[PATCH] D118613: [DebugInfo][InstrRef][NFC] Bypass a frequently-noop loop

[Jeremy Morse via Phabricator via llvm-commits]

jmorse created this revision.
jmorse added reviewers: StephenTozer, Orlando, TWeaver.
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Bypass a loop if it would do nothing -- if there are no register masks to be examined, there's no point looking at each location to see if the location has been def'd. Awkwardly, this was responsible for almost an entire half a percent of performance improvement on the tracker [0].

[0] http://llvm-compile-time-tracker.com/compare.php?from=47a80eaac7a4fb1b115a5fb467ce03b51182fc56&to=19a73bdd29dcc0f522aba1e722f7a3a16cbea4dd&stat=instructions


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D118613

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
@@ -1374,18 +1374,20 @@
 
   // Look for any clobbers performed by a register mask. Only test locations
   // that are actually being tracked.
-  for (auto L : MTracker->locations()) {
-    // Stack locations can't be clobbered by regmasks.
-    if (MTracker->isSpill(L.Idx))
-      continue;
+  if (!RegMaskPtrs.empty()) {
+    for (auto L : MTracker->locations()) {
+      // Stack locations can't be clobbered by regmasks.
+      if (MTracker->isSpill(L.Idx))
+        continue;
 
-    Register Reg = MTracker->LocIdxToLocID[L.Idx];
-    if (IgnoreSPAlias(Reg))
-      continue;
+      Register Reg = MTracker->LocIdxToLocID[L.Idx];
+      if (IgnoreSPAlias(Reg))
+        continue;
 
-    for (auto *MO : RegMaskPtrs)
-      if (MO->clobbersPhysReg(Reg))
-        TTracker->clobberMloc(L.Idx, MI.getIterator(), false);
+      for (auto *MO : RegMaskPtrs)
+        if (MO->clobbersPhysReg(Reg))
+          TTracker->clobberMloc(L.Idx, MI.getIterator(), false);
+    }
   }
 
   // Tell TTracker about any folded stack store.


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