Don't internalize __llvm_profile_counter_bias

Currently, LLVM profiling runtime counter relocation cannot be
used by rust during LTO because symbols are being internalized
before all symbol information is known.

This mode makes LLVM emit a __llvm_profile_counter_bias symbol
which is referenced by the profiling initialization, which itself
is pulled in by the rust driver here [1].

It is enabled with -Cllvm-args=-runtime-counter-relocation for
platforms which are opt-in to this mode like Linux. On these
platforms there will be no link error, rather just surprising
behavior for a user which request runtime counter relocation.
The profiling runtime will not see that symbol go on as if it
were never there. On Fuchsia, the profiling runtime must have
this symbol which will cause a hard link error.

As an aside, I don't have enough context as to why rust's LTO
model is how it is. AFAICT, the internalize pass is only safe
to run at link time when all symbol information is actually
known, this being an example as to why. I think special casing
this symbol as a known one that LLVM can emit which should not
have it's visbility de-escalated should be fine given how
seldom this pattern of defining an undefined symbol to get
initilization code pulled in is. From a quick grep,
__llvm_profile_runtime is the only symbol that rustc does this
for.

[1] https://github.com/rust-lang/rust/blob/0265a3e93bf1b89d97cae113ed214954d5c35e22/compiler/rustc_codegen_ssa/src/back/linker.rs#L598

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() {}