I am working on a legacy project that has a large amount of files dating back to pre-OS X days. It's data has been 16 bit aligned for > 15 years. I would like to move to a full LLVM compilation but I can't seem to get 2 byte alignment working. Are there any compiler level options available for this? (previously using -malign-mac68k)
I am aware of the #pragma pack(2) option here. However that would require me to modify upwards of 1000 source files to include this. That it's a worst-case option, but it seems like a hack. Besides, if this is possible then surely there is a default option to set the alignment?
According to clang's sources, it does support mac68k alignment rules. It seems that right now you can enable it via "#pragma options align=mac68k" only. If you're ok with small clang hacking, then you can implement the cmdline option as well and submit the patch to clang.
I would suggest looking at #pragma pack (see http://msdn.microsoft.com/en-us/library/2e70t5y1%28v=vs.80%29.aspx and http://gcc.gnu.org/onlinedocs/gcc/Structure_002dPacking-Pragmas.html). It's relatively easy to use, and should work with any compiler Apple ships.
i use the latest gcc/gortran on osx compiled by the guys at ibm, if you read the gcc manual there are at least 8 different alignment optimizations to consider not just the bulk malign
Because system-level headers assume default alignment (they don't have directives to override alignment in most cases), changing the alignment for everything will break OS calls. As such, you don't want to be doing this.
Just write a script to apply your #pragma pack(2) to the source files in question, avoiding #includes. It's relatively easy and unlikely to cause unfortunate side-effects.
来源：https://stackoverflow.com/questions/6430214/getting-llvm-clang-to-be-16-bit-aligned