Any comment on revised patch? At least, in finish_decl, decl global attributes are populated.
On Tue, Jul 14, 2020 at 8:37 AM Sunil Pandey <skpg...@gmail.com> wrote: > On Sat, Jul 4, 2020 at 9:11 AM Richard Biener > <richard.guent...@gmail.com> wrote: > > > > On July 3, 2020 11:16:46 PM GMT+02:00, Jason Merrill <ja...@redhat.com> > wrote: > > >On 6/29/20 5:00 AM, Richard Biener wrote: > > >> On Fri, Jun 26, 2020 at 10:11 PM H.J. Lu <hjl.to...@gmail.com> wrote: > > >>> > > >>> On Thu, Jun 25, 2020 at 1:10 AM Richard Biener > > >>> <richard.guent...@gmail.com> wrote: > > >>>> > > >>>> On Thu, Jun 25, 2020 at 2:53 AM Sunil Pandey <skpg...@gmail.com> > > >wrote: > > >>>>> > > >>>>> On Wed, Jun 24, 2020 at 12:30 AM Richard Biener > > >>>>> <richard.guent...@gmail.com> wrote: > > >>>>>> > > >>>>>> On Tue, Jun 23, 2020 at 5:31 PM Sunil K Pandey via Gcc-patches > > >>>>>> <gcc-patches@gcc.gnu.org> wrote: > > >>>>>>> > > >>>>>>> From: Sunil K Pandey <skpg...@gmail.com> > > >>>>>>> > > >>>>>>> Default for this hook is NOP. For x86, in 32 bit mode, this hook > > >>>>>>> sets alignment of long long on stack to 32 bits if preferred > > >stack > > >>>>>>> boundary is 32 bits. > > >>>>>>> > > >>>>>>> - This patch fixes > > >>>>>>> gcc.target/i386/pr69454-2.c > > >>>>>>> gcc.target/i386/stackalign/longlong-1.c > > >>>>>>> - Regression test on x86-64, no new fail introduced. > > >>>>>> > > >>>>>> I think the name is badly chosen, > > >TARGET_LOWER_LOCAL_DECL_ALIGNMENT > > >>>>> > > >>>>> Yes, I can change the target hook name. > > >>>>> > > >>>>>> would be better suited (and then asks for LOCAL_DECL_ALIGNMENT to > > >be > > >>>>>> renamed to INCREASE_LOCAL_DECL_ALIGNMENT). > > >>>>> > > >>>>> It seems like LOCAL_DECL_ALIGNMENT macro documentation is > > >incorrect. > > >>>>> It increases as well as decreases alignment based on > > >condition(-m32 > > >>>>> -mpreferred-stack-boundary=2) > > >>>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95885 > > >>>>> > > >>>>>> > > >>>>>> You're calling it from do_type_align which IMHO is dangerous > > >since that's > > >>>>>> invoked from FIELD_DECL layout as well. Instead invoke it from > > >>>>>> layout_decl itself where we do > > >>>>>> > > >>>>>> if (code != FIELD_DECL) > > >>>>>> /* For non-fields, update the alignment from the type. */ > > >>>>>> do_type_align (type, decl); > > >>>>>> > > >>>>>> and invoke the hook _after_ do_type_align. Also avoid > > >>>>>> invoking the hook on globals or hard regs and only > > >>>>>> invoke it on VAR_DECLs, thus only > > >>>>>> > > >>>>>> if (VAR_P (decl) && !is_global_var (decl) && > > >!DECL_HARD_REGISTER (decl)) > > >>>>> > > >>>>> It seems like decl property is not fully populated at this point > > >call > > >>>>> to is_global_var (decl) on global variable return false. > > >>>>> > > >>>>> $ cat foo.c > > >>>>> long long x; > > >>>>> int main() > > >>>>> { > > >>>>> if (__alignof__(x) != 8) > > >>>>> __builtin_abort(); > > >>>>> return 0; > > >>>>> } > > >>>>> > > >>>>> Breakpoint 1, layout_decl (decl=0x7ffff7ffbb40, known_align=0) > > >>>>> at /local/skpandey/gccwork/gccwork/gcc/gcc/stor-layout.c:674 > > >>>>> 674 do_type_align (type, decl); > > >>>>> Missing separate debuginfos, use: dnf debuginfo-install > > >>>>> gmp-6.1.2-10.fc31.x86_64 isl-0.16.1-9.fc31.x86_64 > > >>>>> libmpc-1.1.0-4.fc31.x86_64 mpfr-3.1.6-5.fc31.x86_64 > > >>>>> zlib-1.2.11-20.fc31.x86_64 > > >>>>> (gdb) call debug_tree(decl) > > >>>>> <var_decl 0x7ffff7ffbb40 x > > >>>>> type <integer_type 0x7fffea801888 long long int DI > > >>>>> size <integer_cst 0x7fffea7e8d38 constant 64> > > >>>>> unit-size <integer_cst 0x7fffea7e8d50 constant 8> > > >>>>> align:64 warn_if_not_align:0 symtab:0 alias-set -1 > > >>>>> canonical-type 0x7fffea801888 precision:64 min <integer_cst > > >>>>> 0x7fffea7e8fd8 -9223372036854775808> max <integer_cst > > >0x7fffea806000 > > >>>>> 9223372036854775807> > > >>>>> pointer_to_this <pointer_type 0x7fffea8110a8>> > > >>>>> DI foo.c:1:11 size <integer_cst 0x7fffea7e8d38 64> unit-size > > >>>>> <integer_cst 0x7fffea7e8d50 8> > > >>>>> align:1 warn_if_not_align:0> > > >>>>> > > >>>>> (gdb) p is_global_var(decl) > > >>>>> $1 = false > > >>>>> (gdb) > > >>>>> > > >>>>> > > >>>>> What about calling hook here > > >>>>> > > >>>>> 603 do_type_align (tree type, tree decl) > > >>>>> 604 { > > >>>>> 605 if (TYPE_ALIGN (type) > DECL_ALIGN (decl)) > > >>>>> 606 { > > >>>>> 607 SET_DECL_ALIGN (decl, TYPE_ALIGN (type)); > > >>>>> 608 if (TREE_CODE (decl) == FIELD_DECL) > > >>>>> 609 DECL_USER_ALIGN (decl) = TYPE_USER_ALIGN (type); > > >>>>> 610 else > > >>>>> 611 /* Lower local decl alignment */ > > >>>>> 612 if (VAR_P (decl) > > >>>>> 613 && !is_global_var (decl) > > >>>>> 614 && !DECL_HARD_REGISTER (decl) > > >>>>> 615 && cfun != NULL) > > >>>>> 616 targetm.lower_local_decl_alignment (decl); > > >>>>> 617 } > > >>>> > > >>>> But that doesn't change anything (obviously). layout_decl > > >>>> is called quite early, too early it looks like. > > >>>> > > >>>> Now there doesn't seem to be any other good place where > > >>>> we are sure to catch the decl before we evaluate things > > >>>> like __alignof__ > > >>>> > > >>>> void __attribute__((noipa)) > > >>>> foo (__SIZE_TYPE__ align, long long *p) > > >>>> { > > >>>> if ((__SIZE_TYPE__)p & (align-1)) > > >>>> __builtin_abort (); > > >>>> } > > >>>> int main() > > >>>> { > > >>>> long long y; > > >>>> foo (_Alignof y, &y); > > >>>> return 0; > > >>>> } > > >>>> > > >>>> Joseph/Jason - do you have a good recommendation > > >>>> how to deal with targets where natural alignment > > >>>> is supposed to be lowered for optimization purposes? > > >>>> (this case is for i?86 to avoid dynamic stack re-alignment > > >>>> to align long long to 8 bytes with -mpreferred-stack-boundary=2) > > >>>> > > >>>> I note that for -mincoming-stack-boundary=2 we do perform > > >>>> dynamic stack re-alignment already. > > >>>> > > >>>> I can't find a suitable existing target macro/hook for this, > > >>>> but my gut feeling is that the default alignment should > > >>>> instead be the lower one and instead the alignment for > > >>>> globals should be raised as optimization? > > >>>> > > >>> > > >>> Here is the updated patch from Sunil. > > >> > > >> It does not address the fundamental issue that during > > >> do_type_align the is_global_var predicate is not > > >> reliable. This means that for > > >> > > >> int main() > > >> { > > >> extern long z; > > >> } > > >> > > >> the new hook (with -m32 -mpreferred-stack-boundary=2) > > >> will lower the alignment of 'z' which looks wrong. During > > >> layout_decl we can unfortunately not distinguish between > > >> locals and globals. We need to find another spot to adjust > > >> alignment of locals. For C that might be in finish_decl, > > >> for C++ there's probably another suitable place. > > > > > >cp_finish_decl could work, but layout_decl seems like the right spot; > > >if > > >the problem is that the appropriate flags currently aren't being set in > > > > > >time, can't we fix that? > > > > The first and usually only call to layout_decl is from build_decl which > gets nothing more than the type... But yes, I also initially thought that's > the correct spot but it turns out it isn't. > > I added a new function lower_decl_alignment and invoked from > cp_decl_finish/decl_finish. Attached is revised patch. > > > > >> Note it needs to be a place before the frontends possibly > > >> inspect the alignment of the decl > > >> In C++ constexpr evalualtion might also expose alignment > > >> "early" so we really need a frontend solution here. > > > > > >Yes, we need to know the alignment right after the declaration. > > > > > >Jason > > >
