#[used] attribute

src/doc/unstable-book/src/SUMMARY.md

@@ -203,6 +203,7 @@
 - [unwind_attributes](unwind-attributes.md)
 - [update_panic_count](update-panic-count.md)
 - [use_extern_macros](use-extern-macros.md)
+- [used](used.md)
 - [utf8_error_error_len](utf8-error-error-len.md)
 - [vec_remove_item](vec-remove-item.md)
 - [windows_c](windows-c.md)

src/doc/unstable-book/src/used.md

@@ -0,0 +1,152 @@
+# `used`
+
+The tracking issue for this feature is: 40289.
+
+------------------------
+
+The `#[used]` attribute can be applied to `static` variables to prevent the Rust
+compiler from optimizing them away even if they appear to be unused by the crate
+(appear to be "dead code").
+
+``` rust
+#![feature(used)]
+
+#[used]
+static FOO: i32 = 1;
+
+static BAR: i32 = 2;
+
+fn main() {}
+```
+
+If you compile this program into an object file, you'll see that `FOO` makes it
+to the object file but `BAR` doesn't. Neither static variable is used by the
+program.
+
+``` text
+$ rustc -C opt-level=3 --emit=obj used.rs
+
+$ nm -C used.o
+0000000000000000 T main
+                 U std::rt::lang_start
+0000000000000000 r used::FOO
+0000000000000000 t used::main
+```
+
+Note that the *linker* knows nothing about the `#[used]` attribute and will
+remove `#[used]` symbols if they are not referenced by other parts of the
+program:
+
+``` text
+$ rustc -C opt-level=3 used.rs
+
+$ nm -C used | grep FOO
+```
+
+"This doesn't sound too useful then!" you may think but keep reading.
+
+To preserve the symbols all the way to the final binary, you'll need the
+cooperation of the linker. Here's one example:
+
+The ELF standard defines two special sections, `.init_array` and
+`.pre_init_array`, that may contain function pointers which will be executed
+*before* the `main` function is invoked. The linker will preserve symbols placed
+in these sections (at least when linking programs that target the `*-*-linux-*`
+targets).
+
+``` rust
+#![feature(used)]
+
+extern "C" fn before_main() {
+    println!("Hello, world!");
+}
+
+#[link_section = ".init_array"]
+#[used]
+static INIT_ARRAY: [extern "C" fn(); 1] = [before_main];
+
+fn main() {}
+```
+
+So, `#[used]` and `#[link_section]` can be combined to obtain "life before
+main".
+
+``` text
+$ rustc -C opt-level=3 before-main.rs
+
+$ ./before-main
+Hello, world!
+```
+
+Another example: ARM Cortex-M microcontrollers need their reset handler, a
+pointer to the function that will executed right after the microcontroller is
+turned on, to be placed near the start of their FLASH memory to boot properly.
+
+This condition can be met using `#[used]` and `#[link_section]` plus a linker
+script.
+
+``` rust
+#![feature(lang_items)]
+#![feature(used)]
+#![no_main]
+#![no_std]
+
+extern "C" fn reset_handler() -> ! {
+    loop {}
+}
+
+#[link_section = ".reset_handler"]
+#[used]
+static RESET_HANDLER: extern "C" fn() -> ! = reset_handler;
+
+#[lang = "panic_fmt"]
+fn panic_fmt() {}
+```
+
+``` text
+MEMORY
+{
+  FLASH : ORIGIN = 0x08000000, LENGTH = 128K
+  RAM : ORIGIN = 0x20000000, LENGTH = 20K
+}
+
+SECTIONS
+{
+  .text ORIGIN(FLASH) :
+  {
+    /* Vector table */
+    LONG(ORIGIN(RAM) + LENGTH(RAM)); /* initial SP value */
+    KEEP(*(.reset_handler));
+
+    /* Omitted: The rest of the vector table */
+
+    *(.text.*);
+  } > FLASH
+
+  /DISCARD/ :
+  {
+    /* Unused unwinding stuff */
+    *(.ARM.exidx.*)
+  }
+}
+```
+
+``` text
+$ xargo rustc --target thumbv7m-none-eabi --release -- \
+    -C link-arg=-Tlink.x -C link-arg=-nostartfiles
+
+$ arm-none-eabi-objdump -Cd target/thumbv7m-none-eabi/release/app
+./target/thumbv7m-none-eabi/release/app:     file format elf32-littlearm
+
+
+Disassembly of section .text:
+
+08000000 <app::RESET_HANDLER-0x4>:
+ 8000000:       20005000        .word   0x20005000
+
+08000004 <app::RESET_HANDLER>:
+ 8000004:       08000009                                ....
+
+08000008 <app::reset_handler>:
+ 8000008:       e7fe            b.n     8000008 <app::reset_handler>
+```

src/librustc_trans/base.rs

@@ -50,7 +50,7 @@ use builder::Builder;
 use callee;
 use common::{C_bool, C_bytes_in_context, C_i32, C_uint};
 use collector::{self, TransItemCollectionMode};
-use common::{C_struct_in_context, C_u64, C_undef};
+use common::{C_struct_in_context, C_u64, C_undef, C_array};
 use common::CrateContext;
 use common::{type_is_zero_size, val_ty};
 use common;
@@ -1187,6 +1187,23 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 }
             }
 
+            // Create the llvm.used variable
+            // This variable has type [N x i8*] and is stored in the llvm.metadata section
+            if !ccx.used_statics().borrow().is_empty() {
+                let name = CString::new("llvm.used").unwrap();
+                let section = CString::new("llvm.metadata").unwrap();
+                let array = C_array(Type::i8(&ccx).ptr_to(), &*ccx.used_statics().borrow());
+
+                unsafe {
+                    let g = llvm::LLVMAddGlobal(ccx.llmod(),
+                                                val_ty(array).to_ref(),
+                                                name.as_ptr());
+                    llvm::LLVMSetInitializer(g, array);
+                    llvm::LLVMRustSetLinkage(g, llvm::Linkage::AppendingLinkage);
+                    llvm::LLVMSetSection(g, section.as_ptr());
+                }
+            }
+
             // Finalize debuginfo
             if ccx.sess().opts.debuginfo != NoDebugInfo {
                 debuginfo::finalize(&ccx);

src/librustc_trans/consts.rs

@@ -276,6 +276,12 @@ pub fn trans_static<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>,
 
         base::set_link_section(ccx, g, attrs);
 
+        if attr::contains_name(attrs, "used") {
+            // This static will be stored in the llvm.used variable which is an array of i8*
+            let cast = llvm::LLVMConstPointerCast(g, Type::i8p(ccx).to_ref());
+            ccx.used_statics().borrow_mut().push(cast);
+        }
+
         Ok(g)
     }
 }

src/librustc_trans/context.rs

@@ -132,6 +132,10 @@ pub struct LocalCrateContext<'tcx> {
     /// to constants.)
     statics_to_rauw: RefCell<Vec<(ValueRef, ValueRef)>>,
 
+    /// Statics that will be placed in the llvm.used variable
+    /// See http://llvm.org/docs/LangRef.html#the-llvm-used-global-variable for details
+    used_statics: RefCell<Vec<ValueRef>>,
+
     lltypes: RefCell<FxHashMap<Ty<'tcx>, Type>>,
     llsizingtypes: RefCell<FxHashMap<Ty<'tcx>, Type>>,
     type_hashcodes: RefCell<FxHashMap<Ty<'tcx>, String>>,
@@ -587,6 +591,7 @@ impl<'tcx> LocalCrateContext<'tcx> {
                 impl_method_cache: RefCell::new(FxHashMap()),
                 closure_bare_wrapper_cache: RefCell::new(FxHashMap()),
                 statics_to_rauw: RefCell::new(Vec::new()),
+                used_statics: RefCell::new(Vec::new()),
                 lltypes: RefCell::new(FxHashMap()),
                 llsizingtypes: RefCell::new(FxHashMap()),
                 type_hashcodes: RefCell::new(FxHashMap()),
@@ -754,6 +759,10 @@ impl<'b, 'tcx> CrateContext<'b, 'tcx> {
         &self.local().statics_to_rauw
     }
 
+    pub fn used_statics<'a>(&'a self) -> &'a RefCell<Vec<ValueRef>> {
+        &self.local().used_statics
+    }
+
     pub fn lltypes<'a>(&'a self) -> &'a RefCell<FxHashMap<Ty<'tcx>, Type>> {
         &self.local().lltypes
     }

src/libsyntax/feature_gate.rs

@@ -337,11 +337,15 @@ declare_features! (
     // `extern "x86-interrupt" fn()`
     (active, abi_x86_interrupt, "1.17.0", Some(40180)),
 
+
     // Allows the `catch {...}` expression
     (active, catch_expr, "1.17.0", Some(31436)),
 
     // See rust-lang/rfcs#1414. Allows code like `let x: &'static u32 = &42` to work.
     (active, rvalue_static_promotion, "1.15.1", Some(38865)),
+
+    // Used to preserve symbols (see llvm.used)
+    (active, used, "1.18.0", Some(40289)),
 );
 
 declare_features! (
@@ -748,6 +752,10 @@ pub const BUILTIN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeG
                                   "unwind_attributes",
                                   "#[unwind] is experimental",
                                   cfg_fn!(unwind_attributes))),
+    ("used", Whitelisted, Gated(
+        Stability::Unstable, "used",
+        "the `#[used]` attribute is an experimental feature",
+        cfg_fn!(used))),
 
     // used in resolve
     ("prelude_import", Whitelisted, Gated(Stability::Unstable,

src/test/compile-fail/feature-gate-used.rs

@@ -0,0 +1,15 @@
+// Copyright 2017 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.
+
+#[used]
+fn foo() {}
+//~^^ ERROR the `#[used]` attribute is an experimental feature
+
+fn main() {}

src/test/run-make/used/Makefile

@@ -0,0 +1,11 @@
+-include ../tools.mk
+
+ifdef IS_WINDOWS
+# Do nothing on MSVC.
+all:
+	exit 0
+else
+all:
+	$(RUSTC) -C opt-level=3 --emit=obj used.rs
+	nm -C $(TMPDIR)/used.o | grep FOO
+endif

src/test/run-make/used/used.rs

@@ -0,0 +1,17 @@
+// Copyright 2017 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.
+
+#![crate_type = "lib"]
+#![feature(used)]
+
+#[used]
+static FOO: u32 = 0;
+
+static BAR: u32 = 0;