Support for disabling the PLT on ELF targets

Disable the PLT where possible to improve performance
for indirect calls into shared libraries.

This optimization is enabled by default where possible.

- Add the `NonLazyBind` attribute to `rustllvm`:
  This attribute informs LLVM to skip PLT calls in codegen.

- Disable PLT unconditionally:
  Apply the `NonLazyBind` attribute on every function.

- Only enable no-plt when full relro is enabled:
  Ensures we only enable it when we have linker support.

- Add `-Z plt` as a compiler option

src/test/codegen/naked-functions.rs

@@ -15,7 +15,7 @@
 #![crate_type = "lib"]
 #![feature(naked_functions)]
 
-// CHECK: Function Attrs: naked uwtable
+// CHECK: Function Attrs: naked
 // CHECK-NEXT: define void @naked_empty()
 #[no_mangle]
 #[naked]
@@ -24,7 +24,7 @@ pub fn naked_empty() {
     // CHECK-NEXT: ret void
 }
 
-// CHECK: Function Attrs: naked uwtable
+// CHECK: Function Attrs: naked
 #[no_mangle]
 #[naked]
 // CHECK-NEXT: define void @naked_with_args(i{{[0-9]+}})
@@ -35,7 +35,7 @@ pub fn naked_with_args(a: isize) {
     // CHECK: ret void
 }
 
-// CHECK: Function Attrs: naked uwtable
+// CHECK: Function Attrs: naked
 // CHECK-NEXT: define i{{[0-9]+}} @naked_with_return()
 #[no_mangle]
 #[naked]
@@ -45,7 +45,7 @@ pub fn naked_with_return() -> isize {
     0
 }
 
-// CHECK: Function Attrs: naked uwtable
+// CHECK: Function Attrs: naked
 // CHECK-NEXT: define i{{[0-9]+}} @naked_with_args_and_return(i{{[0-9]+}})
 #[no_mangle]
 #[naked]
@@ -57,7 +57,7 @@ pub fn naked_with_args_and_return(a: isize) -> isize {
     a
 }
 
-// CHECK: Function Attrs: naked uwtable
+// CHECK: Function Attrs: naked
 // CHECK-NEXT: define void @naked_recursive()
 #[no_mangle]
 #[naked]

src/test/codegen/no-plt.rs

@@ -0,0 +1,27 @@
+// Copyright 2018 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.
+
+// compile-flags: -C relocation-model=pic -Z plt=no
+
+#![crate_type = "lib"]
+
+// We need a function which is normally called through the PLT.
+extern "C" {
+    // CHECK: Function Attrs: nounwind nonlazybind
+    fn getenv(name: *const u8) -> *mut u8;
+}
+
+// Ensure the function gets referenced.
+pub unsafe fn call_through_plt() -> *mut u8 {
+    getenv(b"\0".as_ptr())
+}
+
+// Ensure intrinsics also skip the PLT
+// CHECK: !"RtLibUseGOT"