[Error] Add runtime flag BreakOnError and a debug trap in the Error class

[sgraenitz]

Summary

Small debugging utility to stop immediately when an Error occurs and not only in abort() far up the stack.
Enable/disable in your debug session with one of these:

(lldb) expr BreakOnError = true
(lldb) expr llvm::Error::BreakOnError = true

[lhames]

Hi Stefan,

I usually find it better to put a breakpoint on the ErrorInfoBase constructor: that way you don't hit breakpoints when errors are just passed around.

Would using a breakpoint on ErrorInfoBase address your use case?

[sgraenitz]

Move BreakOnError check to ErrorInfoBase constructor

[sgraenitz]

Hi Lang, thanks for your feedback. Very good point. Changed that!

The additional static variable is not considered ABI-breaking right?

[sgraenitz]

In fact, (lldb) b ErrorInfoBase::ErrorInfoBase is just as good and doesn't need any code change. Is it documented somewhere?

[lhames]

Hi Stefan,

The additional static variable is not considered ABI-breaking right?

No, it would not be.

In fact, (lldb) b ErrorInfoBase::ErrorInfoBase is just as good and doesn't need any code change. Is it documented somewhere?

No, I do not think it is. That would be a good tip to add to the programmer's manual.

include/llvm/Support/Error.h

#endif
   }
 
   void setPtr(ErrorInfoBase *EI) {
 #if LLVM_ENABLE_ABI_BREAKING_CHECKS
     Payload = reinterpret_cast<ErrorInfoBase*>(
                 (reinterpret_cast<uintptr_t>(EI) &
                  ~static_cast<uintptr_t>(0x1)) |
                 (reinterpret_cast<uintptr_t>(Payload) & 0x1));
+
+# if !defined(NDEBUG)
+    if (BreakOnError && EI != nullptr) {
+      LLVM_BUILTIN_DEBUGTRAP;
+    }
+# endif
 #else
     Payload = EI;
 #endif
   }
 
   bool getChecked() const {
 #if LLVM_ENABLE_ABI_BREAKING_CHECKS
     return (reinterpret_cast<uintptr_t>(Payload) & 0x1) == 0;
friend raw_ostream &operator<<(raw_ostream &OS, const Error &E) {
     if (auto P = E.getPtr())
       P->log(OS);
     else
       OS << "success";
     return OS;
   }
 
   ErrorInfoBase *Payload = nullptr;
+
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS && !defined(NDEBUG)
+  // Set to true in debug sessions to immediately break when an error occcurs.
+  static bool BreakOnError;
+#endif
 };
 
 /// Subclass of Error for the sole purpose of identifying the success path in
 /// the type system. This allows to catch invalid conversion to Expected<T> at
 /// compile time.
 class ErrorSuccess : public Error {};
 
 inline ErrorSuccess Error::success() { return ErrorSuccess(); }

lib/Support/Error.cpp

 namespace llvm {
 
 void ErrorInfoBase::anchor() {}
 char ErrorInfoBase::ID = 0;
 char ErrorList::ID = 0;
 char ECError::ID = 0;
 char StringError::ID = 0;
 
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS && !defined(NDEBUG)
+bool Error::BreakOnError = false;
+#endif
+
 void logAllUnhandledErrors(Error E, raw_ostream &OS, Twine ErrorBanner) {
   if (!E)
     return;
   OS << ErrorBanner;
   handleAllErrors(std::move(E), [&](const ErrorInfoBase &EI) {
     EI.log(OS);
     OS << "\n";
   });