From 91fb66cf91ae557d45bde7931cbb364f1c08d245 Mon Sep 17 00:00:00 2001
From: Jeremy Morse <jeremy.morse@sony.com>
Date: Tue, 1 Feb 2022 18:55:08 +0000
Subject: [PATCH] [DebugInfo][InstrRef][NFC] Don't build a map of un-needed
 values

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% on CTMark, woo.

Differential Revision: https://reviews.llvm.org/D118597
---
 .../CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp  | 24 ++++++++++++++--------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
index 90cd448eefc4..61d127d7a294 100644
--- a/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
+++ b/llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp
@@ -274,6 +274,13 @@ public:
 
     // 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 @@ public:
       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 +320,7 @@ public:
       // 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->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())
-- 
2.11.4.GIT