diff --git a/CMakeLists.txt b/CMakeLists.txt
index bf194358..8f01ecaa 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -159,9 +159,18 @@ else()
     add_definitions(-fno-tree-vectorize)
 endif()
 
+# compile a target with -O2 in Debug mode
+function(upx_compile_target_debug_with_O2 t)
+    if(MSVC)
+        # msvc uses some Debug compile options like -RTC1 that are incompatible with -O2
+    else()
+        target_compile_options(${t} PRIVATE $<$<CONFIG:Debug>:-O2>)
+    endif()
+endfunction()
+
 function(upx_sanitize_target t)
     if(NOT UPX_CONFIG_DISABLE_SANITIZE AND NOT MSVC)
-        if(CMAKE_C_PLATFORM_ID MATCHES "^MinGW")
+        if(CMAKE_C_PLATFORM_ID MATCHES "^MinGW" OR MINGW OR CYGWIN)
             # avoid link errors with current MinGW-w64 versions
             # see https://www.mingw-w64.org/contribute/#sanitizers-asan-tsan-usan
         else()
@@ -175,6 +184,7 @@ endfunction()
 
 set(t upx_vendor_ucl)
 target_include_directories(${t} PRIVATE vendor/ucl/include vendor/ucl)
+upx_compile_target_debug_with_O2(${t})
 upx_sanitize_target(${t})
 if(MSVC)
     target_compile_options(${t} PRIVATE -J -W4 ${warn_WX})
@@ -183,6 +193,7 @@ else()
 endif()
 
 set(t upx_vendor_zlib)
+upx_compile_target_debug_with_O2(${t})
 upx_sanitize_target(${t})
 if(MSVC)
     target_compile_options(${t} PRIVATE -DHAVE_STDARG_H -DHAVE_VSNPRINTF -J -W3 ${warn_WX})
@@ -194,6 +205,7 @@ endif()
 
 if(NOT UPX_CONFIG_DISABLE_ZSTD)
 set(t upx_vendor_zstd)
+upx_compile_target_debug_with_O2(${t})
 upx_sanitize_target(${t})
 target_compile_options(${t} PRIVATE -DDYNAMIC_BMI2=0 -DZSTD_DISABLE_ASM)
 if(MSVC)
@@ -212,6 +224,7 @@ if(GITREV_SHORT)
         target_compile_definitions(${t} PRIVATE UPX_VERSION_GIT_DESCRIBE="${GIT_DESCRIBE}")
     endif()
 endif()
+#upx_compile_target_debug_with_O2(${t})
 upx_sanitize_target(${t})
 if(MSVC)
     target_compile_options(${t} PRIVATE -Zc:__cplusplus -EHsc -J -W4 ${warn_WX})
diff --git a/README.SRC b/README.SRC
index 31875007..4a5ff4ad 100644
--- a/README.SRC
+++ b/README.SRC
@@ -44,8 +44,8 @@ Short overview
 Tools needed to build/modify the UPX sources
 --------------------------------------------
 
-  - A C++ compiler that supports C++ 17: clang 5.0, gcc 8.0 or msvc 2019 16.11
-    (older or other compilers may work, use at your own risk)
+  - A C++ compiler that supports C++ 17: clang-5, gcc-8 or msvc-2019 16.11
+    (older or other compilers may work but are unsupported, use at your own risk)
 
   - GNU make