The Windows Linker has 3 main parts that this patch changes.
- Identification and classification of sections
- Adding of symbols to the symbols tables
- Reallocation of sections
Previously section identification used to be done on a whitelisted basis. It was also exclusively being done based on the names of the sections. This meant that there was a bit of a cat and mouse game between GCC and GHC. Every time GCC added new sections there was a good chance GHC would break. Luckily this hasn't happened much in the past because the GCC versions GHC used were largely unchanged.
The new code instead treats all new section as CODE or DATA sections, and changes the classifications based on the Characteristics flag in the PE header. By doing so we no longer have the fragility of changing section names. The one exception to this is the .ctors section, which has no differentiating flag in the PE header, but we know we need to treat it as initialization data.
The check to see if the sections are aligned by 4 has been removed. The reason is that debug sections often time are 1 aligned but do have relocation symbols. In order to support relocations of .debug sections this check needs to be gone. Crucially this assumption doesn't seem to be in the rest of the code. We only check if there are at least 4 bytes to realign further down the road.
The second loop is iterating of all the symbols in the file and trying to add them to the symbols table.
Because the classification of the sections we did previously are (currently) not available in this phase we still have to exclude the sections by hand. If they don't we will load in symbols from sections we've explicitly ignored the in # 1. This whole part should rewritten to avoid this. But didn't want to do it in this commit.
Finally the sections are relocated. But for some reason the PE files contain a Linux relocation constant in them 0x0011 This constant as far as I can tell does not come from GHC (or I couldn't find where it's being set). I believe this is probably a bug in GAS. But because the constant is in the output we have to handle it. I am thus mapping it to the constant I think it should be 0x0003.
Finally, static linking *should* work, but won't. At least not if you want to statically link libgcc with exceptions support. Doing so would require you to link libgcc and libstd++ but also libmingwex. The problem is that libmingwex also defines a lot of symbols that the RTS automatically injects into the symbol table. Presumably because they're symbols that it needs. like coshf. The these symbols are not in a section that is declared with weak symbols support. So if we ever want to get this working, we should either a) Ask mingw to declare the section as such, or b) treat all a imported symbols as being weak. Though this doesn't seem like it's a good idea..