fix: initializing order of gpu compile options (#4581)

Summary:
# issue
I met gpu_compile_options initialization order issue over gcc 8.4.0. Maybe related with LTO bug?

# solution
make it deterministic, no "crossing fingers" required. 😸

Pull Request resolved: https://github.com/facebookresearch/faiss/pull/4581

Reviewed By: subhadeepkaran

Differential Revision: D82647834

Pulled By: pankajsingh88

fbshipit-source-id: ead9515dc6e84987e6c8f5471318a34cc5b81086

faiss/gpu/GpuIndex.cu

@@ -748,20 +748,19 @@ bool isGpuIndexImplemented(faiss::Index* index) {
 } // namespace gpu
 
 // This is the one defined in utils.cpp
-// Crossing fingers that the InitGpuCompileOptions_instance will
-// be instanciated after this global variable
-extern std::string gpu_compile_options;
+extern std::string& ref_gpu_compile_options();
 
 struct InitGpuCompileOptions {
     InitGpuCompileOptions() {
-        gpu_compile_options = "GPU ";
+        ref_gpu_compile_options() = std::string("GPU ") +
 #ifdef USE_NVIDIA_CUVS
-        gpu_compile_options += "NVIDIA_CUVS ";
+                "NVIDIA_CUVS " +
 #endif
 
 #ifdef USE_AMD_ROCM
-        gpu_compile_options += "AMD_ROCM ";
+                "AMD_ROCM " +
 #endif
+                "";
     }
 };
 

faiss/utils/utils.cpp

@@ -102,7 +102,10 @@ int sgemv_(
 namespace faiss {
 
 // this will be set at load time from GPU Faiss
-std::string gpu_compile_options;
+std::string& ref_gpu_compile_options() {
+    static std::string gpu_compile_options;
+    return gpu_compile_options;
+}
 
 std::string get_compile_options() {
     std::string options;
@@ -124,7 +127,7 @@ std::string get_compile_options() {
     options += "GENERIC ";
 #endif
 
-    options += gpu_compile_options;
+    options += ref_gpu_compile_options();
 
     return options;
 }