Fix linker-lto for amdgpu

This allows linking to amd device-libs:
https://github.com/ROCm/llvm-project/tree/amd-staging/amd/device-libs

Author: Flakebi

compiler/rustc_codegen_llvm/src/context.rs

@@ -19,10 +19,10 @@ use rustc_middle::ty::layout::{
 };
 use rustc_middle::ty::{self, Instance, Ty, TyCtxt};
 use rustc_middle::{bug, span_bug};
-use rustc_session::Session;
 use rustc_session::config::{
     BranchProtection, CFGuard, CFProtection, CrateType, DebugInfo, FunctionReturn, PAuthKey, PacRet,
 };
+use rustc_session::{Session, config};
 use rustc_span::source_map::Spanned;
 use rustc_span::{DUMMY_SP, Span};
 use rustc_target::spec::{HasTargetSpec, RelocModel, SmallDataThresholdSupport, Target, TlsModel};
@@ -266,6 +266,11 @@ pub(crate) unsafe fn create_module<'ll>(
         );
     }
 
+    // Disable ThinLTO if fat lto is requested.
+    if sess.lto() == config::Lto::Fat {
+        llvm::add_module_flag_u32(llmod, llvm::ModuleFlagMergeBehavior::Override, "ThinLTO", 0);
+    }
+
     // Add "kcfi" module flag if KCFI is enabled. (See https://reviews.llvm.org/D119296.)
     if sess.is_sanitizer_kcfi_enabled() {
         llvm::add_module_flag_u32(llmod, llvm::ModuleFlagMergeBehavior::Override, "kcfi", 1);

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -2498,8 +2498,10 @@ fn add_order_independent_options(
         // as it appears to be unused. This can then cause the PGO profile file to lose
         // some functions. If we are generating a profile we shouldn't strip those metadata
         // sections to ensure we have all the data for PGO.
-        let keep_metadata =
-            crate_type == CrateType::Dylib || sess.opts.cg.profile_generate.enabled();
+        let keep_metadata = crate_type == CrateType::Dylib
+            || sess.opts.cg.profile_generate.enabled()
+            // TODO Needed to get the <kernel>.kd symbol?
+            || (sess.target.arch == "amdgpu" && sess.opts.cg.linker_plugin_lto.enabled());
         if crate_type != CrateType::Executable || !sess.opts.unstable_opts.export_executable_symbols
         {
             cmd.gc_sections(keep_metadata);

compiler/rustc_codegen_ssa/src/back/linker.rs

@@ -839,7 +839,15 @@ impl<'a> Linker for GccLinker<'a> {
         } else {
             let mut arg = OsString::from("--version-script=");
             arg.push(path);
-            self.link_arg(arg).link_arg("--no-undefined-version");
+            self.link_arg(arg);
+            if self.sess.target.arch == "amdgpu" && self.sess.opts.cg.linker_plugin_lto.enabled() {
+                // The .kd symbol needs to be visible.
+                // Workaround because we cannot export a symbol that does not exist before linking.
+                // At the same time, if we do not specify the .kd symbol, it will not be exported.
+                self.link_arg("--undefined-version");
+            } else {
+                self.link_arg("--no-undefined-version");
+            }
         }
     }