[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

@@ -61,23 +61,21 @@ static void insertCallAtAllFunctionExitPoints(Function &Fn,
       insertCallBeforeInstruction(Fn, I, InsertFnName, FunctionArgs);
 }
 
-static PreservedAnalyses rtsanPreservedCFGAnalyses() {
-  PreservedAnalyses PA;
-  PA.preserveSet<CFGAnalyses>();
-  return PA;
-}
+static void runSanitizeRealtime(Function &Fn) {
+  if (Fn.empty())
+    return;
 
-static PreservedAnalyses runSanitizeRealtime(Function &Fn) {
   insertCallAtFunctionEntryPoint(Fn, "__rtsan_realtime_enter", {});
   insertCallAtAllFunctionExitPoints(Fn, "__rtsan_realtime_exit", {});
-  return rtsanPreservedCFGAnalyses();
 }
 
-static PreservedAnalyses runSanitizeRealtimeBlocking(Function &Fn) {
+static void runSanitizeRealtimeBlocking(Function &Fn) {
+  if (Fn.empty())
+    return;
+
   IRBuilder<> Builder(&Fn.front().front());
   Value *Name = Builder.CreateGlobalString(demangle(Fn.getName()));
   insertCallAtFunctionEntryPoint(Fn, "__rtsan_notify_blocking_call", {Name});
-  return rtsanPreservedCFGAnalyses();
 }
 
 PreservedAnalyses RealtimeSanitizerPass::run(Module &M,

llvm/test/Instrumentation/RealtimeSanitizer/rtsan_attrib_declare.ll

@@ -0,0 +1,12 @@
+; 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:
+