rip out link guards

As discussed in
#32293 (comment),
adding link guards are a heuristic that is causing undue complications:

- the link guards inject extra public symbols, which is not always OK.
- link guards as implemented could be a non-trivial performance hit,
  because no attempt is made to "de-duplicate" the dependency graph,
  so at worst you have O(N!) calls to the link guard functions.

Nonetheless, link guards are very helpful in detecting errors, so it may
be worth adding them back in some modified form in the future.

Author: nikomatsakis

src/librustc_trans/back/linker.rs

@@ -23,7 +23,6 @@ use session::config::CrateTypeDylib;
 use session::config;
 use syntax::ast;
 use trans::CrateTranslation;
-use trans::link_guard;
 
 /// Linker abstraction used by back::link to build up the command to invoke a
 /// linker.
@@ -361,25 +360,6 @@ impl<'a> Linker for MsvcLinker<'a> {
                 writeln!(f, "  {}", symbol)?;
             }
 
-            // Add link-guard symbols
-            {
-                // local crate
-                let symbol = link_guard::link_guard_name(&trans.link.crate_name[..],
-                                                         &trans.link.crate_hash);
-                try!(writeln!(f, "  {}", symbol));
-            }
-            // statically linked dependencies
-            for (i, format) in formats[&CrateTypeDylib].iter().enumerate() {
-                if *format == Linkage::Static {
-                    let cnum = (i + 1) as ast::CrateNum;
-                    let crate_name = cstore.original_crate_name(cnum);
-                    let svh = cstore.crate_hash(cnum);
-
-                    let symbol = link_guard::link_guard_name(&crate_name[..], &svh);
-                    try!(writeln!(f, "  {}", symbol));
-                }
-            }
-
             Ok(())
         })();
         if let Err(e) = res {

src/librustc_trans/trans/base.rs

@@ -79,7 +79,6 @@ use trans::expr;
 use trans::glue;
 use trans::inline;
 use trans::intrinsic;
-use trans::link_guard;
 use trans::machine;
 use trans::machine::{llalign_of_min, llsize_of, llsize_of_real};
 use trans::meth;
@@ -2384,7 +2383,6 @@ pub fn create_entry_wrapper(ccx: &CrateContext, sp: Span, main_llfn: ValueRef) {
         unsafe {
             llvm::LLVMPositionBuilderAtEnd(bld, llbb);
 
-            link_guard::insert_reference_to_link_guard(ccx, llbb);
             debuginfo::gdb::insert_reference_to_gdb_debug_scripts_section_global(ccx);
 
             let (start_fn, args) = if use_start_lang_item {
@@ -2763,8 +2761,6 @@ pub fn trans_crate<'tcx>(tcx: &TyCtxt<'tcx>,
         symbol_names_test::report_symbol_names(&ccx);
     }
 
-    emit_link_guard_if_necessary(&shared_ccx);
-
     for ccx in shared_ccx.iter() {
         if ccx.sess().opts.debuginfo != NoDebugInfo {
             debuginfo::finalize(&ccx);
@@ -2825,8 +2821,6 @@ pub fn trans_crate<'tcx>(tcx: &TyCtxt<'tcx>,
     if sess.entry_fn.borrow().is_some() {
         reachable_symbols.push("main".to_string());
     }
-    reachable_symbols.push(link_guard::link_guard_name(&link_meta.crate_name,
-                                                       &link_meta.crate_hash));
 
     // For the purposes of LTO, we add to the reachable set all of the upstream
     // reachable extern fns. These functions are all part of the public ABI of
@@ -2870,24 +2864,6 @@ pub fn trans_crate<'tcx>(tcx: &TyCtxt<'tcx>,
     }
 }
 
-fn emit_link_guard_if_necessary(shared_ccx: &SharedCrateContext) {
-    let link_meta = shared_ccx.link_meta();
-    let link_guard_name = link_guard::link_guard_name(&link_meta.crate_name,
-                                                      &link_meta.crate_hash);
-    let link_guard_name = CString::new(link_guard_name).unwrap();
-
-    // Check if the link-guard has already been emitted in a codegen unit
-    let link_guard_already_emitted = shared_ccx.iter().any(|ccx| {
-        let link_guard = unsafe { llvm::LLVMGetNamedValue(ccx.llmod(),
-                                                          link_guard_name.as_ptr()) };
-        !link_guard.is_null()
-    });
-
-    if !link_guard_already_emitted {
-        link_guard::get_or_insert_link_guard(&shared_ccx.get_ccx(0));
-    }
-}
-
 /// We visit all the items in the krate and translate them.  We do
 /// this in two walks. The first walk just finds module items. It then
 /// walks the full contents of those module items and translates all

src/librustc_trans/trans/link_guard.rs

@@ -53,7 +53,6 @@ mod expr;
 mod glue;
 mod inline;
 mod intrinsic;
-pub mod link_guard;
 mod machine;
 mod _match;
 mod meth;