update LQ generator vector insert order (#1009)

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**Context:**
Currently in `GateImplementationsLM.hpp` some generator functions
performs two std::vector insertions to the same vector, both to the
beginning of the destination vectors.

**Description of the Change:**
The order is swapped, and the second vector is inserted to the end of
the vector. This should be more performant.

**Benefits:**

**Possible Drawbacks:**

**Related GitHub Issues:**

[sc-78933]

---------

Co-authored-by: ringo-but-quantum <[email protected]>

Author: josephleekl

.github/CHANGELOG.md

@@ -23,6 +23,9 @@
 
 ### Improvements
 
+* Reverse Lightning Qubit generators vector insertion order.
+  [(#1009)](https://github.com/PennyLaneAI/pennylane-lightning/pull/1009)
+
 * Update Kokkos version support to 4.4.1 and enable Lightning-Kokkos[CUDA] C++ tests on CI.
   [(#1000)](https://github.com/PennyLaneAI/pennylane-lightning/pull/1000)
 

pennylane_lightning/core/_version.py

@@ -16,4 +16,4 @@
    Version number (major.minor.patch[-label])
 """
 
-__version__ = "0.40.0-dev18"
+__version__ = "0.40.0-dev19"

pennylane_lightning/core/src/simulators/lightning_qubit/gates/cpu_kernels/GateImplementationsLM.hpp

@@ -1946,9 +1946,10 @@ class GateImplementationsLM : public PauliGenerator<GateImplementationsLM> {
 
         std::vector<std::size_t> all_wires;
         all_wires.reserve(nw_tot);
-        all_wires.insert(all_wires.begin(), wires.begin(), wires.end());
         all_wires.insert(all_wires.begin(), controlled_wires.begin(),
                          controlled_wires.end());
+        all_wires.insert(all_wires.begin() + n_contr, wires.begin(),
+                         wires.end());
         const auto revs = reverseWires(num_qubits, all_wires, {});
         const auto &rev_wires = revs.first;
         const std::vector<std::size_t> parity =
@@ -2166,9 +2167,10 @@ class GateImplementationsLM : public PauliGenerator<GateImplementationsLM> {
 
         std::vector<std::size_t> all_wires;
         all_wires.reserve(nw_tot);
-        all_wires.insert(all_wires.begin(), wires.begin(), wires.end());
         all_wires.insert(all_wires.begin(), controlled_wires.begin(),
                          controlled_wires.end());
+        all_wires.insert(all_wires.begin() + n_contr, wires.begin(),
+                         wires.end());
         const auto revs = reverseWires(num_qubits, all_wires, {});
         const auto &rev_wires = revs.first;
         const std::vector<std::size_t> parity =
@@ -2448,9 +2450,10 @@ class GateImplementationsLM : public PauliGenerator<GateImplementationsLM> {
 
         std::vector<std::size_t> all_wires;
         all_wires.reserve(nw_tot);
-        all_wires.insert(all_wires.begin(), wires.begin(), wires.end());
         all_wires.insert(all_wires.begin(), controlled_wires.begin(),
                          controlled_wires.end());
+        all_wires.insert(all_wires.begin() + n_contr, wires.begin(),
+                         wires.end());
         const auto revs = reverseWires(num_qubits, all_wires, {});
         const auto &rev_wires = revs.first;
         const std::vector<std::size_t> parity =