Add instructions for a conda environment on mac that can successfully

build FTorch and utilise the MPS backend on Apple Silicon.

Co-authored-by: Karl Harrison <[email protected]>

README.md

@@ -115,9 +115,12 @@ Note that LibTorch is not supported for the GNU Fortran compiler with MinGW.
 
 #### Apple Silicon Support
 
-At the time of writing, LibTorch is only officially available for x86 architectures
-(according to https://pytorch.org/). However, the version of PyTorch provided by
-`pip install torch` uses an ARM binary for LibTorch which works on Apple Silicon.
+At the time of writing [there are issues](https://github.com/pytorch/pytorch/issues/143571)
+building FTorch on apple silicon when linking to downloaded `LibTorch` binaries or
+pip-installed PyTorch.
+FTorch can successfully be built, including utilising the MPS backend, from inside a
+conda environment using the environment files and instructions in
+[`conda/`](https://github.com/Cambridge-ICCS/FTorch/tree/main/conda).
 
 #### Conda Support
 

conda/README.md

@@ -74,6 +74,32 @@ cmake --build . --target install
 Note: There is the option of using `--parallel` to speed this up as described in
 the main documentation.
 
+### Mac and Mps
+
+At the time of writing [there are issues](https://github.com/pytorch/pytorch/issues/143571)
+building FTorch when linking to downloaded `LibTorch` binaries or pip-installed PyTorch.
+FTorch can successfully be built, including utilising the MPS backend, from inside a
+conda environment using the environment files provided here.
+
+From a conda base environment run:
+```sh
+conda env create -f environment_mac.yaml
+```
+from this directory to create the environment and install dependencies.
+We install PyTorch using `pip` from within the conda environment which should include
+the Mps backend.
+
+FTorch can then be build with a CMake command similar to the following:
+```sh
+cmake \
+    -DCMAKE_INSTALL_PREFIX=$CONDA_PREFIX \
+    -DCMAKE_PREFIX_PATH=$(python -c 'import torch;print(torch.utils.cmake_prefix_path)') \
+    -DCMAKE_BUILD_TYPE=Release \
+    ..
+cmake --build . --target install
+```
+Note: There is the option of using `--parallel` to speed this up as described in
+the main documentation.
 
 ### Other Backends
 

conda/environment_mac.yaml

@@ -0,0 +1,13 @@
+name: ftorch-mac
+channels:
+  - conda-forge
+  - nodefaults
+dependencies:
+  - fortran-compiler
+  - cxx-compiler
+  - cmake >=3.15
+  - openmpi >=5.0.6  # For pFUnit
+  - pip
+  - pip:  # We install PyTorch using pip as recommended by PyTorch
+    - torch
+    - torchvision