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Add identity removal pass to presets #13363

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merged 4 commits into from
Nov 6, 2024
Merged

Add identity removal pass to presets #13363

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360d625
Add identity removal pass to presets
mtreinish Oct 23, 2024
c31a602
Fix lint
mtreinish Oct 29, 2024
356ac6f
Merge remote-tracking branch 'origin/main' into use-drop-neglible
mtreinish Oct 29, 2024
33635ca
Merge branch 'main' into use-drop-neglible
mtreinish Nov 6, 2024
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16 changes: 16 additions & 0 deletions qiskit/transpiler/preset_passmanagers/builtin_plugins.py
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Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,7 @@
CommutativeCancellation,
ConsolidateBlocks,
InverseCancellation,
RemoveIdentityEquivalent,
)
from qiskit.transpiler.passes import Depth, Size, FixedPoint, MinimumPoint
from qiskit.transpiler.passes.utils.gates_basis import GatesInBasis
Expand Down Expand Up @@ -156,6 +157,13 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
if pass_manager_config.routing_method != "none":
init.append(ElidePermutations())
init.append(RemoveDiagonalGatesBeforeMeasure())
# Target not set on RemoveIdentityEquivalent because we haven't applied a Layout
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I think I would expect the transpiler to remove Ry(0) & co even as a very basic optimization, should we also include this target-free run in optimization level 1?

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My concern with doing it for level 1 was more about small angles like Ry(1e-8) will be removed and I was worried that might be surprising to people especially at level 1. But I don't feel super strongly about it, it's easy enough to add it to level 1 if you think that's better.

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Hm you're right, I didn't think that these angles are already removed already... as rule we should maybe not increase the level of approximation that is already happening (e.g. cutoffs that unitary re-synthesis does). On level 1 we don't do re-synthesis, right? So then I agree we shouldn't add it one 1 yet 🙂

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Level 1 will run 1q re-synthesis, but via Optikize1qGatesDecomposition not UnitarySynthesis. I can't remember how that pass handles tolerances, I'll have to check.

# yet so doing anything relative to an error rate in the target is not valid.
init.append(
RemoveIdentityEquivalent(
approximation_degree=pass_manager_config.approximation_degree
)
)
init.append(
InverseCancellation(
[
Expand Down Expand Up @@ -580,6 +588,10 @@ def _opt_control(property_set):

elif optimization_level == 2:
_opt = [
RemoveIdentityEquivalent(
approximation_degree=pass_manager_config.approximation_degree,
target=pass_manager_config.target,
),
Optimize1qGatesDecomposition(
basis=pass_manager_config.basis_gates, target=pass_manager_config.target
),
Expand All @@ -602,6 +614,10 @@ def _opt_control(property_set):
plugin_config=pass_manager_config.unitary_synthesis_plugin_config,
target=pass_manager_config.target,
),
RemoveIdentityEquivalent(
approximation_degree=pass_manager_config.approximation_degree,
target=pass_manager_config.target,
),
Optimize1qGatesDecomposition(
basis=pass_manager_config.basis_gates, target=pass_manager_config.target
),
Expand Down
8 changes: 8 additions & 0 deletions releasenotes/notes/add-identity-pass-builtin-2061b29b53b928d3.yaml
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@@ -0,0 +1,8 @@
---
features_transpiler:
- |
The :class:`.RemoveIdentityEquivalent` transpiler pass is now run as part
of the preset pass managers at optimization levels 2 and 3. The pass is
run in the ``init`` stage and the ``optimization`` stage, because the
optimizations it applies are valid in both stages and the pass is
fast to execute.
28 changes: 18 additions & 10 deletions test/python/compiler/test_transpiler.py
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Expand Up @@ -1796,21 +1796,29 @@ def test_no_infinite_loop(self, optimization_level):
seed_transpiler=42,
)

# Expect a -pi/2 global phase for the U3 to RZ/SX conversion, and
# a -0.5 * theta phase for RZ to P twice, once at theta, and once at 3 pi
# for the second and third RZ gates in the U3 decomposition.
expected = QuantumCircuit(
1, global_phase=-np.pi / 2 - 0.5 * (-0.2 + np.pi) - 0.5 * 3 * np.pi
)
expected.p(-np.pi, 0)
expected.sx(0)
expected.p(np.pi - 0.2, 0)
expected.sx(0)
if optimization_level == 1:
# Expect a -pi/2 global phase for the U3 to RZ/SX conversion, and
# a -0.5 * theta phase for RZ to P twice, once at theta, and once at 3 pi
# for the second and third RZ gates in the U3 decomposition.
expected = QuantumCircuit(
1, global_phase=-np.pi / 2 - 0.5 * (-0.2 + np.pi) - 0.5 * 3 * np.pi
)
expected.p(-np.pi, 0)
expected.sx(0)
expected.p(np.pi - 0.2, 0)
expected.sx(0)
else:
expected = QuantumCircuit(1, global_phase=(15 * np.pi - 1) / 10)
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This is slightly unexpected, I guess at some point a gate is now removed and Optimize1qGatesDecomposition optimizes the sequence differently? Or why do you think this happens?

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That was my assumption, I don't remember the exact details though. Let me run it with a callback and see what's going on exactly.

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@mtreinish mtreinish Nov 6, 2024 •
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Looking through each pass it looks like RemoveIdentityEquivalent changes:

global phase: 4.6124
   ┌──────┐┌────┐┌───────────┐┌────┐┌───────┐
q: ┤ P(0) ├┤ √X ├┤ P(3.3416) ├┤ √X ├┤ P(3π) ├
   └──────┘└────┘└───────────┘└────┘└───────┘

to:

global phase: 4.6124
   ┌────┐┌───────────┐┌────┐┌───────┐
q: ┤ √X ├┤ P(3.3416) ├┤ √X ├┤ P(3π) ├
   └────┘└───────────┘└────┘└───────┘

This changes the output of Optimize1qGatesDecomposition which runs immediately after from:

global phase: 4.8124
   ┌───────┐┌────┐┌───────────┐┌────┐
q: ┤ P(-π) ├┤ √X ├┤ P(2.9416) ├┤ √X ├
   └───────┘└────┘└───────────┘└────┘

which is the output on calling the pass on the circuit with P(0)

to:

global phase: 4.6124
   ┌────┐┌───────────┐┌────┐┌───────┐
q: ┤ √X ├┤ P(3.3416) ├┤ √X ├┤ P(3π) ├
   └────┘└───────────┘└────┘└───────┘

which is the pass's output with P(0) removed. What this is coming down to is the preference for Optimize1qGatesDecomposition to defer to the input circuit if it doesn't think the synthesis output is any better. Since the gate counts of either output are the same, the pass views them as equivalent (in the absence of error rates in the target) and prefer to not modify the circuit instead of use the synthesis output. While in the case of without the removal pass being run there is an extra 1q gate in the sequence the pass views the synthesis output as being more efficient because it has 1 fewer gate.

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Ahh that makes sense -- thanks for checking, better to be sure! 🙂

expected.sx(0)
expected.p(1.0 / 5.0 + np.pi, 0)
expected.sx(0)
expected.p(3 * np.pi, 0)

error_message = (
f"\nOutput circuit:\n{out!s}\n{Operator(out).data}\n"
f"Expected circuit:\n{expected!s}\n{Operator(expected).data}"
)
self.assertEqual(Operator(qc), Operator(out))
self.assertEqual(out, expected, error_message)

@data(0, 1, 2, 3)
Expand Down