On Tue, Dec 7, 2021 at 1:01 AM Sandra Loosemore <san...@codesourcery.com> wrote: > > The attached patch fixes an ICE in lto1 at lto-partition.c:215 that > was reported by a customer. Unfortunately I have no test case for > this; the customer's application is a big C++ shared library with lots > of dependencies and proprietary code under NDA. I did try reducing it > with cvise but couldn't end up with anything small or robust enough to > be suitable, and likewise my attempts to write a small testcase by > hand were not successful in reproducing the bug. OTOH, I did track > this down in the debugger and I think I have a pretty good > understanding of what the problem is, namely...
I found PR65995 which looks related (well, the ICE is at the same position). Does the customer use the linker plugin? > The symbol lto-partition was failing on was a vtable symbol that was > in a comdat group but not marked as DECL_COMDAT and without the right > visibility flags for a comdat symbol. The LTO data in the input .o > files is correct and lto1 is creating the symbol correctly when it's > read in, but the problem is that there are two optimization passes > being run before lto-partition that are trying to do conflicting > things with COMDATs. > > The first is the ipa-visibility pass. When this pass is run as part > of lto1, since it has the complete program or shared library (as in > this case) available to analyze, it tries to break up COMDATs and turn > them into ordinary local symbols. But then the ipa-comdats pass, > which is also run as part of regular IPA optimizations at compile > time, tries to do the reverse and move local symbols only referenced > from a COMDAT into that same COMDAT, but in the process it is failing > to set all the flags needed by the LTO partitioner to correctly > process the symbol. It's possible the failure to set the flags > properly is a bug in ipa-comdats, but looking at the bigger picture, > it makes no sense to be running this optimization pass at link time at > all. It is a compile-time optimization intended to reduce code size > by letting the linker keep only one copy of these symbols that may be > redundantly defined in multiple objects. > > So the patch I've come up with disables the ipa-comdats pass in the > same situations in which ipa-visibility localizes COMDAT symbols, to > remove the conflict between the two passes. This fixes the customer > problem (in a GCC 10 build for arm-linux-gnueabi), and regression > tests OK on mainline x86_64-linux-gnu as well. OK for mainline, and > to backport to older branches? It looked to me like none of this > this code had been touched for years. I hope Honza can review this. Thanks, Richard. > > -Sandra
