Refactor code and address review comments

Author: GabrielMajeri

src/librustc/session/config.rs

@@ -1387,7 +1387,7 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
           "output a json file with profiler results"),
     emit_stack_sizes: bool = (false, parse_bool, [UNTRACKED],
           "emits a section containing stack size metadata"),
-    plt: Option<bool> = (Some(false), parse_opt_bool, [TRACKED],
+    plt: Option<bool> = (None, parse_opt_bool, [TRACKED],
           "whether to use the PLT when calling into shared libraries;
           only has effect for PIC code on systems with ELF binaries
           (default: PLT is disabled if full relro is enabled)"),

src/librustc/session/mod.rs

@@ -41,8 +41,7 @@ use syntax::feature_gate::AttributeType;
 use syntax_pos::{MultiSpan, Span};
 use util::profiling::SelfProfiler;
 
-use rustc_target::spec::PanicStrategy;
-use rustc_target::spec::{Target, TargetTriple};
+use rustc_target::spec::{PanicStrategy, RelroLevel, Target, TargetTriple};
 use rustc_data_structures::flock;
 use jobserver::Client;
 
@@ -981,6 +980,23 @@ impl Session {
     pub fn edition(&self) -> Edition {
         self.opts.edition
     }
+
+    /// True if we cannot skip the PLT for shared library calls.
+    pub fn needs_plt(&self) -> bool {
+        let dbg_opts = &self.opts.debugging_opts;
+
+        let relro_level = dbg_opts.relro_level
+            .unwrap_or(self.target.target.options.relro_level);
+
+        // Only enable this optimization by default if full relro is also enabled.
+        // In this case, lazy binding was already unavailable, so nothing is lost.
+        // This also ensures `-Wl,-z,now` is supported by the linker.
+        let full_relro = RelroLevel::Full == relro_level;
+
+        // If user didn't explicitly forced us to use the PLT,
+        // then try to skip it where possible.
+        dbg_opts.plt.unwrap_or(!full_relro)
+    }
 }
 
 pub fn build_session(

src/librustc_codegen_llvm/attributes.rs

@@ -20,7 +20,7 @@ use rustc::ty::layout::HasTyCtxt;
 use rustc::ty::query::Providers;
 use rustc_data_structures::sync::Lrc;
 use rustc_data_structures::fx::FxHashMap;
-use rustc_target::spec::{PanicStrategy, RelroLevel};
+use rustc_target::spec::PanicStrategy;
 
 use attributes;
 use llvm::{self, Attribute};
@@ -138,6 +138,15 @@ pub fn apply_target_cpu_attr(cx: &CodegenCx<'ll, '_>, llfn: &'ll Value) {
             target_cpu.as_c_str());
 }
 
+/// Sets the `NonLazyBind` LLVM attribute on a given function,
+/// assuming the codegen options allow skipping the PLT.
+pub fn non_lazy_bind(sess: &Session, llfn: &'ll Value) {
+    // Don't generate calls through PLT if it's not necessary
+    if !sess.needs_plt() {
+        Attribute::NonLazyBind.apply_llfn(Function, llfn);
+    }
+}
+
 /// Composite function which sets LLVM attributes for function depending on its AST (#[attribute])
 /// attributes.
 pub fn from_fn_attrs(
@@ -174,14 +183,6 @@ pub fn from_fn_attrs(
     set_frame_pointer_elimination(cx, llfn);
     set_probestack(cx, llfn);
 
-    if !cx.sess().opts.debugging_opts.plt.unwrap_or(false) {
-        // Only enable this optimization if full relro is also enabled.
-        // In this case, lazy binding was already unavailable, so nothing is lost.
-        if let RelroLevel::Full = cx.sess().target.target.options.relro_level {
-            Attribute::NonLazyBind.apply_llfn(Function, llfn);
-        }
-    }
-
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::COLD) {
         Attribute::Cold.apply_llfn(Function, llfn);
     }

src/librustc_codegen_llvm/context.rs

@@ -210,7 +210,7 @@ pub unsafe fn create_module(
 
     // If skipping the PLT is enabled, we need to add some module metadata
     // to ensure intrinsic calls don't use it.
-    if !sess.opts.debugging_opts.plt.unwrap_or(false) {
+    if !sess.needs_plt() {
         let avoid_plt = "RtLibUseGOT\0".as_ptr() as *const _;
         llvm::LLVMRustAddModuleFlag(llmod, avoid_plt, 1);
     }

src/librustc_codegen_llvm/declare.rs

@@ -104,6 +104,8 @@ fn declare_raw_fn(
         attributes::unwind(llfn, false);
     }
 
+    attributes::non_lazy_bind(cx.sess(), llfn);
+
     llfn
 }