feat: Classical control flow with mid circuit measurements by speller26 · Pull Request #347 · amazon-braket/amazon-braket-default-simulator-python

speller26 · 2026-02-19T00:49:00Z

Issue #, if available:

Description of changes:

Testing done:

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

General

I have read the CONTRIBUTING doc
I used the commit message format described in CONTRIBUTING
I have updated any necessary documentation, including READMEs and API docs (if appropriate)

Tests

I have added tests that prove my fix is effective or that my feature works (if appropriate)
I have checked that my tests are not configured for a specific region or account (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

More to come

* Squeeze single-element density matrix before computing expectation * Pinned setuptools to fix doc build * Fixed all linter errors

codecov · 2026-02-23T18:56:54Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (aed65bd) to head (7ae10d1).

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #347    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           48        49     +1     
  Lines         4142      4752   +610     
  Branches       426       533   +107     
==========================================
+ Hits          4142      4752   +610

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

src/braket/default_simulator/openqasm/program_context.py

src/braket/default_simulator/gate_operations.py

src/braket/default_simulator/openqasm/program_context.py

src/braket/default_simulator/simulation_strategies/batch_operation_strategy.py

src/braket/default_simulator/simulation_strategies/single_operation_strategy.py

src/braket/default_simulator/simulator.py

yitchen-tim · 2026-03-02T14:06:17Z

Some product-level UX questions:

Several new feature for MCM triggers _uses_advanced_language_features=True. This causes warning "This program uses OpenQASM language features that may not be supported on QPUs or on-demand simulators." This is somewhat confusing to users, as if there is something wrong in their program/execution and they cannot trust the simulation results. Given that we are pushing for official support of MCM. Should we remove these warnings for MCM, and reserve _uses_advanced_language_features=True for something truly experimental?
We are lacking result schema and result class that supports MCM and general variable results. Right now, variables play no role in the result object. Result object always returns the end-of-circuit measurement outcome for all used qubits regardless whether there is an measurement instruction. Is it intentional and expected? Have you thought about the UX around the fact that we are not outputting any variables?

laurencap

Reviewed 1.5 files today. Should have more time tomorrow!

src/braket/default_simulator/openqasm/interpreter.py

laurencap · 2026-03-02T22:40:47Z

src/braket/default_simulator/openqasm/interpreter.py

-        condition = cast_to(BooleanLiteral, self.visit(node.condition))
-        for statement in node.if_block if condition.value else node.else_block:
-            self.visit(statement)
+        self.context.handle_branching_statement(node, self.visit)


Curious about this. Doesn't the visitor usually extract all the important fields, and then call context methods on extracted fields? Why not call visit from this visitor?

This is really annoying, and I'm totally open to better ways to do this. The problem is, the node itself can contain MCM conditionals, so calling visit right away will trigger the branch hander; the recursive nature of these calls means evaluation is completed "bottom-up," while the branch simulations have to be done "top-down."

src/braket/default_simulator/openqasm/interpreter.py

laurencap · 2026-03-02T22:47:51Z

src/braket/default_simulator/openqasm/program_context.py

+            measurement_target (Identifier | IndexedIdentifier | None): The AST node
+                for the classical variable being assigned, e.g. ``b`` in
+                ``b = measure q[0]``. Used by the branched MCM path to update
+                per-path classical variables. None for end-of-circuit measurements.


b[0] = measure q[0] is a valid circuit-final measurement right? In those cases, this is None. Is there no reason to track b[0] for circuit-final measurements?

I guess, because we return results by qubit index order at the end, it's not needed for anything?

Renamed to classical_destination and clarified documentation; None guarantees that it's terminal because the result can't be used (it isn't assigned to anything); however, assignment to a target may be either terminal or feedforward, but we don't know until the result is actually used.

laurencap · 2026-03-02T22:48:42Z

src/braket/default_simulator/openqasm/program_context.py

    def add_verbatim_marker(self, marker) -> None:
        """Add verbatim markers"""

+    def handle_branching_statement(self, node: BranchingStatement, visit_block: Callable) -> None:


Cross commenting from the last file -- visit_block callback feels odd to me. Not that important though. Nice documentation here

laurencap · 2026-03-02T22:53:10Z

src/braket/default_simulator/openqasm/program_context.py

+                with self.enter_scope():
+                    self.declare_variable(node.identifier.name, node.type, i)
+                    visit_block(deepcopy(node.block))
+            except _BreakSignal:


I see, maybe these were moved for ease of integration with _Continue/BreakSignal? Is that basically what changed from interpreter.py, support for these signals?

Yeah, break and continue actually weren't supported at all before, even for static circuits

src/braket/default_simulator/openqasm/program_context.py

laurencap · 2026-03-02T22:57:50Z

src/braket/default_simulator/openqasm/program_context.py

+        but no control flow depended on them) are registered in the circuit
+        as normal end-of-circuit measurements.
+        """
+        if not self._is_branched and self._pending_mcm_targets:


is_branched and circuit properties won't be called until interpretation is complete?

_is_branched ia called frequently, but _pending_measurements will always be empty once the simulation is in branched mode (although that can be optimized).

Support MCM later

src/braket/default_simulator/gate_operations.py

rmshaffer · 2026-03-05T22:43:26Z

src/braket/default_simulator/gate_operations.py

+                    zero_index = i & ~mask
+
+                    # Transfer the amplitude (with proper scaling)
+                    state[zero_index] += state[i]


do you want to copy state into a separate array first, rather than mutating the input array in place? (for consistency with how the Measure operation works)

src/braket/default_simulator/openqasm/program_context.py

MCM tests now all start from an OpenQASM program.

yitchen-tim · 2026-03-09T13:32:23Z

src/braket/default_simulator/openqasm/interpreter.py

-        for statement in node.if_block if condition.value else node.else_block:
-            self.visit(statement)
+        if self.context.supports_midcircuit_measurement:
+            self.context.handle_branching_statement(node)


This is only handled under BranchingStatement would cause the following use case to fail:

OPENQASM 3.0; qubit[3] __qubits__; bit[1] mcm; bit __bit_1__; __bit_1__ = measure __qubits__[1]; mcm[0] = __bit_1__;

when mcm[0] = __bit_1__; is called, __bit_1__ is not yet initialized because it's not a branch statement. It throws NameError: Identifier '__bit_1__' is not initialized.

yitchen-tim · 2026-03-09T14:56:18Z

Opened #351 to fix and allow re-measuring qubits.

shah-rushil and others added 15 commits September 12, 2025 13:56

feat: Mid Circuit Measurement (#293)

259b318

Merge branch 'main' into mcm-experimental

253a450

Merge branch 'main' into mcm-experimental

5d38025

Merge branch 'main' into mcm-experimental

9729a19

Merge branch 'main' into mcm-experimental

90cc443

minor fixes

79ccfe9

formatting

0df15af

Fixed bug and incorrect test case

eb03649

Update .gitignore

b4c2ac8

Delete .vscode/launch.json

4bf80a1

More tests

6f1ec22

Simplified visitor

18e1dc2

More to come

Update branched_interpreter.py

e3c8982

Consolidate branching logic into existing classes

115612f

Fix measurements

4456be9

yitchen-tim self-requested a review February 19, 2026 22:02

fix: Fix build failures

0ae913b

* Squeeze single-element density matrix before computing expectation * Pinned setuptools to fix doc build * Fixed all linter errors

speller26 changed the base branch from main to fix February 21, 2026 00:33

Merge branch 'fix' into mcm-experimental

61f5a46

Base automatically changed from fix to main February 23, 2026 17:29

speller26 added 2 commits February 23, 2026 09:32

Merge branch 'main' into mcm-experimental

4a859cb

Merge branch 'main' into mcm-experimental

96b9b2e

add unit test for an edge case

cabffb7

yitchen-tim reviewed Feb 26, 2026

View reviewed changes

src/braket/default_simulator/openqasm/program_context.py Outdated Show resolved Hide resolved

yitchen-tim reviewed Mar 2, 2026

View reviewed changes

add various edge case tests for reset and MCM

64ab4e9

laurencap reviewed Mar 2, 2026

View reviewed changes

better classical assignment handling

6b6d207

speller26 added 3 commits March 3, 2026 19:53

Update test_branched_mcm.py

5d05c01

Add more tests

1ac7b48

More tests

325c353

speller26 requested a review from aniksd-braket March 4, 2026 04:41

speller26 added 9 commits March 4, 2026 14:11

Prune unreachable assertions

96fa8ab

formatting

4549801

More coverage

98a7f4a

minor fixes

29f5899

More minor fixes

a3abdc4

Simplify branch methods

87319ce

Simplify edge cases

77283e2

Revert batch_operation_strategy

8bca38f

Support MCM later

100% test coverage

f927119

speller26 marked this pull request as ready for review March 5, 2026 22:47

speller26 requested a review from a team as a code owner March 5, 2026 22:47

rmshaffer reviewed Mar 5, 2026

View reviewed changes

speller26 added 3 commits March 5, 2026 17:10

Even more simplifications

a68744d

MCM tests now all start from an OpenQASM program.

rename

08f7329

Update gate_operations.py

8f99f71

yitchen-tim reviewed Mar 9, 2026

View reviewed changes

yitchen-tim closed this Mar 9, 2026

yitchen-tim deleted the mcm-experimental branch March 9, 2026 14:50

yitchen-tim restored the mcm-experimental branch March 9, 2026 14:51

yitchen-tim reopened this Mar 9, 2026

fix: allow re-measurement of qubits when MCM is supported (#351)

77fe95b

rmshaffer mentioned this pull request Mar 12, 2026

fix: call ProgramContext constructor amazon-braket/autoqasm#93

Merged

5 tasks

speller26 added 3 commits March 25, 2026 16:07

Allow using measurements outside branching

2bf6c35

Fix qubit reuse corner case

26babd4

Update program_context.py

7ae10d1

Conversation

speller26 commented Feb 19, 2026

Merge Checklist

General

Tests

Uh oh!

codecov bot commented Feb 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Codecov Report

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

yitchen-tim commented Mar 2, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

laurencap left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

yitchen-tim commented Mar 9, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

5 participants

codecov bot commented Feb 23, 2026 •

edited

Loading

yitchen-tim commented Mar 2, 2026 •

edited

Loading