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[release/6.0] JIT: Abandon loop search if we reach the end of the bbNext chain #69525

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Jun 9, 2022
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[release/6.0] JIT: Abandon loop search if we reach the end of the bbNext chain #69525

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17 changes: 15 additions & 2 deletions src/coreclr/jit/optimizer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1928,13 +1928,26 @@ class LoopSearch
// This blocks is lexically between TOP and BOTTOM, but it does not
// participate in the flow cycle. Check for a run of consecutive
// such blocks.
//
// If blocks have been reordered and bbNum no longer reflects bbNext ordering
// (say by a call to MakeCompactAndFindExits for an earlier loop or unsuccessful
// attempt to find a loop), the bottom block of this loop may now appear earlier
// in the bbNext chain than other loop blocks. So when the previous hasn't reached bottom
// and block is a non-loop block, and we walk the bbNext chain, we may reach the end.
// If so, give up on recognition of this loop.
//
BasicBlock* lastNonLoopBlock = block;
BasicBlock* nextLoopBlock = block->bbNext;
while (!loopBlocks.IsMember(nextLoopBlock->bbNum))
while ((nextLoopBlock != nullptr) && !loopBlocks.IsMember(nextLoopBlock->bbNum))
{
lastNonLoopBlock = nextLoopBlock;
nextLoopBlock = nextLoopBlock->bbNext;
// This loop must terminate because we know BOTTOM is in loopBlocks.
}

if (nextLoopBlock == nullptr)
{
JITDUMP("Did not find expected loop block when walking from " FMT_BB "\n", lastNonLoopBlock->bbNum);
return false;
}

// Choose an insertion point for non-loop blocks if we haven't yet done so.
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