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// Copyright 2016 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! The Rust Linkage Model and Symbol Names //! ======================================= //! //! The semantic model of Rust linkage is, broadly, that "there's no global //! namespace" between crates. Our aim is to preserve the illusion of this //! model despite the fact that it's not *quite* possible to implement on //! modern linkers. We initially didn't use system linkers at all, but have //! been convinced of their utility. //! //! There are a few issues to handle: //! //! - Linkers operate on a flat namespace, so we have to flatten names. //! We do this using the C++ namespace-mangling technique. Foo::bar //! symbols and such. //! //! - Symbols for distinct items with the same *name* need to get different //! linkage-names. Examples of this are monomorphizations of functions or //! items within anonymous scopes that end up having the same path. //! //! - Symbols in different crates but with same names "within" the crate need //! to get different linkage-names. //! //! - Symbol names should be deterministic: Two consecutive runs of the //! compiler over the same code base should produce the same symbol names for //! the same items. //! //! - Symbol names should not depend on any global properties of the code base, //! so that small modifications to the code base do not result in all symbols //! changing. In previous versions of the compiler, symbol names incorporated //! the SVH (Stable Version Hash) of the crate. This scheme turned out to be //! infeasible when used in conjunction with incremental compilation because //! small code changes would invalidate all symbols generated previously. //! //! - Even symbols from different versions of the same crate should be able to //! live next to each other without conflict. //! //! In order to fulfill the above requirements the following scheme is used by //! the compiler: //! //! The main tool for avoiding naming conflicts is the incorporation of a 64-bit //! hash value into every exported symbol name. Anything that makes a difference //! to the symbol being named, but does not show up in the regular path needs to //! be fed into this hash: //! //! - Different monomorphizations of the same item have the same path but differ //! in their concrete type parameters, so these parameters are part of the //! data being digested for the symbol hash. //! //! - Rust allows items to be defined in anonymous scopes, such as in //! `fn foo() { { fn bar() {} } { fn bar() {} } }`. Both `bar` functions have //! the path `foo::bar`, since the anonymous scopes do not contribute to the //! path of an item. The compiler already handles this case via so-called //! disambiguating `DefPaths` which use indices to distinguish items with the //! same name. The DefPaths of the functions above are thus `foo[0]::bar[0]` //! and `foo[0]::bar[1]`. In order to incorporate this disambiguation //! information into the symbol name too, these indices are fed into the //! symbol hash, so that the above two symbols would end up with different //! hash values. //! //! The two measures described above suffice to avoid intra-crate conflicts. In //! order to also avoid inter-crate conflicts two more measures are taken: //! //! - The name of the crate containing the symbol is prepended to the symbol //! name, i.e. symbols are "crate qualified". For example, a function `foo` in //! module `bar` in crate `baz` would get a symbol name like //! `baz::bar::foo::{hash}` instead of just `bar::foo::{hash}`. This avoids //! simple conflicts between functions from different crates. //! //! - In order to be able to also use symbols from two versions of the same //! crate (which naturally also have the same name), a stronger measure is //! required: The compiler accepts an arbitrary "disambiguator" value via the //! `-C metadata` commandline argument. This disambiguator is then fed into //! the symbol hash of every exported item. Consequently, the symbols in two //! identical crates but with different disambiguators are not in conflict //! with each other. This facility is mainly intended to be used by build //! tools like Cargo. //! //! A note on symbol name stability //! ------------------------------- //! Previous versions of the compiler resorted to feeding NodeIds into the //! symbol hash in order to disambiguate between items with the same path. The //! current version of the name generation algorithm takes great care not to do //! that, since NodeIds are notoriously unstable: A small change to the //! code base will offset all NodeIds after the change and thus, much as using //! the SVH in the hash, invalidate an unbounded number of symbol names. This //! makes re-using previously compiled code for incremental compilation //! virtually impossible. Thus, symbol hash generation exclusively relies on //! DefPaths which are much more robust in the face of changes to the code base. use common::SharedCrateContext; use monomorphize::Instance; use rustc::middle::weak_lang_items; use rustc::hir::def_id::LOCAL_CRATE; use rustc::hir::map as hir_map; use rustc::ty::{self, Ty, TypeFoldable}; use rustc::ty::fold::TypeVisitor; use rustc::ty::item_path::{self, ItemPathBuffer, RootMode}; use rustc::ty::subst::Substs; use rustc::hir::map::definitions::{DefPath, DefPathData}; use rustc::util::common::record_time; use syntax::attr; use syntax::symbol::{Symbol, InternedString}; fn get_symbol_hash<'a, 'tcx>(scx: &SharedCrateContext<'a, 'tcx>, // path to the item this name is for def_path: &DefPath, // type of the item, without any generic // parameters substituted; this is // included in the hash as a kind of // safeguard. item_type: Ty<'tcx>, // values for generic type parameters, // if any. substs: Option<&'tcx Substs<'tcx>>) -> String { debug!("get_symbol_hash(def_path={:?}, parameters={:?})", def_path, substs); let tcx = scx.tcx(); let mut hasher = ty::util::TypeIdHasher::<u64>::new(tcx); record_time(&tcx.sess.perf_stats.symbol_hash_time, || { // the main symbol name is not necessarily unique; hash in the // compiler's internal def-path, guaranteeing each symbol has a // truly unique path hasher.def_path(def_path); // Include the main item-type. Note that, in this case, the // assertions about `needs_subst` may not hold, but this item-type // ought to be the same for every reference anyway. assert!(!item_type.has_erasable_regions()); hasher.visit_ty(item_type); // also include any type parameters (for generic items) if let Some(substs) = substs { assert!(!substs.has_erasable_regions()); assert!(!substs.needs_subst()); substs.visit_with(&mut hasher); // If this is an instance of a generic function, we also hash in // the ID of the instantiating crate. This avoids symbol conflicts // in case the same instances is emitted in two crates of the same // project. if substs.types().next().is_some() { hasher.hash(scx.tcx().crate_name.as_str()); hasher.hash(scx.sess().local_crate_disambiguator().as_str()); } } }); // 64 bits should be enough to avoid collisions. format!("h{:016x}", hasher.finish()) } impl<'a, 'tcx> Instance<'tcx> { pub fn symbol_name(self, scx: &SharedCrateContext<'a, 'tcx>) -> String { let Instance { def: def_id, substs } = self; debug!("symbol_name(def_id={:?}, substs={:?})", def_id, substs); let node_id = scx.tcx().hir.as_local_node_id(def_id); if let Some(id) = node_id { if scx.sess().plugin_registrar_fn.get() == Some(id) { let svh = &scx.link_meta().crate_hash; let idx = def_id.index; return scx.sess().generate_plugin_registrar_symbol(svh, idx); } if scx.sess().derive_registrar_fn.get() == Some(id) { let svh = &scx.link_meta().crate_hash; let idx = def_id.index; return scx.sess().generate_derive_registrar_symbol(svh, idx); } } // FIXME(eddyb) Precompute a custom symbol name based on attributes. let attrs = scx.tcx().get_attrs(def_id); let is_foreign = if let Some(id) = node_id { match scx.tcx().hir.get(id) { hir_map::NodeForeignItem(_) => true, _ => false } } else { scx.sess().cstore.is_foreign_item(def_id) }; if let Some(name) = weak_lang_items::link_name(&attrs) { return name.to_string(); } if is_foreign { if let Some(name) = attr::first_attr_value_str_by_name(&attrs, "link_name") { return name.to_string(); } // Don't mangle foreign items. return scx.tcx().item_name(def_id).as_str().to_string(); } if let Some(name) = attr::find_export_name_attr(scx.sess().diagnostic(), &attrs) { // Use provided name return name.to_string(); } if attr::contains_name(&attrs, "no_mangle") { // Don't mangle return scx.tcx().item_name(def_id).as_str().to_string(); } let def_path = scx.tcx().def_path(def_id); // We want to compute the "type" of this item. Unfortunately, some // kinds of items (e.g., closures) don't have an entry in the // item-type array. So walk back up the find the closest parent // that DOES have an entry. let mut ty_def_id = def_id; let instance_ty; loop { let key = scx.tcx().def_key(ty_def_id); match key.disambiguated_data.data { DefPathData::TypeNs(_) | DefPathData::ValueNs(_) => { instance_ty = scx.tcx().item_type(ty_def_id); break; } _ => { // if we're making a symbol for something, there ought // to be a value or type-def or something in there // *somewhere* ty_def_id.index = key.parent.unwrap_or_else(|| { bug!("finding type for {:?}, encountered def-id {:?} with no \ parent", def_id, ty_def_id); }); } } } // Erase regions because they may not be deterministic when hashed // and should not matter anyhow. let instance_ty = scx.tcx().erase_regions(&instance_ty); let hash = get_symbol_hash(scx, &def_path, instance_ty, Some(substs)); let mut buffer = SymbolPathBuffer { names: Vec::with_capacity(def_path.data.len()) }; item_path::with_forced_absolute_paths(|| { scx.tcx().push_item_path(&mut buffer, def_id); }); mangle(buffer.names.into_iter(), &hash) } } struct SymbolPathBuffer { names: Vec<InternedString>, } impl ItemPathBuffer for SymbolPathBuffer { fn root_mode(&self) -> &RootMode { const ABSOLUTE: &'static RootMode = &RootMode::Absolute; ABSOLUTE } fn push(&mut self, text: &str) { self.names.push(Symbol::intern(text).as_str()); } } pub fn exported_name_from_type_and_prefix<'a, 'tcx>(scx: &SharedCrateContext<'a, 'tcx>, t: Ty<'tcx>, prefix: &str) -> String { let empty_def_path = DefPath { data: vec![], krate: LOCAL_CRATE, }; let hash = get_symbol_hash(scx, &empty_def_path, t, None); let path = [Symbol::intern(prefix).as_str()]; mangle(path.iter().cloned(), &hash) } // Name sanitation. LLVM will happily accept identifiers with weird names, but // gas doesn't! // gas accepts the following characters in symbols: a-z, A-Z, 0-9, ., _, $ pub fn sanitize(s: &str) -> String { let mut result = String::new(); for c in s.chars() { match c { // Escape these with $ sequences '@' => result.push_str("$SP$"), '*' => result.push_str("$BP$"), '&' => result.push_str("$RF$"), '<' => result.push_str("$LT$"), '>' => result.push_str("$GT$"), '(' => result.push_str("$LP$"), ')' => result.push_str("$RP$"), ',' => result.push_str("$C$"), // '.' doesn't occur in types and functions, so reuse it // for ':' and '-' '-' | ':' => result.push('.'), // These are legal symbols 'a' ... 'z' | 'A' ... 'Z' | '0' ... '9' | '_' | '.' | '$' => result.push(c), _ => { result.push('$'); for c in c.escape_unicode().skip(1) { match c { '{' => {}, '}' => result.push('$'), c => result.push(c), } } } } } // Underscore-qualify anything that didn't start as an ident. if !result.is_empty() && result.as_bytes()[0] != '_' as u8 && ! (result.as_bytes()[0] as char).is_xid_start() { return format!("_{}", &result[..]); } return result; } fn mangle<PI: Iterator<Item=InternedString>>(path: PI, hash: &str) -> String { // Follow C++ namespace-mangling style, see // http://en.wikipedia.org/wiki/Name_mangling for more info. // // It turns out that on OSX you can actually have arbitrary symbols in // function names (at least when given to LLVM), but this is not possible // when using unix's linker. Perhaps one day when we just use a linker from LLVM // we won't need to do this name mangling. The problem with name mangling is // that it seriously limits the available characters. For example we can't // have things like &T in symbol names when one would theoretically // want them for things like impls of traits on that type. // // To be able to work on all platforms and get *some* reasonable output, we // use C++ name-mangling. let mut n = String::from("_ZN"); // _Z == Begin name-sequence, N == nested fn push(n: &mut String, s: &str) { let sani = sanitize(s); n.push_str(&format!("{}{}", sani.len(), sani)); } // First, connect each component with <len, name> pairs. for data in path { push(&mut n, &data); } push(&mut n, hash); n.push('E'); // End name-sequence. n }