This seems like a massive rehash of a discussion Peter Collingbourne and I had about passing -O0 to the linker for -flto=full. I had previously thought of LTO as "link time optimization", but in practice it's useful for (and required for correctness of some) non-optimization IR passes.
In other words, the basic question seems to be: "Should LTO support non-optimization use cases?" I tend (now) to think it should -- having "optimization" in its name is an historical artifact -- because adding another way to run IR passes at link-time seems redundant. Whereas, Paul, it seems like you disagree? (Also, this discussion seems higher level than just the patch at hand... maybe llvm-dev would be more appropriate?) > On 2017-Jan-09, at 16:03, Paul Robinson via Phabricator > <revi...@reviews.llvm.org> wrote: > > probinson added a comment. > > In https://reviews.llvm.org/D28404#640588, @mehdi_amini wrote: > >> Actually, as mentioned before, I could be fine with making `O0` incompatible >> with LTO, however security features like CFI (or other sort of whole-program >> analyses/instrumentations) requires LTO. > > > Well, "requires LTO" is overstating the case, AFAICT from the link you gave > me. Doesn't depend on //optimization// at all. It depends on some > interprocedural analyses given some particular scope/visibility boundary, > which it is convenient to define as a set of linked bitcode modules, that by > some happy chance is the same set of linked bitcode modules that LTO will > operate on. > > If it's important to support combining a bitcode version of my-application > with your-bitcode-library for this CFI or whatever, and you also want to let > me have my-application be unoptimized while your-bitcode-library gets > optimized, NOW we have a use-case. (Maybe that's what you had in mind > earlier, but for some reason I wasn't able to extract that out of any prior > comments. No matter.) > > I'm now thinking along the lines of a `-foptimize-off` flag (bikesheds > welcome) which would set the default for the pragma to 'off'. How is that > different than what you wanted for `-O0`? It is defined in terms of an > existing pragma, which is WAY easier to explain and WAY easier to implement. > And, it still lets us say that `-c -O0 -flto` is a mistake, if that seems > like a useful thing to say. > > Does that seem reasonable? Fit your understanding of the needs? > > > https://reviews.llvm.org/D28404 > > > _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
