dongjianqiang2 wrote:
> > > My understanding is that this will make all calls to global functions > > > into long calls. > > > In AArch64 static linkes are required to insert range extension thunks > > > for out of range BLs. In the best case this is just another direct > > > branch, at worst case for `--pic-veneer` this is just `adrp, add, br`. I > > > would expect that on-demand linker inserted thunks would outperform > > > making all calls long for the majority of programs. I'm interested in any > > > data that shows that long calls works better, and whether that could feed > > > back into the lld thunk generation code. For example are the thunks too > > > far away from the caller which causes page faults etc. > > > I note that with `-ffunction-sections` and certain linker options calls > > > to static functions can go out of range too. These would get handled by > > > linker thunks though. > > > > > > This option is explicitly designed to enable reliable patching workflows > > when compiling object files. It is to guarantee call range safety in > > patches. When modifying/recompiling individual object files (e.g., > > during security patches)，final memory layouts are unknown at compile > > time, patched functions might end up >128MB away from callers. > > -mlong-calls forces all cross-object calls to use 64-bit absolute > > addressing. > > If I've understood object patching, this would mean inserting a new function > implementation, and binary patching all the call-sites to point to the new > implementation. > > As an aside to this patch. > > I'd be tempted to see if I could indirect all the calls via the PLT. Then > you'd be able add the new function and alter the dynamic symbol table entry > to point to the new implementation and the dynamic linker would do the rest. > That might need some fiddling in the linker or compiler driver to force it to > create a PLT entry, --shared would do it, but for an executable we'd need a > PT_INTERPRET section. > > There was a Discourse thread on ROM Patching for embedded systems > https://discourse.llvm.org/t/rfc-a-user-guided-rom-patching-mechanism-for-embedded-applications/78467 > which had a similar idea. Thanks @smithp35 for your solution! I would like to kindly ask for your expertise in reviewing the following code, which implements backend support for `-mlong-calls` on AArch64 targets. It might not need to be merged, just considering it as an optional approach. Thank you once again for your time and consideration. https://github.com/llvm/llvm-project/pull/142982 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
