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SSA shuffler: "all necessary slots reachable" should ignore junk #16609

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clonker merged 2 commits into from
Apr 19, 2026
+56 −2
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SSA shuffler: "all necessary slots reachable" should ignore junk #16609

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b898e0c
ssa shuffler: add tests where the stack has to grow by one by pulling…
clonker Apr 18, 2026
d0192f4
ssa shuffler: allNecessarySlotsReachableOrFinal ignores junk in targe…
clonker Apr 18, 2026
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10 changes: 8 additions & 2 deletions libyul/backends/evm/ssa/StackShuffler.h
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Original file line number Diff line number Diff line change
Expand Up @@ -807,12 +807,18 @@ class StackShuffler
{
if (_state.isArgsCompatible(offset, offset))
continue;

auto const& targetArg = _state.targetArg(offset);
// if the target arg is junk, we can simply push0 and it's fine
if (targetArg.isJunk())
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@clonker clonker Apr 18, 2026

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this condition should in the future, i think, be extended to "is it junk or a lit that is cheap to push or we don't care about bytecode size and simply want fast execution"

continue;

// find first occurrence of the slot
std::optional<StackDepth> depth = _stack.findSlotDepth(_state.targetArg(offset));
std::optional<StackDepth> const depth = _stack.findSlotDepth(targetArg);
if (!depth)
{
// if there is no occurrence of the slot anywhere, we must be able to freely generate it
yulAssert(_stack.canBeFreelyGenerated(_state.targetArg(offset)));
yulAssert(_stack.canBeFreelyGenerated(targetArg));
}
else
{
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12 changes: 12 additions & 0 deletions test/libyul/ssa/stackShuffler/stack_top_is_junk.stack
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@@ -0,0 +1,12 @@
initial: [JUNK, JUNK, v0, v0, lit0, v0, lit0, v0, v0, v0, lit0, v0, v0, v0, lit0, v0, lit0, lit0, v0]
targetStackTop: [JUNK, lit0, v0, lit0, v0, JUNK, v0, lit0, JUNK, v0, v0, lit0, v0, JUNK, lit0, v0, JUNK]
targetStackTailSet: {v0}
targetStackSize: 20
// ----
// | 0 1 2 | 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
// +--------------------- +-----------------------------------------------------------------------------------------------------------------------
// (initial)| * * v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit0 v0
// PUSH JUNK| * * v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit0 v0 *
// +--------------------- +-----------------------------------------------------------------------------------------------------------------------
// (target)| {v0} | * lit0 v0 lit0 v0 * v0 lit0 * v0 v0 lit0 v0 * lit0 v0 *
// Status: Admissible
18 changes: 18 additions & 0 deletions test/libyul/ssa/stackShuffler/stack_top_is_lit.stack
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@@ -0,0 +1,18 @@
initial: [lit1, lit1, v0, v0, lit0, v0, lit0, v0, v0, v0, lit0, v0, v0, v0, lit0, v0, lit0, lit0, v0]
targetStackTop: [JUNK, lit0, v0, lit0, v0, JUNK, v0, lit0, JUNK, v0, v0, lit0, v0, JUNK, lit0, v0, lit1]
targetStackTailSet: {v0}
targetStackSize: 20
Comment on lines +1 to +4
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@blishko blishko Apr 18, 2026

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I am a bit confused.
Why is it trying to reach for lit1 instead of just doing PUSH lit1?
This should be just one instruction, similar to the JUNK case.

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@blishko blishko Apr 18, 2026

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Oh, now I understand your comment.
PUSH lit1 is potentially expensive, if lit1 is large.
As you say, at some point it would be good to come up with some heuristic to re-create the literal instead of trying to dup/swap existing one.

// ----
// | 0 1 2 | 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
// +--------------------- +-----------------------------------------------------------------------------------------------------------------------
// (initial)| lit1 lit1 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit0 v0
// SWAP1| lit1 lit1 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 v0 lit0
// POP| lit1 lit1 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 v0
// SWAP16| lit1 v0 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit1
// DUP2| lit1 v0 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit1 lit0
// SWAP1| lit1 v0 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit0 lit1
// DUP4| lit1 v0 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit0 lit1 v0
// SWAP1| lit1 v0 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit0 v0 lit1
// +--------------------- +-----------------------------------------------------------------------------------------------------------------------
// (target)| {v0} | * lit0 v0 lit0 v0 * v0 lit0 * v0 v0 lit0 v0 * lit0 v0 lit1
// Status: Admissible
18 changes: 18 additions & 0 deletions test/libyul/ssa/stackShuffler/stack_top_is_var.stack
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@@ -0,0 +1,18 @@
initial: [v1, v1, v0, v0, lit0, v0, lit0, v0, v0, v0, lit0, v0, v0, v0, lit0, v0, lit0, lit0, v0]
targetStackTop: [JUNK, lit0, v0, lit0, v0, JUNK, v0, lit0, JUNK, v0, v0, lit0, v0, JUNK, lit0, v0, v1]
targetStackTailSet: {v0}
targetStackSize: 20
// ----
// | 0 1 2 | 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
// +--------------------- +-----------------------------------------------------------------------------------------------------------------------
// (initial)| v1 v1 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit0 v0
// SWAP1| v1 v1 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 v0 lit0
// POP| v1 v1 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 v0
// SWAP16| v1 v0 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 v1
// DUP2| v1 v0 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 v1 lit0
// SWAP1| v1 v0 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit0 v1
// DUP4| v1 v0 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit0 v1 v0
// SWAP1| v1 v0 v0 | v0 lit0 v0 lit0 v0 v0 v0 lit0 v0 v0 v0 lit0 v0 lit0 lit0 v0 v1
// +--------------------- +-----------------------------------------------------------------------------------------------------------------------
// (target)| {v0} | * lit0 v0 lit0 v0 * v0 lit0 * v0 v0 lit0 v0 * lit0 v0 v1
// Status: Admissible