Richard Biener <richard.guent...@gmail.com> writes: > On Sun, Jul 7, 2019 at 9:04 PM Jeff Law <l...@redhat.com> wrote: >> >> On 7/7/19 3:43 AM, Richard Sandiford wrote: >> > This patch stops gimple and rtl DSE from running by default at -Og. >> > The idea is both to improve compile time and to stop us from deleting >> > stores that we can't track in debug info. >> > >> > We could rein this back in future for stores to local variables >> > with is_gimple_reg_type, but at the moment we don't have any >> > infrastructure for switching between binds to specific values >> > and binds to evolving memory locations. Even then, location >> > tracking only works for direct references to the variables, and doesn't >> > for example help with printing dereferenced pointers (see the next patch >> > in the series for an example). >> > >> > I'm also not sure that DSE is important enough for -Og to justify the >> > compile time cost -- especially in the case of RTL DSE, which is pretty >> > expensive. >> > >> > Tested on aarch64-linux-gnu and x86_64-linux-gnu. OK to install? >> > >> > Richard >> > >> > >> > 2019-07-07 Richard Sandiford <richard.sandif...@arm.com> >> > >> > gcc/ >> > * common.opt (Og): Change the initial value of flag_dse to 0. >> > * opts.c (default_options_table): Move OPT_ftree_dse from >> > OPT_LEVELS_1_PLUS to OPT_LEVELS_1_PLUS_NOT_DEBUG. Also add >> > OPT_fdse to OPT_LEVELS_1_PLUS_NOT_DEBUG. Put the OPT_ftree_pta >> > entry before the OPT_ftree_sra entry. >> > * doc/invoke.texi (Og): Add -fdse and -ftree-dse to the list >> > of flags disabled by Og. >> > >> > gcc/testsuite/ >> > * c-c++-common/guality/Og-global-dse-1.c: New test. >> OK. I doubt DSE is at all important if someone has asked for Og. > > Hmm. -Og also asks for runtime performance. Can we instead restrict > DSE to artificial decls? I also fear that this leaves a _lot_ of abstraction > for C++ code given we also do not run SRA with -Og even though that > has means to preserve debug info.
Following up your question in the other patch: I see a 2.5% slowdown for cc1plus bulit at -Og when compiling tree-into-ssa.ii at -O2 -g. I think it mostly comes from removing RTL DSE. I get a very similar slowdown with gimple DSE reenabled. There again, we're not very C++-heavy, so cc1plus might not be a good test for that. Richard
