Auto merge of #87570 - nikic:llvm-13, r=nagisa

Upgrade to LLVM 13

Work in progress update to LLVM 13. Main changes:

 * InlineAsm diagnostics reported using SrcMgr diagnostic kind are now handled. Previously these used a separate diag handler.
 * Codegen tests are updated for additional attributes.
 * Some data layouts have changed.
 * Switch `#[used]` attribute from `llvm.used` to `llvm.compiler.used` to avoid SHF_GNU_RETAIN flag introduced in https://reviews.llvm.org/D97448, which appears to trigger a bug in older versions of gold.
 * Set `LLVM_INCLUDE_TESTS=OFF` to avoid Python 3.6 requirement.

Upstream issues:

 * ~~https://bugs.llvm.org/show_bug.cgi?id=51210 (InlineAsm diagnostic reporting for module asm)~~ Fixed by https://github.com/llvm/llvm-project/commit/1558bb80c01b695ce12642527cbfccf16cf54ece.
 * ~~https://bugs.llvm.org/show_bug.cgi?id=51476 (Miscompile on AArch64 due to incorrect comparison elimination)~~ Fixed by https://github.com/llvm/llvm-project/commit/81b106584f2baf33e09be2362c35c1bf2f6bfe94.
 * https://bugs.llvm.org/show_bug.cgi?id=51207 (Can't set custom section flags anymore). Problematic change reverted in our fork, https://reviews.llvm.org/D107216 posted for upstream revert.
 * https://bugs.llvm.org/show_bug.cgi?id=51211 (Regression in codegen for #83623). This is an optimization regression that we may likely have to eat for this release. The fix for #83623 was based on an incorrect premise, and this needs to be properly addressed in the MergeICmps pass.

The [compile-time impact](https://perf.rust-lang.org/compare.html?start=ef9549b6c0efb7525c9b012148689c8d070f9bc0&end=0983094463497eec22d550dad25576a894687002) is mixed, but quite positive as LLVM upgrades go.

The LLVM 13 final release is scheduled for Sep 21st. The current nightly is scheduled for stable release on Oct 21st.

r? `@ghost`

2 files changed, 10 insertions, 10 deletions

diff --git a/compiler/rustc_codegen_ssa/src/traits/misc.rs b/compiler/rustc_codegen_ssa/src/traits/misc.rs
index 46f2adbe552..4266e42ec2b 100644
--- a/compiler/rustc_codegen_ssa/src/traits/misc.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/misc.rs
@@ -16,9 +16,11 @@ pub trait MiscMethods<'tcx>: BackendTypes {
     fn sess(&self) -> &Session;
     fn codegen_unit(&self) -> &'tcx CodegenUnit<'tcx>;
     fn used_statics(&self) -> &RefCell<Vec<Self::Value>>;
+    fn compiler_used_statics(&self) -> &RefCell<Vec<Self::Value>>;
     fn set_frame_pointer_type(&self, llfn: Self::Function);
     fn apply_target_cpu_attr(&self, llfn: Self::Function);
     fn create_used_variable(&self);
+    fn create_compiler_used_variable(&self);
     /// Declares the extern "C" main function for the entry point. Returns None if the symbol already exists.
     fn declare_c_main(&self, fn_type: Self::Type) -> Option<Self::Function>;
 }
diff --git a/compiler/rustc_codegen_ssa/src/traits/statics.rs b/compiler/rustc_codegen_ssa/src/traits/statics.rs
index 817fc02d166..a2a3cb56c78 100644
--- a/compiler/rustc_codegen_ssa/src/traits/statics.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/statics.rs
@@ -6,17 +6,15 @@ pub trait StaticMethods: BackendTypes {
     fn static_addr_of(&self, cv: Self::Value, align: Align, kind: Option<&str>) -> Self::Value;
     fn codegen_static(&self, def_id: DefId, is_mutable: bool);
 
-    /// Mark the given global value as "used", to prevent a backend from potentially removing a
-    /// static variable that may otherwise appear unused.
-    ///
-    /// Static variables in Rust can be annotated with the `#[used]` attribute to direct the `rustc`
-    /// compiler to mark the variable as a "used global".
-    ///
-    /// ```no_run
-    /// #[used]
-    /// static FOO: u32 = 0;
-    /// ```
+    /// Mark the given global value as "used", to prevent the compiler and linker from potentially
+    /// removing a static variable that may otherwise appear unused.
     fn add_used_global(&self, global: Self::Value);
+
+    /// Same as add_used_global(), but only prevent the compiler from potentially removing an
+    /// otherwise unused symbol. The linker is still permitted to drop it.
+    ///
+    /// This corresponds to the semantics of the `#[used]` attribute.
+    fn add_compiler_used_global(&self, global: Self::Value);
 }
 
 pub trait StaticBuilderMethods: BackendTypes {