[PATCH] Add TARGET_LOWER_LOCAL_DECL_ALIGNMENT [PR95237]

[H.J. Lu]

On Thu, Jun 25, 2020 at 1:10 AM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Thu, Jun 25, 2020 at 2:53 AM Sunil Pandey <skpgkp1@gmail.com> wrote:
> >
> > On Wed, Jun 24, 2020 at 12:30 AM Richard Biener
> > <richard.guenther@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 <skpgkp1@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.

-- 
H.J.
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