C++ – Can optimizations affect the ability to debug a VC++ app using its PDB

coptimizationpdb-filesvisual studio

In order to be able to properly debug release builds a PDB file is needed. Can the PDB file become less usable when the compiler uses different kinds of optimizations (FPO, PGO, intrinsic functions, inlining etc.)? If so, is the effect of optimization severe or merely cause adjacent lines of code to get mixed up?

(I'm using VC2005, and will always choose debugability over optimized performance – but the question is general)

Best Answer

Yes, optimized code is less debuggable. Not only is some information missing, some information will be very misleading.

The biggest issue in my opinion is local variables. The compiler may use the same stack address or register for multiple variables throughout a function. As other posters mentioned, sometimes even figuring out what the "this" pointer is can take a bit of time. When debugging optimized code you may see the current line jumping around as you single step, since the compiler reorganized the generated code. If you use PGO, this jumping around will probably just get worse.

FPO shouldn't affect debuggability too much provided you have a PDB since the PDB contains all the info necessary to unwind the stack for FPO frames. FPO can be a problem when using tools that need to take stack traces without symbols. For many projects, the perf benefit of FPO nowadays doesn't outweigh the hit for diagnosability; for this reason, MS decided not to build Windows Vista with the FPO optimization (http://blogs.msdn.com/larryosterman/archive/2007/03/12/fpo.aspx).

I prefer to debug unoptimized code but this isn't always possible - some problems only repro with optimized code, customer crash dumps are from the released build, and getting a debug private deployed sometimes isn't possible. Often when debugging optimized code, I use the disassembly view - dissasembly never lies.

This all applies to windbg since I do all native code debugging with it. Visual Studio's debugger might handle some of these cases better.

Related Solutions

C++ – How to get the list of files in a directory using C or C++

UPDATE 2017:

In C++17 there is now an official way to list files of your file system: std::filesystem. There is an excellent answer from Shreevardhan below with this source code (This code may throw):

#include <string>
#include <iostream>
#include <filesystem>
namespace fs = std::filesystem;

int main()
{
    std::string path = "/path/to/directory";
    for (const auto & entry : fs::directory_iterator(path))
        std::cout << entry.path() << std::endl;
}

Old Answer:

In small and simple tasks I do not use boost, I use dirent.h. It is available as a standard header in UNIX, and also available for Windows via a compatibility layer created by Toni Ronkko.

DIR *dir;
struct dirent *ent;
if ((dir = opendir ("c:\\src\\")) != NULL) {
  /* print all the files and directories within directory */
  while ((ent = readdir (dir)) != NULL) {
    printf ("%s\n", ent->d_name);
  }
  closedir (dir);
} else {
  /* could not open directory */
  perror ("");
  return EXIT_FAILURE;
}

It is just a small header file and does most of the simple stuff you need without using a big template-based approach like boost(no offence, I like boost!).

Visual-studio – The Following Module was built either with optimizations enabled or without debug information

Try disabling "Enable Just My Code" in Tools/Options/Debugging/Options.

Best Answer

Related Solutions

C++ – How to get the list of files in a directory using C or C++

Visual-studio – The Following Module was built either with optimizations enabled or without debug information

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