> While a plain Firefox -flto build works fine. LTO/PGO build fails with: > > lto1: internal compiler error: in ipa_merge_profiles, at ipa-utils.c:540 > 0x7d6165 ipa_merge_profiles(cgraph_node*, cgraph_node*) > ../../gcc/gcc/ipa-utils.c:540 > 0xf10c41 ipa_icf::sem_function::merge(ipa_icf::sem_item*) > ../../gcc/gcc/ipa-icf.c:753 > 0xf15206 ipa_icf::sem_item_optimizer::merge_classes(unsigned int) > ../../gcc/gcc/ipa-icf.c:2706 > 0xf1c1f4 ipa_icf::sem_item_optimizer::execute() > ../../gcc/gcc/ipa-icf.c:2098 > 0xf1d3f1 ipa_icf_driver > ../../gcc/gcc/ipa-icf.c:2784 > 0xf1d3f1 ipa_icf::pass_ipa_icf::execute(function*) > ../../gcc/gcc/ipa-icf.c:2831 > > > The pass is also very memory hungry (from 3GB without ICF to 4GB during > libxul link), while the code size savings are in the 1% range.
Thnks for checking. I was just thinking about doing that myself. Would you mind posting -ftime-report of firefox WPA stage? It seems that in this case we reject too many of equality candidates? It think the original numbers was about 4-5% but later some equivalences was disabled because of devirt/aliasing issues. Do you compare it with gold ICF enabled? There are quite few obvious improvements to the analysis that can be done, but I guess we need to analyze the interesting cases one by one. One thing that Martin can try is to hook into lto-symtab and try to check that the COMDAT functions that are known to be same pass the equality check. I suppose we will learn interesting things this way. I think the patch adds quite important infrastructure for gimple semantic equality checking and function merging. I went through the majority of code and I think it is mostly ready to mainline (i.e. cleaner than what we have in tree-ssa-tailmerge) so hope we can finish the review process next week. We will need to get better cost/benefits ratio to enable it for -O2 that is someting I would really like to see for 5.0, but it seems to be easier to handle this incrementally.... Honza
