From 96916e41d683400b41610e510d5d8ee3f82a6c03 Mon Sep 17 00:00:00 2001
From: Olivier BICHLER <olivier.bichler@cea.fr>
Date: Sun, 3 Dec 2023 18:56:20 +0100
Subject: [PATCH] Initial concept for a Convert operator (UNTESTED)

---
 include/aidge/backend/TensorImpl.hpp       | 71 +++++++++++++++++-
 include/aidge/data/Tensor.hpp              | 18 +++--
 include/aidge/graph/GraphView.hpp          |  4 +-
 include/aidge/operator/Add.hpp             |  6 +-
 include/aidge/operator/AvgPooling.hpp      |  6 +-
 include/aidge/operator/BatchNorm.hpp       | 12 +--
 include/aidge/operator/Concat.hpp          |  6 +-
 include/aidge/operator/Conv.hpp            |  8 +-
 include/aidge/operator/ConvDepthWise.hpp   |  8 +-
 include/aidge/operator/Convert.hpp         | 87 ++++++++++++++++++++++
 include/aidge/operator/Div.hpp             |  8 +-
 include/aidge/operator/FC.hpp              | 10 +--
 include/aidge/operator/GenericOperator.hpp |  2 +-
 include/aidge/operator/Identity.hpp        |  2 +-
 include/aidge/operator/LeakyReLU.hpp       |  6 +-
 include/aidge/operator/MatMul.hpp          |  8 +-
 include/aidge/operator/MaxPooling.hpp      |  6 +-
 include/aidge/operator/MetaOperator.hpp    |  4 +-
 include/aidge/operator/Mul.hpp             |  8 +-
 include/aidge/operator/Operator.hpp        |  2 +-
 include/aidge/operator/Pad.hpp             |  6 +-
 include/aidge/operator/Pow.hpp             |  8 +-
 include/aidge/operator/Producer.hpp        |  4 +-
 include/aidge/operator/ReLU.hpp            |  6 +-
 include/aidge/operator/Scaling.hpp         |  6 +-
 include/aidge/operator/Slice.hpp           |  6 +-
 include/aidge/operator/Softmax.hpp         |  6 +-
 include/aidge/operator/Sqrt.hpp            |  6 +-
 include/aidge/operator/Sub.hpp             |  8 +-
 src/graph/GraphView.cpp                    |  8 +-
 src/operator/Convert.cpp                   | 83 +++++++++++++++++++++
 31 files changed, 336 insertions(+), 93 deletions(-)
 create mode 100644 include/aidge/operator/Convert.hpp
 create mode 100644 src/operator/Convert.cpp

diff --git a/include/aidge/backend/TensorImpl.hpp b/include/aidge/backend/TensorImpl.hpp
index dfe3d932a..1a036a0d5 100644
--- a/include/aidge/backend/TensorImpl.hpp
+++ b/include/aidge/backend/TensorImpl.hpp
@@ -14,15 +14,81 @@
 
 #include <cstddef>
 #include <cstdio>
+#include "aidge/data/Data.hpp"
 #include "aidge/utils/Types.h"
 
 namespace Aidge {
 class TensorImpl {
 public:
     TensorImpl() = delete;
-    TensorImpl(const char *backend) : mBackend(backend){};
+    TensorImpl(const char *backend, int device = 0) : mBackend(backend), mDevice(device){};
+
+    /**
+     * Return the (backend, device) pair for this implementation.
+    */
+    std::pair<std::string, int> device() const { return std::make_pair(mBackend, mDevice); }
+
+    /**
+     * Set the device ID for current backend.
+     * @param device New device ID on current backend.
+    */
+    virtual void setDevice(int device) = 0;
+
+    /**
+     * Copy data from the same device.
+     * @param src Pointer on current implementation device.
+     * @param length Number of bytes to copy.
+    */
     virtual void copy(const void *src, NbElts_t length) = 0;
-    virtual void *rawPtr() = 0;
+
+    /**
+     * Copy-convert data from the same device.
+     * @param srcDt Source data type.
+     * @param src Pointer on current implementation device.
+     * @param length Number of bytes to copy.
+    */
+    virtual void copyCast(const void *src, NbElts_t length, const DataType srcDt) = 0;
+
+    /**
+     * Copy data from an other device on the same backend.
+     * @param device (backend, device) pair to copy from. The backend must match current implementation backend.
+     * @param src Pointer on current implementation backend.
+     * @param length Number of bytes to copy.
+    */
+    virtual void copyFromDevice(const void *src, NbElts_t length, const std::pair<std::string, int>& device) = 0;
+
+    /**
+     * Copy data from host.
+     * @param src Host pointer to copy from.
+     * @param length Number of bytes to copy.
+    */
+    virtual void copyFromHost(const void *src, NbElts_t length) = 0;
+
+    /**
+     * Copy data to host.
+     * @param src Host pointer to copy to.
+     * @param length Number of bytes to copy.
+    */
+    virtual void copyToHost(void *dst, NbElts_t length) = 0;
+
+    /**
+     * Return the raw device pointer.
+     * The raw pointer is garanteed to be valid only on the *same* device.
+    */
+    virtual void* rawPtr() = 0;
+
+    /**
+     * Return the host pointer.
+     * If the implementation does not have a valid host pointer, nullptr is returned.
+    */
+    virtual void* hostPtr() = 0;
+
+    /**
+     * Sets the device pointer.
+     * UNSAFE: directly setting the device pointer may lead to undefined behavior
+     * if it does not match the required storage.
+     * @param ptr A valid device pointer.
+    */
     virtual void setRawPtr(void* /*ptr*/)
     {
         printf("Cannot set raw pointer for backend %s\n", mBackend);
@@ -37,6 +103,7 @@ public:
 
 private:
     const char *mBackend;
+    int mDevice;
 };
 
 } // namespace Aidge
diff --git a/include/aidge/data/Tensor.hpp b/include/aidge/data/Tensor.hpp
index f8c3a48f7..9fabd2d4e 100644
--- a/include/aidge/data/Tensor.hpp
+++ b/include/aidge/data/Tensor.hpp
@@ -301,7 +301,7 @@ class Tensor : public Data,
         resize(t.dims());
         setDataType(t.dataType());
         if (t.hasImpl()) {
-            setBackend(t.mImpl->backend());
+            setBackend(t.mImpl->backend(), t.mImpl->device().second);
             mImpl->copy(t.mImpl->rawPtr(), size());
         }
         else {
@@ -327,17 +327,23 @@ class Tensor : public Data,
      * @details Create and initialized an implementation if non was associated.
      * @param name
      */
-    inline void setBackend(const std::string &name) {
+    inline void setBackend(const std::string &name, int device = 0) {
         if (mImpl) {
-            if (strcmp(mImpl->backend(), name.c_str()) != 0) {
+            if (mImpl->device() != std::make_pair(name, device)) {
                 // Backend change: create new impl, copy from old to new and replace
                 // impl
                 std::unique_ptr<TensorImpl> newImpl = Registrar<Tensor>::create({name, mDataType})(*this);
-                newImpl->copy(mImpl->rawPtr(), size());
-                mImpl = std::move(newImpl);
+                newImpl->setDevice(device);
+
+                //TODO: FIXME: copy() work only on same device!
+                //newImpl->copy(mImpl->rawPtr(), size());
+                //mImpl = std::move(newImpl);
             }
-        } else
+        }
+        else {
             mImpl = Registrar<Tensor>::create({name, mDataType})(*this);
+            mImpl->setDevice(device);
+        }
     }
 
     /**
diff --git a/include/aidge/graph/GraphView.hpp b/include/aidge/graph/GraphView.hpp
index bf23ef9f0..2a367b4da 100644
--- a/include/aidge/graph/GraphView.hpp
+++ b/include/aidge/graph/GraphView.hpp
@@ -203,7 +203,7 @@ public:
      * If not, add a Transpose Operator.
      * 4 - Propagate Tensor dimensions through the consecutive Operators.
      */
-    void compile(const std::string& backend, const Aidge::DataType datatype);
+    void compile(const std::string& backend, const Aidge::DataType datatype, int device = 0);
 
     /**
      * @brief Compute dimensions of input/output Tensors for each Operator of the
@@ -212,7 +212,7 @@ public:
     void forwardDims();
 
     /** @brief Set the same backend for each Operator of the GraphView object's Nodes. */
-    void setBackend(const std::string &backend);
+    void setBackend(const std::string &backend, int device = 0);
     /** @brief Set the same backend for each Operator of the GraphView object's Nodes. */
     void setDataType(const DataType &datatype);
 
diff --git a/include/aidge/operator/Add.hpp b/include/aidge/operator/Add.hpp
index 0c2854029..59188fcf2 100644
--- a/include/aidge/operator/Add.hpp
+++ b/include/aidge/operator/Add.hpp
@@ -76,13 +76,13 @@ public:
     // }
 
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<Add_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
         for (std::size_t i = 0; i < nbInputs(); ++i) {
-            getInput(i)->setBackend(name);
+            getInput(i)->setBackend(name, device);
         }
     }
 
diff --git a/include/aidge/operator/AvgPooling.hpp b/include/aidge/operator/AvgPooling.hpp
index f0f9f6c54..483f8a60b 100644
--- a/include/aidge/operator/AvgPooling.hpp
+++ b/include/aidge/operator/AvgPooling.hpp
@@ -130,12 +130,12 @@ public:
     // }
 
 
-    void setBackend(const std::string &name) override {
+    void setBackend(const std::string &name, int device = 0) override {
         mImpl = Registrar<AvgPooling_Op<DIM>>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
+        getInput(0)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/BatchNorm.hpp b/include/aidge/operator/BatchNorm.hpp
index 09a9bb9ef..43a096534 100644
--- a/include/aidge/operator/BatchNorm.hpp
+++ b/include/aidge/operator/BatchNorm.hpp
@@ -94,15 +94,15 @@ public:
         }
     }
 
-    void setBackend(const std::string &name) override {
+    void setBackend(const std::string &name, int device = 0) override {
         mImpl = Registrar<BatchNorm_Op<DIM>>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(1)->setBackend(name);
-        getInput(2)->setBackend(name);
-        getInput(3)->setBackend(name);
-        getInput(4)->setBackend(name);
+        getInput(1)->setBackend(name, device);
+        getInput(2)->setBackend(name, device);
+        getInput(3)->setBackend(name, device);
+        getInput(4)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName() {
diff --git a/include/aidge/operator/Concat.hpp b/include/aidge/operator/Concat.hpp
index 01d590aa7..c6b679cf5 100644
--- a/include/aidge/operator/Concat.hpp
+++ b/include/aidge/operator/Concat.hpp
@@ -101,13 +101,13 @@ public:
         }
     }
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<Concat_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
         for (std::size_t i = 0; i < nbInputs(); ++i) {
-            getInput(i)->setBackend(name);
+            getInput(i)->setBackend(name, device);
         }
     }
 
diff --git a/include/aidge/operator/Conv.hpp b/include/aidge/operator/Conv.hpp
index 4f0fb1ea2..8ae56e9b9 100644
--- a/include/aidge/operator/Conv.hpp
+++ b/include/aidge/operator/Conv.hpp
@@ -169,13 +169,13 @@ public:
     //     AIDGE_THROW_OR_ABORT(std::runtime_error, "Given outputDim out of range or output dim not forwarded yet.");
     // }
 
-    void setBackend(const std::string &name) override {
+    void setBackend(const std::string &name, int device = 0) override {
         mImpl = Registrar<Conv_Op<DIM>>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(1)->setBackend(name);
-        getInput(2)->setBackend(name);
+        getInput(1)->setBackend(name, device);
+        getInput(2)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/ConvDepthWise.hpp b/include/aidge/operator/ConvDepthWise.hpp
index ca6401e0e..591f7c6ed 100644
--- a/include/aidge/operator/ConvDepthWise.hpp
+++ b/include/aidge/operator/ConvDepthWise.hpp
@@ -151,13 +151,13 @@ public:
     //     AIDGE_THROW_OR_ABORT(std::runtime_error, "Given outputDim out of range or output dim not forwarded yet.");
     // }
 
-    void setBackend(const std::string &name) override {
+    void setBackend(const std::string &name, int device = 0) override {
         mImpl = Registrar<ConvDepthWise_Op<DIM>>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(1)->setBackend(name);
-        getInput(2)->setBackend(name);
+        getInput(1)->setBackend(name, device);
+        getInput(2)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/Convert.hpp b/include/aidge/operator/Convert.hpp
new file mode 100644
index 000000000..19ffb9a0d
--- /dev/null
+++ b/include/aidge/operator/Convert.hpp
@@ -0,0 +1,87 @@
+/********************************************************************************
+ * Copyright (c) 2023 CEA-List
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0.
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ ********************************************************************************/
+
+#ifndef AIDGE_CORE_OPERATOR_CONVERT_H_
+#define AIDGE_CORE_OPERATOR_CONVERT_H_
+
+#include <cassert>
+#include <memory>
+#include <vector>
+
+#include "aidge/utils/Registrar.hpp"
+#include "aidge/operator/OperatorTensor.hpp"
+#include "aidge/backend/OperatorImpl.hpp"
+#include "aidge/data/Tensor.hpp"
+#include "aidge/graph/Node.hpp"
+#include "aidge/utils/Types.h"
+
+namespace Aidge {
+
+class Convert_Op : public OperatorTensor,
+    public Registrable<Convert_Op, std::tuple<std::string, std::string>, std::unique_ptr<OperatorImpl>(const Convert_Op&)> {
+public:
+    static constexpr const char* Type = "Convert";
+
+    Convert_Op() : OperatorTensor(Type, 1, 0, 1) {}
+
+    /**
+     * @brief Copy-constructor. Copy the operator attributes and its output tensor(s), but not its input tensors (the new operator has no input associated).
+     * @param op Operator to copy.
+     */
+    Convert_Op(const Convert_Op& op)
+        : OperatorTensor(op)
+    {
+        mImpl = op.mImpl ? Registrar<Convert_Op>::create({mInputs[0]->getImpl()->backend(), mOutputs[0]->getImpl()->backend()})(*this) : nullptr;
+    }
+
+    /**
+     * @brief Clone the operator using its copy-constructor.
+     * @see Operator::Convert_Op
+     */
+    std::shared_ptr<Operator> clone() const override {
+        return std::make_shared<Convert_Op>(*this);
+    }
+
+    void setBackend(const std::string& name, int device = 0) override {
+        if (Registrar<Convert_Op>::exists({mInputs[0]->getImpl()->backend(), name})) {
+            mImpl = Registrar<Convert_Op>::create({mInputs[0]->getImpl()->backend(), name})(*this);
+        }
+        mOutputs[0]->setBackend(name, device);
+    }
+
+    void setDataType(const DataType& dataType) const override {
+        mOutputs[0]->setDataType(dataType);
+    }
+
+    void forward() override;
+
+    static const std::vector<std::string> getInputsName(){
+        return {"data_input"};
+    }
+    static const std::vector<std::string> getOutputsName(){
+        return {"data_output"};
+    }
+
+private:
+    /// @brief Store the data to the right type on input device
+    /// Required for any type conversion.
+    std::shared_ptr<Tensor> mConvertedInput;
+    /// @brief Store the data to the right type on host
+    /// Required if there is no direct link between input and output devices
+    std::shared_ptr<Tensor> mHostBuffer;
+};
+
+inline std::shared_ptr<Node> Convert(const std::string& name = "") {
+    return std::make_shared<Node>(std::make_shared<Convert_Op>(), name);
+}
+}
+
+#endif /* AIDGE_CORE_OPERATOR_CONVERT_H_ */
\ No newline at end of file
diff --git a/include/aidge/operator/Div.hpp b/include/aidge/operator/Div.hpp
index ba76c0bde..933e2e092 100644
--- a/include/aidge/operator/Div.hpp
+++ b/include/aidge/operator/Div.hpp
@@ -54,13 +54,13 @@ public:
     void computeOutputDims() override final;
 
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<Div_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
-        getInput(1)->setBackend(name);
+        getInput(0)->setBackend(name, device);
+        getInput(1)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/FC.hpp b/include/aidge/operator/FC.hpp
index 4cece292c..23f2459af 100644
--- a/include/aidge/operator/FC.hpp
+++ b/include/aidge/operator/FC.hpp
@@ -95,14 +95,14 @@ public:
     }
 
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<FC_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
-        getInput(1)->setBackend(name);
-        getInput(2)->setBackend(name);
+        getInput(0)->setBackend(name, device);
+        getInput(1)->setBackend(name, device);
+        getInput(2)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/GenericOperator.hpp b/include/aidge/operator/GenericOperator.hpp
index 505c53449..56e57a14a 100644
--- a/include/aidge/operator/GenericOperator.hpp
+++ b/include/aidge/operator/GenericOperator.hpp
@@ -97,7 +97,7 @@ public:
 
     ~GenericOperator_Op() = default;
 
-    void setBackend(const std::string & /*name*/) override { printf("setBackend: not available yet.\n"); }
+    void setBackend(const std::string & /*name*/, int /*device*/ = 0) override { printf("setBackend: not available yet.\n"); }
     void setDataType(const DataType& /*datatype*/) const override { printf("setDataType: not available yet.\n"); }
     void forward() override final {
         if(mImpl){
diff --git a/include/aidge/operator/Identity.hpp b/include/aidge/operator/Identity.hpp
index c5cd9bb62..55d7f492f 100644
--- a/include/aidge/operator/Identity.hpp
+++ b/include/aidge/operator/Identity.hpp
@@ -103,7 +103,7 @@ public:
         }
         return mInputs[outputIdx];
     }
-    void setBackend(const std::string& name) override final {
+    void setBackend(const std::string& name, int device = 0) override final {
         // setBackend do nothing, Identity node has no backend it just pass the same Tensor
     }
     void setDataType(const DataType& dataType) const override final {
diff --git a/include/aidge/operator/LeakyReLU.hpp b/include/aidge/operator/LeakyReLU.hpp
index 800c8c61d..d8a608331 100644
--- a/include/aidge/operator/LeakyReLU.hpp
+++ b/include/aidge/operator/LeakyReLU.hpp
@@ -67,12 +67,12 @@ public:
 
 
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<LeakyReLU_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
+        getInput(0)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/MatMul.hpp b/include/aidge/operator/MatMul.hpp
index 23c12d458..e69708947 100644
--- a/include/aidge/operator/MatMul.hpp
+++ b/include/aidge/operator/MatMul.hpp
@@ -83,13 +83,13 @@ public:
     }
 
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<MatMul_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
-        getInput(1)->setBackend(name);
+        getInput(0)->setBackend(name, device);
+        getInput(1)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/MaxPooling.hpp b/include/aidge/operator/MaxPooling.hpp
index ad50a27a9..01658bb04 100644
--- a/include/aidge/operator/MaxPooling.hpp
+++ b/include/aidge/operator/MaxPooling.hpp
@@ -104,12 +104,12 @@ public:
     }
 
 
-    void setBackend(const std::string &name) override {
+    void setBackend(const std::string &name, int device = 0) override {
         mImpl = Registrar<MaxPooling_Op<DIM>>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
+        getInput(0)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/MetaOperator.hpp b/include/aidge/operator/MetaOperator.hpp
index 991c1c60d..3427890b0 100644
--- a/include/aidge/operator/MetaOperator.hpp
+++ b/include/aidge/operator/MetaOperator.hpp
@@ -70,7 +70,7 @@ public:
     }
 
 
-    void setBackend(const std::string &name) override {
+    void setBackend(const std::string &name, int device = 0) override {
         if (Registrar<MetaOperator_Op>::exists({name, type()})) {
             // A custom implementation exists for this meta operator
             mImpl = Registrar<MetaOperator_Op>::create({name, type()})(*this);
@@ -79,7 +79,7 @@ public:
         // The micro-graph should always be set to the right backend, since it
         // shares input/output tensors.
         // Input/output tensors backend are updated here.
-        mGraph->setBackend(name);
+        mGraph->setBackend(name, device);
     }
 
     void setDataType(const DataType &datatype) const override {
diff --git a/include/aidge/operator/Mul.hpp b/include/aidge/operator/Mul.hpp
index 5b9ab4eb8..7eb384f35 100644
--- a/include/aidge/operator/Mul.hpp
+++ b/include/aidge/operator/Mul.hpp
@@ -56,13 +56,13 @@ public:
 
     void computeOutputDims() override final;
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<Mul_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
-        getInput(1)->setBackend(name);
+        getInput(0)->setBackend(name, device);
+        getInput(1)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/Operator.hpp b/include/aidge/operator/Operator.hpp
index b0f8435bd..1dd7c4beb 100644
--- a/include/aidge/operator/Operator.hpp
+++ b/include/aidge/operator/Operator.hpp
@@ -114,7 +114,7 @@ public:
 //        IMPLEMENTATION
 ///////////////////////////////////////////////////////
 
-    virtual void setBackend(const std::string& name) = 0;
+    virtual void setBackend(const std::string& name, int device = 0) = 0;
     virtual void setDataType(const DataType& dataType) const = 0;
 
     /**
diff --git a/include/aidge/operator/Pad.hpp b/include/aidge/operator/Pad.hpp
index 279b8b3d2..ab54853a3 100644
--- a/include/aidge/operator/Pad.hpp
+++ b/include/aidge/operator/Pad.hpp
@@ -97,12 +97,12 @@ public:
         }
     }
 
-    void setBackend(const std::string &name) override {
+    void setBackend(const std::string &name, int device = 0) override {
         mImpl = Registrar<Pad_Op<DIM>>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
+        getInput(0)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/Pow.hpp b/include/aidge/operator/Pow.hpp
index 0b0ae82f0..d437f1e4e 100644
--- a/include/aidge/operator/Pow.hpp
+++ b/include/aidge/operator/Pow.hpp
@@ -54,13 +54,13 @@ public:
     void computeOutputDims() override final;
 
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<Pow_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
-        getInput(1)->setBackend(name);
+        getInput(0)->setBackend(name, device);
+        getInput(1)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/Producer.hpp b/include/aidge/operator/Producer.hpp
index fb6a20403..3c37b3d61 100644
--- a/include/aidge/operator/Producer.hpp
+++ b/include/aidge/operator/Producer.hpp
@@ -73,9 +73,9 @@ public:
 
     inline const std::vector<DimSize_t> dims() const noexcept { return mOutputs[0]->dims(); }
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<Producer_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/ReLU.hpp b/include/aidge/operator/ReLU.hpp
index 3444c25fc..97156fda9 100644
--- a/include/aidge/operator/ReLU.hpp
+++ b/include/aidge/operator/ReLU.hpp
@@ -51,12 +51,12 @@ public:
     }
 
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<ReLU_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
+        getInput(0)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/Scaling.hpp b/include/aidge/operator/Scaling.hpp
index fd6d6bcfc..e56901e1e 100644
--- a/include/aidge/operator/Scaling.hpp
+++ b/include/aidge/operator/Scaling.hpp
@@ -66,11 +66,11 @@ public:
         return std::make_shared<Scaling_Op>(*this);
     }
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<Scaling_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
         // FIXME: temporary workaround
-        mInputs[0]->setBackend(name);
+        mInputs[0]->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName() {
diff --git a/include/aidge/operator/Slice.hpp b/include/aidge/operator/Slice.hpp
index 7bdbd8099..c635ea3f5 100644
--- a/include/aidge/operator/Slice.hpp
+++ b/include/aidge/operator/Slice.hpp
@@ -95,12 +95,12 @@ public:
         mOutputs[0]->resize(outputDims);
     }
 
-    void setBackend(const std::string &name) {
+    void setBackend(const std::string &name, int device = 0) {
         mImpl = Registrar<Slice_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
+        getInput(0)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/Softmax.hpp b/include/aidge/operator/Softmax.hpp
index cc19cb821..08500b2fa 100644
--- a/include/aidge/operator/Softmax.hpp
+++ b/include/aidge/operator/Softmax.hpp
@@ -51,12 +51,12 @@ public:
         return std::make_shared<Softmax_Op>(*this);
     }
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<Softmax_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
+        getInput(0)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/Sqrt.hpp b/include/aidge/operator/Sqrt.hpp
index a4069b59b..7df04f776 100644
--- a/include/aidge/operator/Sqrt.hpp
+++ b/include/aidge/operator/Sqrt.hpp
@@ -56,12 +56,12 @@ public:
         return std::make_shared<Sqrt_Op>(*this);
     }
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<Sqrt_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
+        getInput(0)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/include/aidge/operator/Sub.hpp b/include/aidge/operator/Sub.hpp
index becf98926..d4c8c7ad0 100644
--- a/include/aidge/operator/Sub.hpp
+++ b/include/aidge/operator/Sub.hpp
@@ -59,13 +59,13 @@ public:
     void computeOutputDims() override final;
 
 
-    void setBackend(const std::string& name) override {
+    void setBackend(const std::string& name, int device = 0) override {
         mImpl = Registrar<Sub_Op>::create(name)(*this);
-        mOutputs[0]->setBackend(name);
+        mOutputs[0]->setBackend(name, device);
 
         // FIXME: temporary workaround
-        getInput(0)->setBackend(name);
-        getInput(1)->setBackend(name);
+        getInput(0)->setBackend(name, device);
+        getInput(1)->setBackend(name, device);
     }
 
     static const std::vector<std::string> getInputsName(){
diff --git a/src/graph/GraphView.cpp b/src/graph/GraphView.cpp
index 0b64c518c..01de48b41 100644
--- a/src/graph/GraphView.cpp
+++ b/src/graph/GraphView.cpp
@@ -247,10 +247,10 @@ Aidge::GraphView::inputs(std::string name) const {
   return mNodeRegistry.at(name)->inputs();
 }
 
-void Aidge::GraphView::compile(const std::string& backend, const Aidge::DataType datatype) {
+void Aidge::GraphView::compile(const std::string& backend, const Aidge::DataType datatype, int device) {
     // Backend
     // TODO: add Backend attribute to Operator
-    setBackend(backend);
+    setBackend(backend, device);
     // Data type
     // TODO: manage Datatype attribute in OperatorImpl
     setDataType(datatype);
@@ -320,9 +320,9 @@ void Aidge::GraphView::_forwardDims(std::set<std::shared_ptr<Node>> listNodes) {
     }
 }
 
-void Aidge::GraphView::setBackend(const std::string &backend) {
+void Aidge::GraphView::setBackend(const std::string &backend, int device) {
     for (auto node : getNodes()) {
-        node->getOperator()->setBackend(backend);
+        node->getOperator()->setBackend(backend, device);
     }
 }
 
diff --git a/src/operator/Convert.cpp b/src/operator/Convert.cpp
new file mode 100644
index 000000000..d88fcfed4
--- /dev/null
+++ b/src/operator/Convert.cpp
@@ -0,0 +1,83 @@
+/********************************************************************************
+ * Copyright (c) 2023 CEA-List
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-2.0.
+ *
+ * SPDX-License-Identifier: EPL-2.0
+ *
+ ********************************************************************************/
+
+#include <cassert>
+#include <cstddef>
+#include <vector>
+#include <utility>
+
+#include "aidge/backend/OperatorImpl.hpp"
+#include "aidge/operator/Convert.hpp"
+#include "aidge/utils/Types.h"
+#include "aidge/utils/ErrorHandling.hpp"
+
+void Aidge::Convert_Op::forward() {
+    if (mImpl) {
+        mImpl->forward();
+    }
+    else {
+        // mConvertedInput stores data to the desired (output) type
+        if (mInputs[0]->dataType() != mOutputs[0]->dataType()) {
+            // Different type: create a new tensor on same input device
+            if (!mConvertedInput) {
+                mConvertedInput = std::make_shared<Tensor>(mOutputs[0]->dataType());
+            }
+
+            mConvertedInput->setDataType(mOutputs[0]->dataType());
+            const auto device = mInputs[0]->getImpl()->device();
+            mConvertedInput->setBackend(device.first, device.second);
+            mConvertedInput->resize(mInputs[0]->dims());
+
+            // Copy convert input to mConvertedInput
+            mConvertedInput->getImpl()->copyCast(mInputs[0]->getImpl()->rawPtr(), mInputs[0]->size(), mInputs[0]->dataType());
+        }
+        else {
+            // Same type: mConvertedInput *is* the input
+            mConvertedInput = mInputs[0];
+        }
+
+        // Copy to output device, if necessary
+        if (mConvertedInput->getImpl()->device() != mOutputs[0]->getImpl()->device()) {
+            if (mConvertedInput->getImpl()->backend() == mOutputs[0]->getImpl()->backend()) {
+                // Same backend, but different device
+                mOutputs[0]->getImpl()->copyFromDevice(mConvertedInput->getImpl()->rawPtr(), mConvertedInput->size(), mConvertedInput->getImpl()->device());
+            }
+            else if (mConvertedInput->getImpl()->hostPtr() != nullptr) {
+                // Different backend, but input is valid on host
+                mOutputs[0]->getImpl()->copyFromHost(mConvertedInput->getImpl()->hostPtr(), mConvertedInput->size());
+            }
+            else if (mOutputs[0]->getImpl()->hostPtr() != nullptr) {
+                // Different backend, but output is valid on host
+                mConvertedInput->getImpl()->copyToHost(mOutputs[0]->getImpl()->hostPtr(), mConvertedInput->size());
+            }
+            else {
+                // No direct link possible from input to output device
+                // SLOW SOLUTION: must pass through the host, requires TWO copies
+                const auto availableBackends = Tensor::getAvailableBackends();
+                AIDGE_ASSERT(availableBackends.find("cpu") != availableBackends.end(), "Conversion requires CPU backend");
+
+                if (!mHostBuffer) {
+                    mHostBuffer = std::make_shared<Tensor>(mOutputs[0]->dataType());
+                    mHostBuffer->setBackend("cpu");
+                }
+
+                mConvertedInput->getImpl()->copyToHost(mHostBuffer->getImpl()->rawPtr(), mConvertedInput->size());
+                mOutputs[0]->getImpl()->copyFromHost(mHostBuffer->getImpl()->rawPtr(), mConvertedInput->size());
+            }
+        }
+        else {
+            // Same device: simple copy on device
+            mConvertedInput->getImpl()->copy(mConvertedInput->getImpl()->rawPtr(), mConvertedInput->size());
+        }
+    }
+
+    runHooks();
+}
-- 
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