[PATCH] D92882: [MBP] Prevent rotating a chain contains entry block

[Guozhi Wei via Phabricator via llvm-commits]

Carrot created this revision.
Carrot added reviewers: davidxl, xur.
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The entry block should always be the first BB in a function. So we should not rotate a chain contains the entry block.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D92882

Files:
  llvm/lib/CodeGen/MachineBlockPlacement.cpp


Index: llvm/lib/CodeGen/MachineBlockPlacement.cpp
===================================================================
--- llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -2308,6 +2308,10 @@
   if (Bottom == ExitingBB)
     return;
 
+  // The entry block should always be the first BB in a function.
+  if (Top == &F->front())
+    return;
+
   bool ViableTopFallthrough = hasViableTopFallthrough(Top, LoopBlockSet);
 
   // If the header has viable fallthrough, check whether the current loop
@@ -2382,6 +2386,11 @@
     BlockChain &LoopChain, const MachineLoop &L,
     const BlockFilterSet &LoopBlockSet) {
   auto RotationPos = LoopChain.end();
+  MachineBasicBlock *ChainHeaderBB = *LoopChain.begin();
+
+  // The entry block should always be the first BB in a function.
+  if (ChainHeaderBB == &F->front())
+    return;
 
   BlockFrequency SmallestRotationCost = BlockFrequency::getMaxFrequency();
 
@@ -2400,7 +2409,6 @@
   // chain head is not the loop header. As we only consider natural loops with
   // single header, this computation can be done only once.
   BlockFrequency HeaderFallThroughCost(0);
-  MachineBasicBlock *ChainHeaderBB = *LoopChain.begin();
   for (auto *Pred : ChainHeaderBB->predecessors()) {
     BlockChain *PredChain = BlockToChain[Pred];
     if (!LoopBlockSet.count(Pred) &&


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