✨ Add a pass promoting qubit-tensor arguments to scalar qubits

CHANGELOG.md

@@ -64,7 +64,7 @@ releases may include breaking changes.
 #### Passes and transformations
 
 - ✨ Add passes for quantum-specific interprocedural optimizations ([#2193],
-  [#2197]) ([**@DRovara**], [**@burgholzer**])
+  [#2197], [#2198]) ([**@DRovara**], [**@burgholzer**])
 - ✨ Add Pauli twirling, quantum loop unrolling, and qubit reuse passes
   ([#1705], [#1718], [#1755], [#1756], [#1923], [#1924], [#2039], [#2118],
   [#2216], [#2224]) ([**@MatthiasReumann**], [**@DRovara**], [**@burgholzer**],
@@ -890,6 +890,7 @@ for previous changelogs._
 [#2216]: https://github.com/munich-quantum-toolkit/core/pull/2216
 [#2203]: https://github.com/munich-quantum-toolkit/core/pull/2203
 [#2214]: https://github.com/munich-quantum-toolkit/core/pull/2214
+[#2198]: https://github.com/munich-quantum-toolkit/core/pull/2198
 [#2197]: https://github.com/munich-quantum-toolkit/core/pull/2197
 [#2196]: https://github.com/munich-quantum-toolkit/core/pull/2196
 [#2194]: https://github.com/munich-quantum-toolkit/core/pull/2194

mlir/include/mlir/Dialect/QCO/Transforms/Passes.td

@@ -350,6 +350,26 @@ def ContextSensitiveSpecialization
                            "mlir::qco::QCODialect"];
 }
 
+def QuantumArgumentPromotion
+    : Pass<"quantum-argument-promotion", "mlir::ModuleOp"> {
+  let summary = "Replace qubit-tensor arguments by the qubits a callee uses";
+  let description = [{
+        A tensor argument whose elements are taken out and put back at
+        compile-time constant indices is split into one qubit argument and one
+        qubit result per touched element, so untouched elements never cross the
+        call boundary. Call sites extract before and re-insert after the call.
+
+        The callee has to hand the tensor back as its first result, every
+        reference to it has to be a direct call, and elements taken out but
+        never put back prevent promotion.
+    }];
+
+  let dependentDialects = ["::mlir::func::FuncDialect",
+                           "::mlir::arith::ArithDialect",
+                           "::mlir::qtensor::QTensorDialect",
+                           "mlir::qco::QCODialect"];
+}
+
 def RemoveDeadGates : Pass<"remove-dead-gates", "mlir::ModuleOp"> {
   let dependentDialects = ["mlir::qco::QCODialect"];
   let summary = "Remove quantum gates whose results cannot be observed";