[llvm-branch-commits] [llvm] d50d7c3 - [MBP] Prevent rotating a chain contains entry block

[Guozhi Wei via llvm-branch-commits]

Author: Guozhi Wei
Date: 2020-12-14T12:48:55-08:00
New Revision: d50d7c37a159802c89454a6c53c0ec2e7949d84a

URL: https://github.com/llvm/llvm-project/commit/d50d7c37a159802c89454a6c53c0ec2e7949d84a
DIFF: https://github.com/llvm/llvm-project/commit/d50d7c37a159802c89454a6c53c0ec2e7949d84a.diff

LOG: [MBP] Prevent rotating a chain contains entry block

The entry block should always be the first BB in a function.
So we should not rotate a chain contains the entry block.

Differential Revision: https://reviews.llvm.org/D92882

Added: 
    

Modified: 
    llvm/lib/CodeGen/MachineBlockPlacement.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/MachineBlockPlacement.cpp b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
index bd4640822a63..42586cbe06e0 100644
--- a/llvm/lib/CodeGen/MachineBlockPlacement.cpp
+++ b/llvm/lib/CodeGen/MachineBlockPlacement.cpp
@@ -2306,6 +2306,10 @@ void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
   if (Bottom == ExitingBB)
     return;
 
+  // The entry block should always be the first BB in a function.
+  if (Top->isEntryBlock())
+    return;
+
   bool ViableTopFallthrough = hasViableTopFallthrough(Top, LoopBlockSet);
 
   // If the header has viable fallthrough, check whether the current loop
@@ -2380,6 +2384,11 @@ void MachineBlockPlacement::rotateLoopWithProfile(
     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->isEntryBlock())
+    return;
 
   BlockFrequency SmallestRotationCost = BlockFrequency::getMaxFrequency();
 
@@ -2398,7 +2407,6 @@ void MachineBlockPlacement::rotateLoopWithProfile(
   // 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) &&