[PATCH] D86153: [DwarfDebug] Improve validThroughout performance (4/4)

[Orlando Cazalet-Hyams via Phabricator via llvm-commits]

Orlando created this revision.
Orlando added reviewers: vsk, djtodoro, dblaikie, aprantl.
Orlando added projects: LLVM, debug-info.
Herald added subscribers: llvm-commits, hiraditya.
Orlando requested review of this revision.

Almost NFC (see end).

The backwards scan in validThroughout significantly contributed to compile time
for a pathological case, causing the 'X86 Assembly Printer' pass to account for
roughly 70% of the run time. This patch guards the loop against running
unnecessarily for a common case, bringing the pass contribution down to 4%.

Almost NFC:
There is a hack in validThroughout which promotes single constant value
DBG_VALUEs in the prologue to be live throughout the function. We're more
likely to hit this code path with this patch applied. Similarly to the parent patches
there is a small coverage change reported in the order of 10s of bytes.


https://reviews.llvm.org/D86153

Files:
  llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp


Index: llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
@@ -1531,27 +1531,30 @@
   if (LSRange.size() == 0)
     return false;
 
-
-  // Determine if the DBG_VALUE is valid at the beginning of its lexical block.
   const MachineInstr *LScopeBegin = LSRange.front().first;
-  // Early exit if the lexical scope begins outside of the current block.
-  if (LScopeBegin->getParent() != MBB)
-    return false;
-
-  MachineBasicBlock::const_reverse_iterator Pred(DbgValue);
-  for (++Pred; Pred != MBB->rend(); ++Pred) {
-    if (Pred->getFlag(MachineInstr::FrameSetup))
-      break;
-    auto PredDL = Pred->getDebugLoc();
-    if (!PredDL || Pred->isMetaInstruction())
-      continue;
-    // Check whether the instruction preceding the DBG_VALUE is in the same
-    // (sub)scope as the DBG_VALUE.
-    if (DL->getScope() == PredDL->getScope())
-      return false;
-    auto *PredScope = LScopes.findLexicalScope(PredDL);
-    if (!PredScope || LScope->dominates(PredScope))
+  // If the scope starts before the DBG_VALUE then we may have a negative
+  // result. Otherwise the location is live coming into the scope and we
+  // can skip the following checks.
+  if (!Ordering.isBefore(DbgValue, LScopeBegin)) {
+    // Exit if the lexical scope begins outside of the current block.
+    if (LScopeBegin->getParent() != MBB)
       return false;
+
+    MachineBasicBlock::const_reverse_iterator Pred(DbgValue);
+    for (++Pred; Pred != MBB->rend(); ++Pred) {
+      if (Pred->getFlag(MachineInstr::FrameSetup))
+        break;
+      auto PredDL = Pred->getDebugLoc();
+      if (!PredDL || Pred->isMetaInstruction())
+        continue;
+      // Check whether the instruction preceding the DBG_VALUE is in the same
+      // (sub)scope as the DBG_VALUE.
+      if (DL->getScope() == PredDL->getScope())
+        return false;
+      auto *PredScope = LScopes.findLexicalScope(PredDL);
+      if (!PredScope || LScope->dominates(PredScope))
+        return false;
+    }
   }
 
   // If the range of the DBG_VALUE is open-ended, report success.


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