On Mon, Jul 8, 2019 at 4:41 PM Richard Sandiford <richard.sandif...@arm.com> wrote: > > Richard Biener <richard.guent...@gmail.com> writes: > > On Sun, Jul 7, 2019 at 9:07 PM Jeff Law <l...@redhat.com> wrote: > >> > >> On 7/7/19 3:45 AM, Richard Sandiford wrote: > >> > DCE tries to delete dead stores to local data and also tries to insert > >> > debug binds for simple cases: > >> > > >> > /* If this is a store into a variable that is being optimized away, > >> > add a debug bind stmt if possible. */ > >> > if (MAY_HAVE_DEBUG_BIND_STMTS > >> > && gimple_assign_single_p (stmt) > >> > && is_gimple_val (gimple_assign_rhs1 (stmt))) > >> > { > >> > tree lhs = gimple_assign_lhs (stmt); > >> > if ((VAR_P (lhs) || TREE_CODE (lhs) == PARM_DECL) > >> > && !DECL_IGNORED_P (lhs) > >> > && is_gimple_reg_type (TREE_TYPE (lhs)) > >> > && !is_global_var (lhs) > >> > && !DECL_HAS_VALUE_EXPR_P (lhs)) > >> > { > >> > tree rhs = gimple_assign_rhs1 (stmt); > >> > gdebug *note > >> > = gimple_build_debug_bind (lhs, unshare_expr (rhs), stmt); > >> > gsi_insert_after (i, note, GSI_SAME_STMT); > >> > } > >> > } > >> > > >> > But this doesn't help for things like "print *ptr" when ptr points > >> > to the local variable (tests Og-dce-1.c and Og-dce-2.c). It also tends > >> > to make the *live* -- and thus useful -- values optimised out, because > >> > we can't yet switch back to tracking the memory location as it evolves > >> > over time (test Og-dce-3.c). > >> > > >> > So for -Og I think it'd be better not to delete any stmts with > >> > vdefs for now. This also means that we can avoid the potentially > >> > expensive vop walks (which already have a cut-off, but still). > >> > > >> > The patch also fixes the Og failures in gcc.dg/guality/pr54970.c > >> > (PR 86638). > >> > > >> > Tested on aarch64-linux-gnu and x86_64-linux-gnu. OK to install? > >> > > >> > Richard > >> > > >> > > >> > 2019-07-07 Richard Sandiford <richard.sandif...@arm.com> > >> > > >> > gcc/ > >> > PR debug/86638 > >> > * tree-ssa-dce.c (keep_all_vdefs_p): New function. > >> > (mark_stmt_if_obviously_necessary): Mark all stmts with vdefs as > >> > necessary if keep_all_vdefs_p is true. > >> > (mark_aliased_reaching_defs_necessary): Add a gcc_checking_assert > >> > that keep_all_vdefs_p is false. > >> > (mark_all_reaching_defs_necessary): Likewise. > >> > (propagate_necessity): Skip the vuse scan if keep_all_vdefs_p is > >> > true. > >> > > >> > gcc/testsuite/ > >> > * c-c++-common/guality/Og-dce-1.c: New test. > >> > * c-c++-common/guality/Og-dce-2.c: Likewise. > >> > * c-c++-common/guality/Og-dce-3.c: Likewise. > >> OK > > > > I wonder how code size (and compile-time) is affected by the DSE/DCE patch? > > Say just look at -Og built cc1? > > Overall I see a ~2.5% slowdown and a 4.7% increase in load size. > That comes almost entirely from the (RTL) DSE side; this patch > and gimple DSE part don't seem to make much difference. > > If I keep the gimple passes as-is and just disable RTL DSE, the slowdown > is still ~2.5% and there's a 4.4% increase in load size. > > These are all measuring cc1plus (built from post-patch sources) > and using -O2 -g tree-into-ssa.ii for the speed checks. > > > Can you restrict the keep-all-vdefs to user variables (and measure the > > difference this makes)? > > In order to avoid wrong debug for pointer dereferences, I think it would > have to be keep-all-vdefs for writes to either user variables or unknown > locations. But as above, I can't measure a significant difference with > the patch. > > > Again I wonder if this makes C++ with -Og impractical runtime-wise. > > Got a particular test in mind?
Nothing specific - there are a few C/C++ benchmarks in SPEC and there's also tramp3d-v4. I guess SRA is much more important for the abstraction penalty than DSE - FRE should be able to remove the abstraction, just the dead stores will remain (but they'd probably nicely execute out-of-order). Anyway, the biggest runtime penalty from -Og is probably not running any loop optimization (invariant motion mostly). Richard. > > Richard