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[release/6.0] JIT: fix scalability issue in redundant branch optimizer #68972

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May 14, 2022
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[release/6.0] JIT: fix scalability issue in redundant branch optimizer #68972

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18 changes: 15 additions & 3 deletions src/coreclr/jit/redundantbranchopts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -106,9 +106,11 @@ bool Compiler::optRedundantBranch(BasicBlock* const block)
// Walk up the dom tree and see if any dominating block has branched on
// exactly this tree's VN...
//
BasicBlock* prevBlock = block;
BasicBlock* domBlock = block->bbIDom;
int relopValue = -1;
BasicBlock* prevBlock = block;
BasicBlock* domBlock = block->bbIDom;
int relopValue = -1;
unsigned matchCount = 0;
const unsigned matchLimit = 4;

if (domBlock == nullptr)
{
Expand Down Expand Up @@ -140,6 +142,16 @@ bool Compiler::optRedundantBranch(BasicBlock* const block)
//
if (domCmpVN == tree->GetVN(VNK_Liberal))
{
// If we have a long skinny dominator tree we may scale poorly,
// and in particular reachability (below) is costly. Give up if
// we've matched a few times and failed to optimize.
//
if (++matchCount > matchLimit)
{
JITDUMP("Bailing out; %d matches found w/o optimizing\n", matchCount);
return false;
}

// The compare in "tree" is redundant.
// Is there a unique path from the dominating compare?
//
Expand Down