Sorry for being late on my own thread. Trying to reply to most comments at once:
* I am pretty sure that we want to split the "in a comdat" notion from the linkage. I am not married to how this patch does it. * On MachO. This should actually improve things. A plain linkonce_odr will then mean the same thing on ELF and MachO and a comdat in MachO gets rejected early. * One Idea I had was having a selfcomdat syntactic sugar in the .ll, but I think I like rnk's suggestion better: just omit the $value (and flipping the order in the case of globals). * For the idea of using external instead of linkonce linkage. I agree with David. We should be making these things independent. Keeping the linkonce_odr/weak_odr is useful for the 'odr' bit, for saying the symbol should be STB_WEAK and for the can/cannot be dropped distinction. * I like the idea of optimizing the case of a select any that has the same name as an existing symbol, but it is not that simple in cases like $v = comdat any @v = global i32 42 comdat $v @bar = global i32 42 comdat $v When we are building bar, how do we find if there is a "simple comdat $v"? We would have to first lookup a GV named $v. When building @bar first and @v second we would still need to look at the "normal comdat table" when building $v to see if a comdat was already there. Since the main saving is on the symbol names, another idea would be to replace typedef StringMap<Comdat> ComdatSymTabType; With typedef DenseMap<const char *, Comdat> ComdatSymTabType; And have an extra StringMap with the non-symbol names like C5/D5 (or even introduce name -> null mappings in the gv symbol table?). But it would have the main constraint as the full optimization: when dropping the comdat, We have to be sure to drop $v last or we might end up we with a pointer to garbage. So, how about splitting work in: * llvm: Implement the syntatic change for comdats with the same name as the GV, but keep the current representation. * clang: this patch, but with less test change noise. * llvm: stop producing implicit comdats. * llvm: implement and benchmark one of the possible optimizations. On 20 December 2014 at 19:13, David Majnemer <[email protected]> wrote: > > > On Fri, Dec 19, 2014 at 5:43 PM, Richard Smith <[email protected]> > wrote: >> >> On Fri, Dec 19, 2014 at 12:51 PM, Rafael Espíndola >> <[email protected]> wrote: >>> >>> There is quiet a bit of history behind this. >>> >>> The llvm IR until very recently had no support for comdats. This was a >>> problem when targeting C++ on ELF/COFF as just using weak linkage >>> would cause quiet a bit of dead bits to remain on the executable >>> (unless -ffunction-sections, -fdata-sections and --gc-sections were >>> used). >>> >>> To fix the problem, llvm's codegen will just assume that any weak or >>> linkonce that is not in an explicit comdat should be output in one >>> with the same name as the global. >>> >>> This unfortunately breaks cases like pr19848 where a weak symbol is >>> not expected to be part of any comdat. >>> >>> Now that we have explicit comdats in the IR, we can finally get both >>> cases right. >>> >>> This first patch just makes clang give explicit comdats to >>> GlobalValues where it is allowed to. >>> >>> A followup patch to llvm will then stop implicitly producing comdats. >> >> >> I'm not sure this is the right direction, but if it is, this change looks >> reasonable to me. > > > I think this patch is fine, what are your reservations? > >> >> >> Assuming we want to go in this direction, I think we should also consider >> switching from a weak linkage to a strong linkage for entities governed by >> the ODR; using weak linkage is a hack, since these symbols are really not >> weak in the usual sense. > > > Maybe but I think that can be considered separately from this change. > >> >> That exposes a hole in our comdat support: we should have the ability to >> define a discardable comdat (which need not be emitted if none of its >> symbols are used in the current TU); a discardable comdat with an external >> function definition seems like the right representation for an inline >> function. > > > I don't think that we want to abuse linkage like that. We really need to > sit down untangle this mess. Before comdats, there was never a way to say > "give me a global, stick it in a COMDAT but *don't* make it weak", > COMDAT-ness and weak-ness came hand in hand. > > Now we have COMDATs and the world of linkage is unchanged, I don't think > that this is a good place to be. > > We really need to kill linkage and replace it with several things, each of > which individually control the behavior of the global. Things like > @llvm.used are a hack to work around the lack of flexibility linkage has to > offer. > > I'm more than happy to help out with this if we can get consensus on a > design. _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
