[rtsan] Handle attributed IR function declarations

Addresses #169377.

Previously, the RealtimeSanitizer pass only handled attributed function
definitions in IR, and attributed function declarations caused it to
crash. To fix the issue, we must check whether the IR function is empty
before attempting to do any manipulation of its instructions.

Author: davidtrevelyan

llvm/lib/Transforms/Instrumentation/RealtimeSanitizer.cpp

@@ -90,6 +90,9 @@ PreservedAnalyses RealtimeSanitizerPass::run(Module &M,
       [&](Function *Ctor, FunctionCallee) { appendToGlobalCtors(M, Ctor, 0); });
 
   for (Function &F : M) {
+    if (F.empty())
+      continue;
+
     if (F.hasFnAttribute(Attribute::SanitizeRealtime))
       runSanitizeRealtime(F);
 

llvm/test/Instrumentation/RealtimeSanitizer/rtsan_attrib_declare.ll

@@ -0,0 +1,11 @@
+; RUN: opt < %s -passes='rtsan' -S | FileCheck %s
+
+declare void @declared_realtime_function() sanitize_realtime #0
+
+declare void @declared_blocking_function() sanitize_realtime_blocking #0
+
+; RealtimeSanitizer pass should ignore attributed functions that are just declarations
+; CHECK: declared_realtime_function
+; CHECK-EMPTY:
+; CHECK: declared_blocking_function
+; CHECK-EMPTY: