Hi, (this message is a part of the thread originating with https://gcc.gnu.org/ml/gcc-patches/2018-09/msg01018.html) On Thu, Sep 27 2018, Jan Hubicka wrote: >> > If you make this to be INTERPOSABLE (which means it can be replaced by different >> > implementation by linker and that is probably what we want for live patching) >> > then also inliner, ipa-sra and other optimization will give up on these. >> >> do you suggest that to set the global function as AVAIL_INTERPOSABLE when -finline-only-static >> is present? then we should avoid all issues? > > It seems to be reasonable direction I think, because it is what really happens > (well AVAIL_INTERPOSABLE still does not assume that the interposition will > happen at runtime, but it is an approximation and we may introduce something like > AVAIL_RUNTIME_INTERPOSABLE if there is need for better difference). > I wonder if -finline-only-static is good name for the flag though, because it > does a lot more than that. Maybe something like -flive-patching? > How much is this all tied to one particular implementation of the feature? We have just had a quick discussion with two upstream maintainers of Linux kernel live-patching about this and the key points were: 1. SUSE live-patch creators (and I assume all that use the upstream live-patching method) use Martin Liska's (somewhat under-documented) -fdump-ipa-clones option and a utility he wrote (https://github.com/marxin/kgraft-analysis-tool) to deal with all kinds of inlining, IPA-CP and generally all IPA optimizations that internally create a clone. The tool tells them what happened and also lists all callers that need to be live-patched. 2. However, there is growing concern about other IPA analyses that do not create a clone but still affect code generation in other functions. Kernel developers have identified and disabled IPA-RA but there is more of them such as IPA-modref analysis, stack alignment propagation and possibly quite a few others which extract information from one function and use it a caller or perhaps even some almost-unrelated functions (such as detection of read-only and write-only static global variables). The kernel live-patching community would welcome if GCC had an option that could disable all such optimizations/analyses for which it cannot provide a list of all affected functions (i.e. which ones need to be live-patched if a particular function is). I assume this is orthogonal to the proposed -finline-only-static option, but the above approach seems superior in all respects. 3. The community would also like to be involved in these discussions, and therefore I am adding live-patching@xxxxxxxxxxxxxxx to CC. On a related note, they will also have a live-patching mini-summit at the Linux Plumbers conference in Vancouver in November where they plan to discuss what they would like GCC to provide. Thanks, Martin ... >> >> >> >>> For example comdat that was cloned by IPA-SRA. See can_be_local_p and >> >>> comdat_can_be_unshared_p predicates. Similar problem happens to clones created >> >>> by ipa-cp. >> >>> >> >>> I guess we want to disable localization and cloning in this case as well. >> >>> I wonder what else. >> >> >> >> Yes, I think we should make -finline-only-static incompatible with cloning and tree-sra too. >> >> >> >> Qing >> >>> >> >>> Honza >>
