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[MBP] Prevent rotating a chain contains entry block

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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.

Differential Revision: https://reviews.llvm.org/D92882
This commit is contained in:
Guozhi Wei 2020-12-14 12:48:55 -08:00
parent 569f60bb57
commit 43d0dff6c4
1 changed files with 9 additions and 1 deletions
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10
lib/CodeGen/MachineBlockPlacement.cpp
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@ -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) &&