Avoid linking multiple OMP runtimes in libtorch_cpu.so if BLAS used is OpenBLAS. (#147725)

vinithakv · pytorchmergebot · commit e872bf8f888b · 2025-04-29T23:39:48.000Z
When PyTorch is built with OpenBLAS support and libopenblas is ldrectly linked with libgomp.so the libtorch_cpu.so ends up getting multiple omp runtimes linked against it. This may result in unexpected runtime behaviour /regression. This patch fixes this by avoiding linking against libomp.so if OpenBLAS is linked against libgomp.so Fixes #146603 Pull Request resolved: #147725 Approved by: https://github.com/albanD
diff --git a/cmake/Modules/FindOpenBLAS.cmake b/cmake/Modules/FindOpenBLAS.cmake
@@ -59,6 +59,32 @@ ELSE (OpenBLAS_FOUND)
   ENDIF (OpenBLAS_FIND_REQUIRED)
 ENDIF (OpenBLAS_FOUND)
 
+IF(OpenBLAS_LIB)
+ # Run ldd on the OpenBLAS library
+execute_process(
+  COMMAND ldd "${OpenBLAS_LIB}"
+  OUTPUT_VARIABLE LDD_OUTPUT
+  ERROR_VARIABLE LDD_ERROR
+  RESULT_VARIABLE LDD_RESULT
+  OUTPUT_STRIP_TRAILING_WHITESPACE
+)
+
+if(NOT LDD_RESULT EQUAL 0)
+  message(WARNING "ldd failed on ${OpenBLAS_LIB}: ${LDD_ERROR}")
+endif()
+
+# Check if the output contains "libgomp"
+string(FIND "${LDD_OUTPUT}" "libgomp" LIBGOMP_FOUND_INDEX)
+if(LIBGOMP_FOUND_INDEX GREATER -1)
+  message(STATUS "OpenBLAS is directly linked against libgomp")
+  set(OPENBLAS_USES_LIBGOMP TRUE CACHE BOOL "OpenBLAS uses libgomp")
+else()
+  message(STATUS "OpenBLAS is not directly linked against libgomp")
+  set(OPENBLAS_USES_LIBGOMP FALSE CACHE BOOL "OpenBLAS uses libgomp")
+endif()
+
+ENDIF(OpenBLAS_LIB)
+
 MARK_AS_ADVANCED(
     OpenBLAS_INCLUDE_DIR
     OpenBLAS_LIB
diff --git a/cmake/Modules/FindOpenMP.cmake b/cmake/Modules/FindOpenMP.cmake
@@ -277,6 +277,18 @@ function(_OPENMP_GET_FLAGS LANG FLAG_MODE OPENMP_FLAG_VAR OPENMP_LIB_NAMES_VAR)
       mark_as_advanced(OpenMP_libomp_LIBRARY)
     endif()
 
+    # Check if we are using  OpenBLAS which is linked against libgomp
+    # we may end up with  multiple omp runtimes linked
+    # against libtorch_cpu.so
+    if(OpenBLAS_LIB AND OPENBLAS_USES_LIBGOMP)
+      find_library(OpenMP_libomp_LIBRARY
+        NAMES gomp
+        HINTS ${CMAKE_${LANG}_IMPLICIT_LINK_DIRECTORIES}
+        DOC "libomp location for OpenMP"
+      )
+      mark_as_advanced(OpenMP_libomp_LIBRARY)
+    endif()
+
     if (NOT OpenMP_libomp_LIBRARY)
       find_library(OpenMP_libomp_LIBRARY
         NAMES omp gomp iomp5
