xnnpack: skip the file-backed weights cache without a filesystem

The weights cache can back its packed buffers with a file, using open(),
flock(), ftruncate() and mmap(). Those are guarded for Windows already; a
bare-metal target such as Zephyr is in the same position, and more
fundamentally has no filesystem to put the cache on, so it cannot compile that
path either.

Give the condition a name and include Zephyr in it. Such builds keep the heap
path, which is also what the POSIX path falls back to whenever the cache file
cannot be opened.

Author: npitre

backends/xnnpack/runtime/XNNWeightsCache.cpp

@@ -9,7 +9,17 @@
 #include <executorch/backends/xnnpack/runtime/XNNWeightsCache.h>
 #include <executorch/runtime/core/error.h>
 #include <executorch/runtime/core/memory_allocator.h>
-#ifndef _WIN32
+/*
+ * The file-backed weights cache relies on POSIX file APIs (open/flock/
+ * ftruncate) plus mmap.  Windows lacks them, and so do bare-metal targets such
+ * as Zephyr, which have no filesystem to back the cache with in the first
+ * place.  Those builds use the heap path only.
+ */
+#if defined(_WIN32) || defined(__ZEPHYR__)
+#define ET_XNN_NO_FILE_BACKED_CACHE 1
+#endif
+
+#ifndef ET_XNN_NO_FILE_BACKED_CACHE
 #include <fcntl.h>
 #include <sys/file.h>
 #include <sys/mman.h>
@@ -48,7 +58,7 @@ XNNWeightsCache::XNNWeightsCache() {
 }
 
 XNNWeightsCache::~XNNWeightsCache() {
-#ifndef _WIN32
+#ifndef ET_XNN_NO_FILE_BACKED_CACHE
   for (auto& region : mmap_regions_) {
     if (region.addr != nullptr && region.addr != MAP_FAILED) {
       munmap(region.addr, region.size);
@@ -76,7 +86,7 @@ static T read_le(const uint8_t* src) {
   return value;
 }
 
-#ifndef _WIN32
+#ifndef ET_XNN_NO_FILE_BACKED_CACHE
 // Open the cache file and take an advisory exclusive lock. Returns the
 // fd, or -1 if open/flock failed (logs the failure). The caller decides
 // how to recover (typically: skip the mmap path for this init).
@@ -135,7 +145,7 @@ Error XNNWeightsCache::initialize_for_runtime(
   named_data_map_ = named_data_map;
   is_finalized_ = false;
 
-#ifndef _WIN32
+#ifndef ET_XNN_NO_FILE_BACKED_CACHE
   if (packed_cache_path_.empty() || packed_file_fd_ >= 0) {
     return Error::Ok;
   }
@@ -212,7 +222,7 @@ Result<std::vector<std::string>> XNNWeightsCache::finalize_for_runtime() {
     }
   }
 
-#ifndef _WIN32
+#ifndef ET_XNN_NO_FILE_BACKED_CACHE
   // Synchronous flush for newly added regions. MS_SYNC blocks until the
   // dirty pages are written to disk and marked clean
   if (mmap_regions_.size() > mmap_regions_synced_) {
@@ -258,7 +268,7 @@ Result<const uint8_t*> XNNWeightsCache::load_unpacked_data(
 
 void XNNWeightsCache::release_entry(void* packed_data_ptr) {
   packed_pointer_to_container_.erase(packed_data_ptr);
-#ifndef _WIN32
+#ifndef ET_XNN_NO_FILE_BACKED_CACHE
   // Per-entry file-backed mmap region: munmap to release VM. The
   // packed_data_ptrs_ slot is nulled by the caller so existing offsets
   // stay valid.
@@ -276,7 +286,7 @@ void XNNWeightsCache::release_entry(void* packed_data_ptr) {
 }
 
 void XNNWeightsCache::full_unload() {
-#ifndef _WIN32
+#ifndef ET_XNN_NO_FILE_BACKED_CACHE
   for (auto& region : mmap_regions_) {
     if (region.addr != nullptr && region.addr != MAP_FAILED) {
       munmap(region.addr, region.size);
@@ -376,7 +386,7 @@ size_t XNNWeightsCache::look_up(
 }
 
 void* XNNWeightsCache::reserve_space(XNNWeightsCache* context, size_t n) {
-#ifndef _WIN32
+#ifndef ET_XNN_NO_FILE_BACKED_CACHE
   if (context->last_lookup_unnamed_ || context->loaded_from_disk_) {
     return context->reserve_space_heap(n);
   }
@@ -538,7 +548,7 @@ void XNNWeightsCache::set_packed_cache_path(const std::string& path) {
 }
 
 Error XNNWeightsCache::save_packed_index() {
-#ifndef _WIN32
+#ifndef ET_XNN_NO_FILE_BACKED_CACHE
   if (packed_file_fd_ < 0) {
     return Error::Ok;
   }
@@ -638,7 +648,7 @@ Error XNNWeightsCache::save_packed_index() {
 }
 
 bool XNNWeightsCache::load_packed_cache() {
-#ifndef _WIN32
+#ifndef ET_XNN_NO_FILE_BACKED_CACHE
   int fd = open(packed_cache_path_.c_str(), O_RDONLY);
   if (fd < 0) {
     return false;