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Fix SSA CFG shuffler oscillation #16607

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Merged
clonker merged 1 commit into from
Apr 17, 2026
Merged

Fix SSA CFG shuffler oscillation #16607

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1 change: 1 addition & 0 deletions libyul/backends/evm/ssa/Stack.h
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Original file line number Diff line number Diff line change
Expand Up @@ -252,6 +252,7 @@ class Stack
bool dupReachable(Depth const& _depth) const noexcept { return _depth < size() && _depth.value + 1 <= reachableStackDepth; }
bool isValidSwapTarget(Offset const& _offset) const noexcept { return isValidSwapTarget(offsetToDepth(_offset)); }
bool isValidSwapTarget(Depth const& _depth) const noexcept { return _depth < size() && 1 <= _depth.value && _depth.value <= reachableStackDepth; }
bool isBeyondSwapRange(Offset const& _offset) const noexcept { return isBeyondSwapRange(offsetToDepth(_offset)); }
bool isBeyondSwapRange(Depth const& _depth) const noexcept { return _depth > reachableStackDepth; }

void declareJunk(Offset const& _offset) { (*m_data)[_offset.value] = Slot::makeJunk(); }
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3 changes: 2 additions & 1 deletion libyul/backends/evm/ssa/StackShuffler.h
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Expand Up @@ -471,7 +471,8 @@ class StackShuffler
// swap up any slot in args that is out of position and has a slot available in args that it can occupy
for (StackOffset offset: _state.stackArgsRange())
{
bool const reachable = _stack.isValidSwapTarget(offset);
// when offset is already top no swap-up is needed, so it doesn't have to be a valid swap target itself
bool const reachable = !_stack.isBeyondSwapRange(offset);
bool const identical = _state.isArgsCompatible(offset, stackTop) && !_state.targetArbitrary(stackTop);
if (
reachable &&
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34 changes: 3 additions & 31 deletions test/libyul/ssa/stackShuffler/loop-fuzzer.stack
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Expand Up @@ -6,36 +6,8 @@ targetStackSize: 7
// | 0 1 | 2 3 4 5 6
// +-------------- +-----------------------------------
// (initial)| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// POP| v0 phi0 | phi0 lit0 v0 v0
// DUP4| v0 phi0 | phi0 lit0 v0 v0 phi0
// ...|
// SWAP2| v0 phi0 | phi0 v0 v0 lit0
// DUP4| v0 phi0 | phi0 v0 v0 lit0 phi0
// +-------------- +-----------------------------------
// (target)| {v0} | phi0 v0 v0 * phi0
// Status: MaxIterationsReached
// Status: Admissible