Add fallbacks for 0 cpuinfo queries.

Fallback to 1 thread for when cpuinfo_get_cores_count()==0 and core
count when cpuinfo_get_l2_caches_count()==0.
Issue #4654 is tracking making this better.

iree/task/topology.c

@@ -113,7 +113,7 @@ static iree_status_t iree_task_topology_ensure_cpuinfo_available() {
 }
 
 static bool iree_task_topology_is_cpuinfo_available() {
-  return cpuinfo_initialize();
+  return cpuinfo_initialize() && cpuinfo_get_cores_count() > 0;
 }
 
 // Returns the core of the calling thread or NULL if not supported.
@@ -346,8 +346,10 @@ void iree_task_topology_initialize_from_physical_cores_with_filter(
 
 void iree_task_topology_initialize_from_unique_l2_cache_groups(
     iree_host_size_t max_group_count, iree_task_topology_t* out_topology) {
-  if (!iree_task_topology_is_cpuinfo_available()) {
-    iree_task_topology_initialize_fallback(max_group_count, out_topology);
+  if (!iree_task_topology_is_cpuinfo_available() ||
+      !cpuinfo_get_l2_caches_count()) {
+    iree_task_topology_initialize_from_physical_cores(max_group_count,
+                                                      out_topology);
     return;
   }
 

iree/task/topology.h

@@ -169,8 +169,11 @@ void iree_task_topology_initialize_from_physical_cores_with_filter(
 void iree_task_topology_initialize_from_unique_l2_cache_groups(
     iree_host_size_t max_group_count, iree_task_topology_t* out_topology);
 
-// TODO(benvanik): more? or just make users implement as desired? Ideas:
+// TODO(#4654): more helpers and better defaults for the platforms we support.
+// Users can always make their own but just using these is the common path.
+// Ideas:
 // - _from_unique_l2_cache_groups but with a min/max count (N% utilization)
+// - cluster filtering (big/little cores on ARM)
 
 #ifdef __cplusplus
 }  // extern "C"