[PATCH] D118597: [DebugInfo][InstrRef][NFC] Don't build mappings of values to locations for un-needed values

[Jeremy Morse via Phabricator via llvm-commits]

jmorse created this revision.
jmorse added reviewers: StephenTozer, Orlando, TWeaver.
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When finding locations for variable values at the start of a block, we build a large map of every value to every location, and then pick out the locations for values that are desired. This takes up quite a lot of time, because, unsurprisingly, there are usually more values in registers and stack slots than there are variables.

This patch instead creates a map of desired values to their locations, which are initially illegal locations. Then, as we examine every available value, we can select locations for values we care about, and ignore those that we don't. This substantially reduces the amount of work done (i.e., building a map up of values to locations that nothing wants or needs).

Geomean performance improvement of 1%: http://llvm-compile-time-tracker.com/compare.php?from=427eff43fcd470caa5cf44e10188c50f89b93af8&to=842aefd941f8e5f2d73b3178fdbbd7d27df9c55a&stat=instructions


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D118597

Files:
  llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp


Index: llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
===================================================================
--- llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
+++ llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
@@ -274,6 +274,13 @@
 
     // Map of the preferred location for each value.
     DenseMap<ValueIDNum, LocIdx> ValueToLoc;
+
+    // Initialized the preferred-location map with illegal locations, to be
+    // filled in later.
+    for (auto &VLoc : VLocs)
+      if (VLoc.second.Kind == DbgValue::Def)
+        ValueToLoc.insert({VLoc.second.ID, LocIdx::MakeIllegalLoc()});
+
     ActiveMLocs.reserve(VLocs.size());
     ActiveVLocs.reserve(VLocs.size());
 
@@ -285,21 +292,20 @@
       ValueIDNum &VNum = MLocs[Idx.asU64()];
       VarLocs.push_back(VNum);
 
-      // Short-circuit unnecessary preferred location update.
-      if (VLocs.empty())
+      // Is there a variable that wants a location for this value? If not, skip.
+      auto VIt = ValueToLoc.find(VNum);
+      if (VIt == ValueToLoc.end())
         continue;
 
-      auto it = ValueToLoc.find(VNum);
+      LocIdx CurLoc = VIt->second;
       // In order of preference, pick:
       //  * Callee saved registers,
       //  * Other registers,
       //  * Spill slots.
-      if (it == ValueToLoc.end() || MTracker->isSpill(it->second) ||
-          (!isCalleeSaved(it->second) && isCalleeSaved(Idx.asU64()))) {
+      if (CurLoc.isIllegal() || MTracker->isSpill(CurLoc) ||
+          (!isCalleeSaved(CurLoc) && isCalleeSaved(Idx.asU64()))) {
         // Insert, or overwrite if insertion failed.
-        auto PrefLocRes = ValueToLoc.insert(std::make_pair(VNum, Idx));
-        if (!PrefLocRes.second)
-          PrefLocRes.first->second = Idx;
+        VIt->second = Idx;
       }
     }
 
@@ -314,7 +320,8 @@
       // If the value has no location, we can't make a variable location.
       const ValueIDNum &Num = Var.second.ID;
       auto ValuesPreferredLoc = ValueToLoc.find(Num);
-      if (ValuesPreferredLoc == ValueToLoc.end()) {
+      if (ValuesPreferredLoc == ValueToLoc.end() ||
+          ValuesPreferredLoc->second.isIllegal()) {
         // If it's a def that occurs in this block, register it as a
         // use-before-def to be resolved as we step through the block.
         if (Num.getBlock() == (unsigned)MBB.getNumber() && !Num.isPHI())


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