, false,
+                let fcx = new_fn_ctxt(ccx, llfdecl, ast::DUMMY_NODE_ID, false,
                                       ty::mk_u64(), &empty_param_substs,
                                       None, &arena, TranslateItems);
                 let bcx = init_function(&fcx, false, ty::mk_u64());
@@ -321,9 +321,9 @@ impl<'a, 'b> Reflector<'a, 'b> {
                 let arg = get_param(llfdecl, fcx.arg_pos(0u) as c_uint);
                 let arg = BitCast(bcx, arg, llptrty);
                 let ret = adt::trans_get_discr(bcx, &*repr, arg, Some(Type::i64(ccx)));
-                let ret_alloca = alloca(bcx, Type::i64(ccx), "ret_slot");
-                Store(bcx, ret, ret_alloca);
-                Store(bcx, ret_alloca, fcx.llretslotptr.get().unwrap());
+                assert!(!fcx.needs_ret_allocas);
+                let ret_slot = fcx.get_ret_slot(bcx, ty::mk_u64(), "ret_slot");
+                Store(bcx, ret, ret_slot);
                 match fcx.llreturn.get() {
                     Some(llreturn) => Br(bcx, llreturn),
                     None => {}