[LLVMdev] [PATCH] Implement dbgs()

[David Greene]

On Monday 21 December 2009 10:14, Török Edwin wrote:
> On 2009-12-21 18:06, David Greene wrote:
> > On Saturday 19 December 2009 00:16, Chris Lattner wrote:
> >>> Or I think I can just assume (Yikes!) that if the signal handler is
> >>> invoked it will really be a circular_raw_ostream since the handler
> >>> should (!) only be set up in debug mode.
> >>>
> >>> That scares me a bit, though.
> >>
> >> Why don't you just check #ifndef NDEBUG like the code that sets it up?
> >
> > Next iteration.
>
> Will this setup a signal handler in a Release+Asserts build?
> That is not very nice, a library shouldn't setup signal handlers, except
> in debug mode.

Here's a revision.  It only adds the handler if DisableDebugBuffering is 
false, DebugFlag is true and the debug buffer size is non-zero.

                                  -Dave

Index: include/llvm/Support/Debug.h
===================================================================
--- include/llvm/Support/Debug.h	(revision 91825)
+++ include/llvm/Support/Debug.h	(working copy)
@@ -28,6 +28,8 @@
 
 namespace llvm {
 
+class raw_ostream;
+
 /// DEBUG_TYPE macro - Files can specify a DEBUG_TYPE as a string, which 
causes
 /// all of their DEBUG statements to be activatable with 
-debug-only=thatstring.
 #ifndef DEBUG_TYPE
@@ -58,7 +60,7 @@
 /// this is a debug build, then the code specified as the option to the macro
 /// will be executed.  Otherwise it will not be.  Example:
 ///
-/// DEBUG_WITH_TYPE("bitset", errs() << "Bitset contains: " << Bitset << 
"\n");
+/// DEBUG_WITH_TYPE("bitset", dbgs() << "Bitset contains: " << Bitset << 
"\n");
 ///
 /// This will emit the debug information if -debug is present, and 
-debug-only
 /// is not specified, or is specified as "bitset".
@@ -72,15 +74,28 @@
 #define DEBUG_WITH_TYPE(TYPE, X) do { } while (0)
 #endif
 
+/// DisableDebugBuffering - This defaults to false.  If true, the
+/// debug stream will not install signal handlers and thus will not
+/// buffer any output since there will be no way to get at it.  It
+/// allows clients to disable signal handler install in case of a
+/// conflict
+///
+extern bool DisableDebugBuffering;
+
+/// dbgs() - This returns a reference to a raw_ostream for debugging
+/// messages.  If debugging is disabled it returns dbgs().  Use it
+/// like: dbgs() << "foo" << "bar";
+raw_ostream &dbgs();
+
 // DEBUG macro - This macro should be used by passes to emit debug 
information.
 // In the '-debug' option is specified on the commandline, and if this is a
 // debug build, then the code specified as the option to the macro will be
 // executed.  Otherwise it will not be.  Example:
 //
-// DEBUG(errs() << "Bitset contains: " << Bitset << "\n");
+// DEBUG(dbgs() << "Bitset contains: " << Bitset << "\n");
 //
 #define DEBUG(X) DEBUG_WITH_TYPE(DEBUG_TYPE, X)
-  
+
 } // End llvm namespace
 
 #endif
Index: lib/Support/Debug.cpp
===================================================================
--- lib/Support/Debug.cpp	(revision 91825)
+++ lib/Support/Debug.cpp	(working copy)
@@ -25,6 +25,9 @@
 
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/circular_raw_ostream.h"
+#include "llvm/System/Signals.h"
+
 using namespace llvm;
 
 // All Debug.h functionality is a no-op in NDEBUG mode.
@@ -37,6 +40,16 @@
 Debug("debug", cl::desc("Enable debug output"), cl::Hidden,
       cl::location(DebugFlag));
 
+// -debug-buffer-size - Buffer the last N characters of debug output
+//until program termination.
+static cl::opt<unsigned>
+DebugBufferSize("debug-buffer-size",
+                cl::desc("Buffer the last N characters of debug output"
+                         "until program termination. "
+                         "[default 0 -- immediate print-out]"),
+                cl::Hidden,
+                cl::init(0));
+
 static std::string CurrentDebugType;
 static struct DebugOnlyOpt {
   void operator=(const std::string &Val) const {
@@ -50,6 +63,18 @@
           cl::Hidden, cl::value_desc("debug string"),
           cl::location(DebugOnlyOptLoc), cl::ValueRequired);
 
+// Signal handlers - dump debug output on termination.
+static void debug_user_sig_handler(void *Cookie)
+{
+  // This is a bit sneaky.  Since this is under #ifndef NDEBUG, we
+  // know that debug mode is enabled and dbgs() really is a
+  // circular_raw_ostream.  If NDEBUG is defined, then dbgs() ==
+  // errs() but this will never be invoked.
+  llvm::circular_raw_ostream *dbgout =
+    static_cast<llvm::circular_raw_ostream *>(&llvm::dbgs());
+  dbgout->flushBufferWithBanner();
+}
+
 // isCurrentDebugType - Return true if the specified string is the debug type
 // specified on the command line, or if none was specified on the command 
line
 // with the -debug-only=X option.
@@ -66,9 +91,38 @@
   CurrentDebugType = Type;
 }
 
+/// DisableDebugBuffering - Turn off signal handler installation.
+///
+bool llvm::DisableDebugBuffering = false;
+
+/// dbgs - Return a circular-buffered debug stream.
+raw_ostream &llvm::dbgs() {
+  // Do one-time initialization in a thread-safe way.
+  static struct dbgstream {
+    circular_raw_ostream strm;
+
+    dbgstream() :
+        strm(errs(),
+             (DisableDebugBuffering || !DebugFlag) ? 0 : DebugBufferSize) {
+      if (!DisableDebugBuffering && DebugFlag && DebugBufferSize != 0)
+        // TODO: Add a handler for SIGUSER1-type signals so the user can
+        // force a debug dump.
+        sys::AddSignalHandler(&debug_user_sig_handler, 0);
+      // Otherwise we've already set the debug stream buffer size to
+      // zero, disabling buffering.
+    }
+  } thestrm;
+
+  return thestrm.strm;
+}
+
 #else
 // Avoid "has no symbols" warning.
 namespace llvm {
-int Debug_dummy = 0;
+  /// dbgs - Return dbgs().
+  raw_ostream &dbgs() {
+    return dbgs();
+  }
 }
+
 #endif