[DebugInfo][RemoveDIs] Don't pointlessly scan funcs for debug-info (#79327)

The utility functions this patch modifies are part of cleanly
transitioning from a context where we use dbg.value intrinsics to one
where we use DPValue objects to record debug-info, and back again.
However, this is a waste of time in non-debug builds (i.e. no -g on the
command line). We still have to call the function on all blocks though
to set the IsNewDbgInfoFormat flag.

To reduce the overhead of this, test whether there's any debug-info in
the function by checking whether the function has a DISubprogram, and
pass a flag down to the utility functions indicating whether they can
skip the scan.

It feels a bit dumb to me now that we're scanning and setting a flag in
a load of blocks when we don't have to -- however it's been really
valuable during development for working out where spurious dbg.value
intrinsics leak into a RemoveDIs context. Happily we'll be able to just
delete this flag entirely when RemoveDIs lands and sticks, and the
conversion routines will eventually be pushed down into the debug-info
autoupgrade path.

Author: jmorse

llvm/include/llvm/IR/BasicBlock.h

@@ -81,12 +81,16 @@ class BasicBlock final : public Value, // Basic blocks are data objects also
   /// intrinsics into DPMarker / DPValue records. Deletes all dbg.values in
   /// the process and sets IsNewDbgInfoFormat = true. Only takes effect if
   /// the UseNewDbgInfoFormat LLVM command line option is given.
-  void convertToNewDbgValues();
+  /// \param HasNoDebugInfo Flag indicating the scan of instructions should be
+  /// skipped as the function is known to have no debug-info.
+  void convertToNewDbgValues(bool HasNoDebugInfo = false);
 
   /// Convert variable location debugging information stored in DPMarkers and
   /// DPValues into the dbg.value intrinsic representation. Sets
   /// IsNewDbgInfoFormat = false.
-  void convertFromNewDbgValues();
+  /// \param HasNoDebugInfo Flag indicating the scan of instructions should be
+  /// skipped as the function is known to have no debug-info.
+  void convertFromNewDbgValues(bool HasNoDebugInfo = false);
 
   /// Ensure the block is in "old" dbg.value format (\p NewFlag == false) or
   /// in the new format (\p NewFlag == true), converting to the desired format

llvm/lib/IR/BasicBlock.cpp

@@ -60,13 +60,17 @@ DPMarker *BasicBlock::createMarker(InstListType::iterator It) {
   return DPM;
 }
 
-void BasicBlock::convertToNewDbgValues() {
+void BasicBlock::convertToNewDbgValues(bool HasNoDebugInfo) {
   // Is the command line option set?
   if (!UseNewDbgInfoFormat)
     return;
 
   IsNewDbgInfoFormat = true;
 
+  // If the caller knows there's no debug-info in this function, do nothing.
+  if (HasNoDebugInfo)
+    return;
+
   // Iterate over all instructions in the instruction list, collecting dbg.value
   // instructions and converting them to DPValues. Once we find a "real"
   // instruction, attach all those DPValues to a DPMarker in that instruction.
@@ -93,10 +97,14 @@ void BasicBlock::convertToNewDbgValues() {
   }
 }
 
-void BasicBlock::convertFromNewDbgValues() {
+void BasicBlock::convertFromNewDbgValues(bool HasNoDebugInfo) {
   invalidateOrders();
   IsNewDbgInfoFormat = false;
 
+  // If the caller knows there's no debug-info in this function, do nothing.
+  if (HasNoDebugInfo)
+    return;
+
   // Iterate over the block, finding instructions annotated with DPMarkers.
   // Convert any attached DPValues to dbg.values and insert ahead of the
   // instruction.

llvm/lib/IR/Function.cpp

@@ -83,15 +83,17 @@ static cl::opt<unsigned> NonGlobalValueMaxNameSize(
 
 void Function::convertToNewDbgValues() {
   IsNewDbgInfoFormat = true;
+  bool HasNoDebugInfo = getSubprogram() == nullptr;
   for (auto &BB : *this) {
-    BB.convertToNewDbgValues();
+    BB.convertToNewDbgValues(HasNoDebugInfo);
   }
 }
 
 void Function::convertFromNewDbgValues() {
   IsNewDbgInfoFormat = false;
+  bool HasNoDebugInfo = getSubprogram() == nullptr;
   for (auto &BB : *this) {
-    BB.convertFromNewDbgValues();
+    BB.convertFromNewDbgValues(HasNoDebugInfo);
   }
 }