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

[WIP] 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%.

## To do

- [ ] Do a perf run to see the effect this has on the compiler (cc @michaelwoerister),
  and possibly run benchmarks on some more crates

- [ ] Add a code gen test

- [ ] Should this be always enabled or should it be behind a command line option?
  If so, what should it be called? `-Z no-plt`? `-Z plt=no`?

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;
+use rustc_target::spec::{PanicStrategy, RelroLevel};
 
 use attributes;
 use llvm::{self, Attribute};
@@ -174,6 +174,12 @@ pub fn from_fn_attrs(
     set_frame_pointer_elimination(cx, llfn);
     set_probestack(cx, llfn);
 
+    // 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/llvm/ffi.rs

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

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");
 }

src/rustllvm/rustllvm.h

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