[PassInstrumentation] Don't insert extra entries in getPassNameForClassName (#150029)

Don't modify ClassToPassName map unless ClassName is found. Instead,
just return empty StringRef if there is no matching entry. This will
prevent possible dangling references in ClassToPassName map in case of
ClassName being freed.
See https://github.com/llvm/llvm-project/pull/145059/files#r2219763671
for more context.

Author: danilaml

llvm/include/llvm/IR/PassInstrumentation.h

@@ -164,7 +164,7 @@ class PassInstrumentationCallbacks {
 
   /// Add a class name to pass name mapping for use by pass instrumentation.
   LLVM_ABI void addClassToPassName(StringRef ClassName, StringRef PassName);
-  /// Get the pass name for a given pass class name.
+  /// Get the pass name for a given pass class name. Empty if no match found.
   LLVM_ABI StringRef getPassNameForClassName(StringRef ClassName);
 
 private:

llvm/lib/IR/PassInstrumentation.cpp

@@ -23,6 +23,7 @@ template struct LLVM_EXPORT_TEMPLATE Any::TypeId<const Loop *>;
 
 void PassInstrumentationCallbacks::addClassToPassName(StringRef ClassName,
                                                       StringRef PassName) {
+  assert(!PassName.empty() && "PassName can't be empty!");
   ClassToPassName.try_emplace(ClassName, PassName.str());
 }
 
@@ -33,7 +34,10 @@ PassInstrumentationCallbacks::getPassNameForClassName(StringRef ClassName) {
       Fn();
     ClassToPassNameCallbacks.clear();
   }
-  return ClassToPassName[ClassName];
+  auto PassNameIter = ClassToPassName.find(ClassName);
+  if (PassNameIter != ClassToPassName.end())
+    return PassNameIter->second;
+  return {};
 }
 
 AnalysisKey PassInstrumentationAnalysis::Key;