Add standalone Link-Time Optimization option to CMake (facebookresearch#2943)

Summary:
Adds `-DFAISS_USE_LTO=ON` option to CMake to enable LTO. LTO increases the linking time, but potentially provides a small boost to the whole library.

Pull Request resolved: facebookresearch#2943

Reviewed By: mengdilin

Differential Revision: D61868553

Pulled By: junjieqi

fbshipit-source-id: f07ade6fdaaa337876f28b9d06bdc5629cc486b0

Author: alexanderguzhva

CMakeLists.txt

@@ -63,6 +63,7 @@ option(FAISS_ENABLE_RAFT "Enable RAFT for GPU indexes." OFF)
 option(FAISS_ENABLE_ROCM "Enable ROCm for GPU indexes." OFF)
 option(FAISS_ENABLE_PYTHON "Build Python extension." ON)
 option(FAISS_ENABLE_C_API "Build C API." OFF)
+option(FAISS_USE_LTO "Enable Link-Time optimization" OFF)
 
 if(FAISS_ENABLE_GPU)
   if(FAISS_ENABLE_ROCM)

INSTALL.md

@@ -122,10 +122,9 @@ Several options can be passed to CMake, among which:
   - `-DCMAKE_BUILD_TYPE=Release` in order to enable generic compiler
   optimization options (enables `-O3` on gcc for instance),
   - `-DFAISS_OPT_LEVEL=avx2` in order to enable the required compiler flags to
-  generate code using optimized SIMD/Vector instructions. possible values are
-  below:
-    - On x86\_64, `generic`, `avx2` and `avx512`, by increasing order of optimization,
-    - On aarch64, `generic` and `sve` , by increasing order of optimization,
+  generate code using optimized SIMD instructions (possible values are `generic`,
+  `avx2` and `avx512`, by increasing order of optimization),
+  - `-DFAISS_USE_LTO=ON` in order to enable [Link-Time Optimization](https://en.wikipedia.org/wiki/Link-time_optimization) (default is `OFF`, possible values are `ON` and `OFF`).
 - BLAS-related options:
   - `-DBLA_VENDOR=Intel10_64_dyn -DMKL_LIBRARIES=/path/to/mkl/libs` to use the
   Intel MKL BLAS implementation, which is significantly faster than OpenBLAS

faiss/CMakeLists.txt

@@ -334,6 +334,20 @@ target_compile_definitions(faiss_avx2 PRIVATE FINTEGER=int)
 target_compile_definitions(faiss_avx512 PRIVATE FINTEGER=int)
 target_compile_definitions(faiss_sve PRIVATE FINTEGER=int)
 
+if(FAISS_USE_LTO)
+  include(CheckIPOSupported)
+  check_ipo_supported(RESULT ipo_supported OUTPUT ipo_error)
+
+  if (ipo_supported)
+    message(STATUS "LTO enabled")
+    set_property(TARGET faiss PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+    set_property(TARGET faiss_avx2 PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+    set_property(TARGET faiss_avx512 PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+  else()
+    message(STATUS "LTO not supported: <${ipo_error}>")
+  endif()
+endif()
+
 find_package(OpenMP REQUIRED)
 target_link_libraries(faiss PRIVATE OpenMP::OpenMP_CXX)
 target_link_libraries(faiss_avx2 PRIVATE OpenMP::OpenMP_CXX)