added some comments on the modifications done into the code

modules/core/include/opencv2/core/utils/allocator_stats.impl.hpp

@@ -21,7 +21,8 @@ class AllocatorStatistics : public AllocatorStatisticsInterface
 {
 protected:
 #ifdef CV_CXX11
-    std::atomic<long> curr, total, total_allocs, peak;      // ESP32 modification (from <long long> to <long>)
+    // std::atomic<long long> curr, total, total_allocs, peak; 
+    std::atomic<long> curr, total, total_allocs, peak;      // esp32 doesn't support hardware 64bit atomic. FIXME: add software support
 #else
     volatile long long curr, total, total_allocs, peak;  // overflow is possible, CV_XADD operates with 'int' only
 #endif

modules/core/src/parallel.cpp

@@ -942,6 +942,7 @@ int cv::getNumberOfCPUs(void)
 
 #endif
 
+// esp32 doesn't support sysconf call. FIXME: add support
 #if !defined(_WIN32) && !defined(__APPLE__) && !defined(ESP32)
 
     static unsigned cpu_count_sysconf = (unsigned)sysconf( _SC_NPROCESSORS_ONLN );

modules/core/src/system.cpp

@@ -1000,6 +1000,7 @@ String tempfile( const char* suffix )
         fname = fname + "__opencv_temp.XXXXXX";
     }
 
+    // FIXME: esp32 compilation only find mktemp and not mkstemp
     //const int fd = mkstemp((char*)fname.c_str());
     //if (fd == -1) return String();
 

modules/imgproc/src/color.simd_helpers.hpp

@@ -136,6 +136,7 @@ public:
     {
     }
 
+// Optimization doesn't works with xtensa cross-compiler
 #pragma GCC push_options
 #pragma GCC optimize ("-O0")
     virtual void operator()(const Range& range) const CV_OVERRIDE

modules/imgproc/src/histogram.cpp

@@ -1777,6 +1777,7 @@ calcSparseBackProj_8u( std::vector<uchar*>& _ptrs, const std::vector<int>& _delt
 
 }
 
+// Optimization doesn't works with xtensa cross-compiler
 #pragma GCC push_options
 #pragma GCC optimize ("-O0")
 void cv::calcBackProject( const Mat* images, int nimages, const int* channels,