[PATCH] D95631: [TailDuplicator] Add TargetInstrInfo hook to modify the TailDuplicateSize default threshold

[Nicholas Guy via Phabricator via llvm-commits]

NickGuy created this revision.
NickGuy added reviewers: SjoerdMeijer, dmgreen.
NickGuy added a project: LLVM.
Herald added a subscriber: hiraditya.
NickGuy requested review of this revision.

Different targets might handle branch performance differently, so this patch allows for
targets to specify the TailDuplicateSize threshold. Said threshold defines how small a branch
can be and still be duplicated to generate straight-line code instead.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D95631

Files:
  llvm/include/llvm/CodeGen/TargetInstrInfo.h
  llvm/lib/CodeGen/TailDuplicator.cpp


Index: llvm/lib/CodeGen/TailDuplicator.cpp
===================================================================
--- llvm/lib/CodeGen/TailDuplicator.cpp
+++ llvm/lib/CodeGen/TailDuplicator.cpp
@@ -564,12 +564,17 @@
   // duplicate only one, because one branch instruction can be eliminated to
   // compensate for the duplication.
   unsigned MaxDuplicateCount;
+
   bool OptForSize = MF->getFunction().hasOptSize() ||
                     llvm::shouldOptimizeForSize(&TailBB, PSI, MBFI);
-  if (TailDupSize == 0)
-    MaxDuplicateCount = TailDuplicateSize;
-  else
+  if (TailDupSize == 0) {
+    if (TailDuplicateSize.getNumOccurrences() > 0)
+      MaxDuplicateCount = TailDuplicateSize;
+    else
+      MaxDuplicateCount = TII->getTailDuplicateSizeOverride();
+  } else
     MaxDuplicateCount = TailDupSize;
+
   if (OptForSize)
     MaxDuplicateCount = 1;
 
Index: llvm/include/llvm/CodeGen/TargetInstrInfo.h
===================================================================
--- llvm/include/llvm/CodeGen/TargetInstrInfo.h
+++ llvm/include/llvm/CodeGen/TargetInstrInfo.h
@@ -1937,6 +1937,10 @@
     return Formatter.get();
   }
 
+  /// Returns the target-specific default value for tail duplication.
+  /// This value will be used if the tail-dup-size argument is not provided.
+  virtual unsigned getTailDuplicateSizeOverride() const { return 2; }
+
 private:
   mutable std::unique_ptr<MIRFormatter> Formatter;
   unsigned CallFrameSetupOpcode, CallFrameDestroyOpcode;


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