From 4c7e27734031d8b57cb51ad498d7f5111032468d Mon Sep 17 00:00:00 2001 From: Jorge Aparicio Date: Mon, 20 Feb 2017 14:42:47 -0500 Subject: [PATCH] add an #[used] attribute similar to GCC's __attribute((used))__. This attribute prevents LLVM from optimizing away a non-exported symbol, within a compilation unit (object file), when there are no references to it. This is better explained with an example: ``` #[used] static LIVE: i32 = 0; static REFERENCED: i32 = 0; static DEAD: i32 = 0; fn internal() {} pub fn exported() -> &'static i32 { &REFERENCED } ``` Without optimizations, LLVM pretty much preserves all the static variables and functions within the compilation unit. ``` $ rustc --crate-type=lib --emit=obj symbols.rs && nm -C symbols.o 0000000000000000 t drop::h1be0f8f27a2ba94a 0000000000000000 r symbols::REFERENCED::hb3bdfd46050bc84c 0000000000000000 r symbols::DEAD::hc2ea8f9bd06f380b 0000000000000000 r symbols::LIVE::h0970cf9889edb56e 0000000000000000 T symbols::exported::h6f096c2b1fc292b2 0000000000000000 t symbols::internal::h0ac1aadbc1e3a494 ``` With optimizations, LLVM will drop dead code. Here `internal` is dropped because it's not a exported function/symbol (i.e. not `pub`lic). `DEAD` is dropped for the same reason. `REFERENCED` is preserved, even though it's not exported, because it's referenced by the `exported` function. Finally, `LIVE` survives because of the `#[used]` attribute even though it's not exported or referenced. ``` $ rustc --crate-type=lib -C opt-level=3 --emit=obj symbols.rs && nm -C symbols.o 0000000000000000 r symbols::REFERENCED::hb3bdfd46050bc84c 0000000000000000 r symbols::LIVE::h0970cf9889edb56e 0000000000000000 T symbols::exported::h6f096c2b1fc292b2 ``` Note that the linker knows nothing about `#[used]` and will drop `LIVE` because no other object references to it. ``` $ echo 'fn main() {}' >> symbols.rs $ rustc symbols.rs && nm -C symbols | grep LIVE ``` At this time, `#[used]` only works on `static` variables. --- src/librustc_trans/base.rs | 20 +++++++++++++++++++- src/librustc_trans/consts.rs | 4 ++++ src/librustc_trans/context.rs | 7 +++++++ src/libsyntax/feature_gate.rs | 8 ++++++++ 4 files changed, 38 insertions(+), 1 deletion(-) diff --git a/src/librustc_trans/base.rs b/src/librustc_trans/base.rs index ec45c5593632e..63258b7453317 100644 --- a/src/librustc_trans/base.rs +++ b/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 +1187,24 @@ pub fn trans_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, } } + // Create llvm.used variable + if !ccx.used_statics().borrow().is_empty() { + debug!("llvm.used"); + + 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); diff --git a/src/librustc_trans/consts.rs b/src/librustc_trans/consts.rs index 0c3d211912add..9974155f7c07d 100644 --- a/src/librustc_trans/consts.rs +++ b/src/librustc_trans/consts.rs @@ -276,6 +276,10 @@ pub fn trans_static<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, base::set_link_section(ccx, g, attrs); + if attr::contains_name(attrs, "used") { + ccx.used_statics().borrow_mut().push(g); + } + Ok(g) } } diff --git a/src/librustc_trans/context.rs b/src/librustc_trans/context.rs index 73602dc420b3f..2eca0a18e2b38 100644 --- a/src/librustc_trans/context.rs +++ b/src/librustc_trans/context.rs @@ -132,6 +132,8 @@ pub struct LocalCrateContext<'tcx> { /// to constants.) statics_to_rauw: RefCell>, + used_statics: RefCell>, + lltypes: RefCell, Type>>, llsizingtypes: RefCell, Type>>, type_hashcodes: RefCell, String>>, @@ -587,6 +589,7 @@ 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 +757,10 @@ impl<'b, 'tcx> CrateContext<'b, 'tcx> { &self.local().statics_to_rauw } + pub fn used_statics<'a>(&'a self) -> &'a RefCell> { + &self.local().used_statics + } + pub fn lltypes<'a>(&'a self) -> &'a RefCell, Type>> { &self.local().lltypes } diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs index 12d25ca4274fe..66a813025c437 100644 --- a/src/libsyntax/feature_gate.rs +++ b/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 + (active, used, "1.18.0", None), ); 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,