khchen added a comment. In D67385#1680959 <https://reviews.llvm.org/D67385#1680959>, @tejohnson wrote:
> In D67385#1680942 <https://reviews.llvm.org/D67385#1680942>, @khchen wrote: > > > @tejohnson for example: > > > > $ clang -flto a.c -c -o a.o > > $ llvm-ar q a.a a.o // archive libraries > > $ clang -flto a.a b.c -O2 -o main -mcmodel=small > > > > > > In above case user need to aware that passing > > `-Wl,-plugin-opt=-code-model=small` to plugin is necessary. > > It seems to me that is inconvenience because I believe user doesn't know > > it. > > > In general, -flto should be transparent to the build process. Think about > what happens in your example if you remove -flto: > > $ clang a.c -c -o a.o > $ llvm-ar q a.a a.o // archive libraries > $ clang a.a b.c -O2 -o main -mcmodel=small > > a.o will be a native code built with the default mcmodel and -O0. > > Generally, adding -flto shouldn't change this - I would argue that it would > be unexpected to apply mcmodel=small as the user chose to compile a.c with > the default mcmodel. We are going more and more toward a model where info is > communicated via the IR from the compile step, as the mcmodel already is. Is > there a compelling reason why a.c should not get the default mcmodel in this > case (without having compiled it mcmodel=small in the first place)? Because this library (a.c) is provided by another user/developer. But you are right, if -flto should be transparent to the build process, user should be aware that. >> I think this case is similar to if a user has a bitcode library compiled >> with -O2 or -Oz, and link a program with -O3 -flto, what is expected >> behavior for user? > > There are a few things that are passed through the link command line to the > plugin for legacy reasons, including -O2/-O3. However, note that in your > above example the -O2 will not cause a.o to be O2 > <https://reviews.llvm.org/owners/package/2/> optimized: a.c is built at -O0 > and this is encoded in the IR via function attributes (noinline and optnone), > so the functions will not get O2 > <https://reviews.llvm.org/owners/package/2/>/O3 > <https://reviews.llvm.org/owners/package/3/> optimizations like inlining, etc. Got it, that's helpful. Thanks for your time and effort. Repository: rC Clang CHANGES SINCE LAST ACTION https://reviews.llvm.org/D67385/new/ https://reviews.llvm.org/D67385 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
