3 ms·

The problem with generating ODR violation diagnostics is that it requires the linker to compare all the duplicate definitions to each other not on bit-by-bit ba

by dfox 10y ago

The problem with generating ODR violation diagnostics is that it requires the linker to compare all the duplicate definitions to each other not on bit-by-bit basis, but somehow discerning whether they are generated from same input source or functionally equivalent, without embedding some hairy metadata in the object file this seems like something that reduces to halting problem. This is also one of the many problems that simply ceases to exists once you move away from the C/C++ model of "preprocessor as module system".

brucedawson 10y ago
I seem to remember that VC++ has a (possibly undocumented?) option to detect ODR violations. It might only work on LTCG builds where functions are stored in an intermediate representation, prior to optimization. But, I can't find any references to it, so maybe I am imagining.
- BudVVeezer 10y ago
  Is it the /d1reportSingleClassLayoutXXX flag, described here? https://blogs.msdn.microsoft.com/vcblog/2007/05/17/diagnosing-hidden-odr-violations-in-visual-c-and-fixing-lnk2022/ https://blogs.msdn.microsoft.com/vcblog/2007/05/17/diagnosin...
  - dfox 10y ago
    This detects differing structure layouts between compilation units which is simple to do given some metadata in the object files. On the other hand it also shows why detecting ODR violations for executable code is non-trivial, because same source code compiled with different compiler flags can lead to different output that might or might not be compatible (enforcing same compiler configuration for all compilation units is certainly non-starter, think JS runtime vs. FFmpeg, 3D renderer vs. physics simulation...).
gpderetta 10y ago
I believe that GCC, when running on LTO mode, embeds exactly this kind of 'hairy metadata' (IIRC the gimple code for the function) and is capable in principle of identifying ODR violations. I have to try this capability myself though, so I don't know how effective it is. edit: reading the docs again, it seems that it only detects ODR violations of the layout of types (and vtables).

by dfox 10y ago

brucedawson 10y ago

I seem to remember that VC++ has a (possibly undocumented?) option to detect ODR violations. It might only work on LTCG builds where functions are stored in an intermediate representation, prior to optimization. But, I can't find any references to it, so maybe I am imagining.

BudVVeezer 10y ago
Is it the /d1reportSingleClassLayoutXXX flag, described here? https://blogs.msdn.microsoft.com/vcblog/2007/05/17/diagnosing-hidden-odr-violations-in-visual-c-and-fixing-lnk2022/ https://blogs.msdn.microsoft.com/vcblog/2007/05/17/diagnosin...
- dfox 10y ago
  This detects differing structure layouts between compilation units which is simple to do given some metadata in the object files. On the other hand it also shows why detecting ODR violations for executable code is non-trivial, because same source code compiled with different compiler flags can lead to different output that might or might not be compatible (enforcing same compiler configuration for all compilation units is certainly non-starter, think JS runtime vs. FFmpeg, 3D renderer vs. physics simulation...).

gpderetta 10y ago

I believe that GCC, when running on LTO mode, embeds exactly this kind of 'hairy metadata' (IIRC the gimple code for the function) and is capable in principle of identifying ODR violations. I have to try this capability myself though, so I don't know how effective it is. edit: reading the docs again, it seems that it only detects ODR violations of the layout of types (and vtables).