Don't internalize __llvm_profile_counter_bias

compiler/rustc_codegen_llvm/src/back/lto.rs

@@ -133,6 +133,10 @@ fn prepare_lto(
         }
     }
 
+    // __llvm_profile_counter_bias is pulled in at link time by an undefined reference to
+    // __llvm_profile_runtime, therefore we won't know until link time if this symbol
+    // should have default visibility.
+    symbols_below_threshold.push(CString::new("__llvm_profile_counter_bias").unwrap());
     Ok((symbols_below_threshold, upstream_modules))
 }
 

src/test/codegen/pgo-counter-bias.rs

@@ -0,0 +1,10 @@
+// Test that __llvm_profile_counter_bias does not get internalized by lto.
+
+// ignore-macos -runtime-counter-relocation not honored on Mach-O
+// compile-flags: -Cprofile-generate -Cllvm-args=-runtime-counter-relocation -Clto=fat
+// needs-profiler-support
+// no-prefer-dynamic
+
+// CHECK: @__llvm_profile_counter_bias = {{.*}}global
+
+pub fn main() {}