Auto merge of #54592 - GabrielMajeri:no-plt, r=<try>

Support for disabling PLT for better function call performance

This PR gives `rustc` the ability to skip the PLT when generating function calls into shared libraries. This can improve performance by reducing branch indirection.

AFAIK, the only advantage of using the PLT is to allow for ELF lazy binding. However, since Rust already [enables full relro for security](#43170), lazy binding was disabled anyway.

This is a little known feature which is supported by [GCC](https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html) and [Clang](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-fplt) as `-fno-plt` (some Linux distros [enable it by default](https://git.archlinux.org/svntogit/packages.git/tree/trunk/makepkg.conf?h=packages/pacman#n40) for all builds).

Implementation inspired by [this patch](https://reviews.llvm.org/D39079#change-YvkpNDlMs_LT) which adds `-fno-plt` support to Clang.

## Performance

I didn't run a lot of benchmarks, but these are the results on my machine for a `clap` [benchmark](https://github.com/clap-rs/clap/blob/master/benches/05_ripgrep.rs):

```
 name              control ns/iter  no-plt ns/iter  diff ns/iter  diff %  speedup
 build_app_long    11,097           10,733                  -364  -3.28%   x 1.03
 build_app_short   11,089           10,742                  -347  -3.13%   x 1.03
 build_help_long   186,835          182,713               -4,122  -2.21%   x 1.02
 build_help_short  80,949           78,455                -2,494  -3.08%   x 1.03
 parse_clean       12,385           12,044                  -341  -2.75%   x 1.03
 parse_complex     19,438           19,017                  -421  -2.17%   x 1.02
 parse_lots        431,493          421,421              -10,072  -2.33%   x 1.02
```

A small performance improvement across the board, with no downsides. It's likely binaries which make a lot of function calls into dynamic libraries could see even more improvements. [This comment](https://patchwork.ozlabs.org/patch/468993/#1028255) suggests that, in some cases, `-fno-plt` could improve PIC/PIE code performance by 10%.

## Security benefits

**Bonus**: some of the speculative execution attacks rely on the PLT, by disabling it we reduce a big attack surface and reduce the need for [`retpoline`](https://reviews.llvm.org/D41723).

## Remaining PLT calls

The compiled binaries still have plenty of PLT calls, coming from C/C++ libraries. Building dependencies with `CFLAGS=-fno-plt CXXFLAGS=-fno-plt` removes them.

Author: bors

Diff for: src/librustc/session/config.rs

@@ -1387,6 +1387,10 @@ 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> = (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)"),
 }
 
 pub fn default_lib_output() -> CrateType {

Diff for: 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(

Diff for: src/librustc_codegen_llvm/attributes.rs

@@ -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(

Diff for: src/librustc_codegen_llvm/context.rs

@@ -208,6 +208,13 @@ pub unsafe fn create_module(
         llvm::LLVMRustSetModulePIELevel(llmod);
     }
 
+    // If skipping the PLT is enabled, we need to add some module metadata
+    // to ensure intrinsic calls don't use it.
+    if !sess.needs_plt() {
+        let avoid_plt = "RtLibUseGOT\0".as_ptr() as *const _;
+        llvm::LLVMRustAddModuleFlag(llmod, avoid_plt, 1);
+    }
+
     llmod
 }
 

Diff for: 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
 }
 

Diff for: src/librustc_codegen_llvm/llvm/ffi.rs

@@ -122,6 +122,7 @@ pub enum Attribute {
     SanitizeThread  = 20,
     SanitizeAddress = 21,
     SanitizeMemory  = 22,
+    NonLazyBind     = 23,
 }
 
 /// LLVMIntPredicate

Diff for: src/rustllvm/RustWrapper.cpp

@@ -178,6 +178,8 @@ static Attribute::AttrKind fromRust(LLVMRustAttribute Kind) {
     return Attribute::SanitizeAddress;
   case SanitizeMemory:
     return Attribute::SanitizeMemory;
+  case NonLazyBind:
+    return Attribute::NonLazyBind;
   }
   report_fatal_error("bad AttributeKind");
 }

Diff for: src/rustllvm/rustllvm.h

@@ -97,6 +97,7 @@ enum LLVMRustAttribute {
   SanitizeThread = 20,
   SanitizeAddress = 21,
   SanitizeMemory = 22,
+  NonLazyBind = 23,
 };
 
 typedef struct OpaqueRustString *RustStringRef;

Diff for: 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]

Diff for: 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"