{"id":8991,"date":"2025-07-24T20:55:00","date_gmt":"2025-07-25T03:55:00","guid":{"rendered":"https:\/\/visualgdb.com\/w\/?p=8991"},"modified":"2025-07-24T20:55:00","modified_gmt":"2025-07-25T03:55:00","slug":"debugging-optimized-code","status":"publish","type":"post","link":"https:\/\/visualgdb.com\/documentation\/debug\/optimized\/","title":{"rendered":"Debugging Optimized Code"},"content":{"rendered":"<p>Debugging C\/C++ works best with the code that has not been optimized. Turning on even the minimal optimization level (-O1) can introduce various debugging glitches:<\/p>\n<ul>\n<li>Breakpoints not setting in some locations<\/li>\n<li>Stepping through functions could appear sporadic, jumping up and down<\/li>\n<li>Some variable values could be completely missing<\/li>\n<\/ul>\n<p>To avoid this, C\/C++ projects usually come with two configurations: <strong>Debug<\/strong> with optimization disabled (<strong>-O0<\/strong>) and <strong>Release<\/strong> with optimizations enabled (<strong>-O3<\/strong> or &#8211;<strong>Os<\/strong>).<\/p>\n<p>If this layout does not work due to some reason, you can try the workarounds described below.<\/p>\n<h3>Updating GCC version<\/h3>\n<p>Newer versions of GCC and GDB tend to handle it slightly better, so updating it could help. It will still be far from perfect though.<\/p>\n<h3>Selectively disabling optimization for individual files<\/h3>\n<p>If you are using MSBuild, you can override the optimization level of individual files by opening MSBuild project properties for the particular files, and changing the optimization level:<a href=\"https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/msbuild.png\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-8992\" src=\"https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/msbuild.png\" alt=\"\" width=\"1255\" height=\"614\" srcset=\"https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/msbuild.png 1255w, https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/msbuild-300x147.png 300w, https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/msbuild-1024x501.png 1024w, https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/msbuild-768x376.png 768w\" sizes=\"(max-width: 1255px) 100vw, 1255px\" \/><\/a>For CMake projects, you can also use the file-level properties, or edit <strong>CMakeLists.txt<\/strong> directly:<\/p>\n<pre>set_source_files_properties(source.cpp PROPERTIES COMPILE_FLAGS -O0)<\/pre>\n<p>Note that unlike MSBuild, where the per-file setting would <strong>override<\/strong> the per-project one, CMake would simply add different optimization flags one after another. According to the <a href=\"https:\/\/gcc.gnu.org\/onlinedocs\/gcc\/Optimize-Options.html\">GCC documentation<\/a>, the last flag in the command line will then take effect:<\/p>\n<pre>If you use multiple <samp class=\"option\">-O<\/samp> options, with or without level numbers, the last such option is the one that is effective.<\/pre>\n<p>You can double-check it by adding an #error directive in the source file and building it. If you are using Ninja, it will output the command line used to build the failing file, so you can double-check which optimization option comes last:<a href=\"https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/optimize.png\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-8994\" src=\"https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/optimize.png\" alt=\"\" width=\"799\" height=\"283\" srcset=\"https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/optimize.png 799w, https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/optimize-300x106.png 300w, https:\/\/visualgdb.com\/w\/wp-content\/uploads\/2025\/07\/optimize-768x272.png 768w\" sizes=\"(max-width: 799px) 100vw, 799px\" \/><\/a><\/p>\n<h3>Selectively disabling optimization for individual functions<\/h3>\n<p>Regardless of the build system used, you can de-optimize individual functions using the <strong>optimize<\/strong> attribute:<\/p>\n<pre>int __attribute__((optimize(\"O0\"))) sum(int x, int y)\r\n{\r\n    return x + y;\r\n}<\/pre>\n<p>This can be defined conditionally to only affect the Debug configuration:<\/p>\n<pre>#ifdef DEBUG\r\n#define UNOPTIMIZED __attribute__((optimize(\"O0\")))\r\n#else\r\n#define UNOPTIMIZED\r\n#endif\r\n\r\nint UNOPTIMIZED sum(int x, int y)\r\n{\r\n    return x + y;\r\n}<\/pre>\n<p>You can also use the <strong>#pragma GCC optimize<\/strong> directive to change the optimization level of all functions within a given block:<\/p>\n<pre>#pragma GCC push_options\r\n#pragma GCC optimize (\"O0\")\r\n\r\nint sum(int x, int y)\r\n{\r\n    return x + y;\r\n}\r\n\r\n#pragma GCC pop_options<\/pre>\n<h3>Other tricks<\/h3>\n<p>If you only care about viewing specific variable values, you can try declaring them as <strong>volatile<\/strong>. This would disable most optimizations for a particular variable, also making it visible in the debugger. Similarly for functions, you can try using <strong>__attribute__((noinline))<\/strong> to prevent inlining of a particular function.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Debugging C\/C++ works best with the code that has not been optimized. Turning on even the minimal optimization level (-O1)<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[227],"tags":[171],"_links":{"self":[{"href":"https:\/\/visualgdb.com\/w\/wp-json\/wp\/v2\/posts\/8991"}],"collection":[{"href":"https:\/\/visualgdb.com\/w\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/visualgdb.com\/w\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/visualgdb.com\/w\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/visualgdb.com\/w\/wp-json\/wp\/v2\/comments?post=8991"}],"version-history":[{"count":2,"href":"https:\/\/visualgdb.com\/w\/wp-json\/wp\/v2\/posts\/8991\/revisions"}],"predecessor-version":[{"id":8995,"href":"https:\/\/visualgdb.com\/w\/wp-json\/wp\/v2\/posts\/8991\/revisions\/8995"}],"wp:attachment":[{"href":"https:\/\/visualgdb.com\/w\/wp-json\/wp\/v2\/media?parent=8991"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/visualgdb.com\/w\/wp-json\/wp\/v2\/categories?post=8991"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/visualgdb.com\/w\/wp-json\/wp\/v2\/tags?post=8991"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}