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Clarification with some comments and another assertion to help understanding the preconditions.

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This commit is contained in:
Daniel Kirchner 2021-08-17 18:59:57 +02:00
parent 55e60bd493
commit 1e3034a0eb
1 changed files with 9 additions and 5 deletions
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14
libyul/backends/evm/StackHelpers.h
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@ -107,8 +107,8 @@ class Shuffler
public: public:
/// Executes the stack shuffling operations. Instantiates an instance of ShuffleOperations /// Executes the stack shuffling operations. Instantiates an instance of ShuffleOperations
/// in each iteration. Each iteration performs exactly one operation that modifies the stack. /// in each iteration. Each iteration performs exactly one operation that modifies the stack.
/// After shuffle, source and target have the same size and all slots in the source layout are /// After `shuffle`, source and target have the same size and all slots in the source layout are
/// compatible to the slots at the same target offset. /// compatible with the slots at the same target offset.
template<typename... Args> template<typename... Args>
static void shuffle(Args&&... args) static void shuffle(Args&&... args)
{ {
@ -129,7 +129,7 @@ private:
// Check if the stack is large enough for anything to potentially become unreachable. // Check if the stack is large enough for anything to potentially become unreachable.
if (_ops.sourceSize() < 15) if (_ops.sourceSize() < 15)
return false; return false;
// Check whether any deep slot might still be needed later. // Check whether any deep slot might still be needed later (i.e. we still need to reach it with a DUP or SWAP).
for (size_t sourceOffset: ranges::views::iota(0u, _ops.sourceSize() - 15)) for (size_t sourceOffset: ranges::views::iota(0u, _ops.sourceSize() - 15))
{ {
// We need another copy of this slot. // We need another copy of this slot.
@ -173,7 +173,7 @@ private:
} }
return false; return false;
} }
/// Finds a slot to dups or pushes with the aim of eventually fixing @a _targetOffset in the target. /// Finds a slot to dup or push with the aim of eventually fixing @a _targetOffset in the target.
/// In the simplest case, the slot at @a _targetOffset has a multiplicity > 0, i.e. it can directly be dupped or pushed /// In the simplest case, the slot at @a _targetOffset has a multiplicity > 0, i.e. it can directly be dupped or pushed
/// and the next iteration will fix @a _targetOffset. /// and the next iteration will fix @a _targetOffset.
/// But, in general, there may already be enough copies of the slot that is supposed to end up at @a _targetOffset /// But, in general, there may already be enough copies of the slot that is supposed to end up at @a _targetOffset
@ -197,7 +197,7 @@ private:
_ops.pushOrDupTarget(offset); _ops.pushOrDupTarget(offset);
return true; return true;
} }
// The desired target slot must already be somewhere else on stack right now. // There must be another slot we can dup/push that will lead to the target slot at ``offset`` to be fixed.
for (auto nextOffset: ranges::views::iota(0u, std::min(_ops.sourceSize(), _ops.targetSize()))) for (auto nextOffset: ranges::views::iota(0u, std::min(_ops.sourceSize(), _ops.targetSize())))
if ( if (
!_ops.isCompatible(nextOffset, nextOffset) && !_ops.isCompatible(nextOffset, nextOffset) &&
@ -258,6 +258,10 @@ private:
return true; return true;
} }
// ops.sourceSize() > ops.targetSize() cannot be true anymore, since if the source top is no longer required,
// we already popped it, and if it is required, we already swapped it down to a suitable target position.
yulAssert(ops.sourceSize() <= ops.targetSize(), "");
// If a lower slot should be removed, try to bring up the slot that should end up there and bring it up. // If a lower slot should be removed, try to bring up the slot that should end up there and bring it up.
// Note that after the cases above, there will always be a target slot to duplicate in this case. // Note that after the cases above, there will always be a target slot to duplicate in this case.
for (size_t offset: ranges::views::iota(0u, ops.sourceSize())) for (size_t offset: ranges::views::iota(0u, ops.sourceSize()))