Run name-anon-globals after LTO passes as well

src/librustc_codegen_llvm/back/lto.rs

@@ -605,6 +605,13 @@ fn run_pass_manager(cgcx: &CodegenContext,
             }
         });
 
+        // We always generate bitcode through ThinLTOBuffers,
+        // which do not support anonymous globals
+        if config.bitcode_needed() {
+            let pass = llvm::LLVMRustFindAndCreatePass("name-anon-globals\0".as_ptr() as *const _);
+            llvm::LLVMRustAddPass(pm, pass.unwrap());
+        }
+
         if config.verify_llvm_ir {
             let pass = llvm::LLVMRustFindAndCreatePass("verify\0".as_ptr() as *const _);
             llvm::LLVMRustAddPass(pm, pass.unwrap());

src/librustc_codegen_llvm/back/write.rs

@@ -337,6 +337,11 @@ impl ModuleConfig {
         self.merge_functions = sess.opts.optimize == config::OptLevel::Default ||
                                sess.opts.optimize == config::OptLevel::Aggressive;
     }
+
+    pub fn bitcode_needed(&self) -> bool {
+        self.emit_bc || self.obj_is_bitcode
+            || self.emit_bc_compressed || self.embed_bitcode
+    }
 }
 
 /// Assembler name and command used by codegen when no_integrated_as is enabled
@@ -564,8 +569,7 @@ unsafe fn optimize(cgcx: &CodegenContext,
             // Some options cause LLVM bitcode to be emitted, which uses ThinLTOBuffers, so we need
             // to make sure we run LLVM's NameAnonGlobals pass when emitting bitcode; otherwise
             // we'll get errors in LLVM.
-            let using_thin_buffers = config.emit_bc || config.obj_is_bitcode
-                || config.emit_bc_compressed || config.embed_bitcode;
+            let using_thin_buffers = config.bitcode_needed();
             let mut have_name_anon_globals_pass = false;
             if !config.no_prepopulate_passes {
                 llvm::LLVMRustAddAnalysisPasses(tm, fpm, llmod);

src/test/ui/issue-51947.rs

@@ -0,0 +1,17 @@
+// compile-pass
+
+#![crate_type = "lib"]
+#![feature(linkage)]
+
+// MergeFunctions will merge these via an anonymous internal
+// backing function, which must be named if ThinLTO buffers are used
+
+#[linkage = "weak"]
+pub fn fn1(a: u32, b: u32, c: u32) -> u32 {
+    a + b + c
+}
+
+#[linkage = "weak"]
+pub fn fn2(a: u32, b: u32, c: u32) -> u32 {
+    a + b + c
+}