[PATCH] D15440: [libc++abi] Use libgcc and libgcc_s to provide _Unwind symbols instead of libgcc_eh.a

[Eric Fiselier via cfe-commits]

EricWF added inline comments.

================
Comment at: src/CMakeLists.txt:37
@@ +36,3 @@
+
+remove_flags(-Wl,-z,defs)
+
----------------
EricWF wrote:
> compnerd wrote:
> > EricWF wrote:
> > > compnerd wrote:
> > > > EricWF wrote:
> > > > > compnerd wrote:
> > > > > > Do we need to worry about an alternative spelling of `-z defs`?
> > > > > Not sure. I only considered the spellings used within the LLVM source tree `llvm/cmake/modules/HandleLLVMOptions.cmake`.
> > > > This is a change in the original behavior.  I think it may be safer to add the `-z defs` and `-zdefs` spellings as well if you want the no undefined symbols behavior.  At least on solaris, I believe that `-no-undefined` is also going to cause this to be emitted.
> > > > This is a change in the original behavior.
> > > 
> > > Yes it is. libc++abi.so used to resolve the missing _Unwind symbols in libgcc_eh. However I'm hesitant to use `remove_flags` more than we need to because it's really dumb. For example calling `remove_flags(-pedantic)` on "-Wno-pedantic -pedantic-errors -pedantic" will result in the string "-Wno- -errors".  For this reason I think its safest to only handle the spelling LLVM uses.
> > > 
> > > Users shouldn't be passing any spelling of "-Wl,-zdefs" to the libc++abi build. I feel like this is one of those instances of "Doctor it hurts when I do this!".
> > > 
> > > Has this swayed your opinion at all? 
> > It has, only in a slightly different direction.  Why not use `--allow-shlib-undefined` instead?  That way we can insert the flag, which would actually mean that we wouldn't need to filter, and by adding it to the end, we don't need to worry about the flags from LLVM/users.
> That seems fairly reasonable. I didn't think of using `--allow-shlib-undefined` since that was already the default behavior. However so long as it actually works it seems like a good solution.
After looking at documentation it would appear that `--allow-shlib-undefined` doesn't do what we though.

http://stackoverflow.com/questions/2356168/force-gcc-to-notify-about-undefined-references-in-shared-libraries


http://reviews.llvm.org/D15440