Re: [patch][version 6] add -ftrivial-auto-var-init and variable attribute "uninitialized" to gcc

[Richard Biener <rguenther@suse.de>]

On Wed, 11 Aug 2021, Qing Zhao wrote:

> 
> 
> > On Aug 11, 2021, at 2:02 AM, Richard Biener <rguenther@suse.de> wrote:
> > 
> > On Tue, 10 Aug 2021, Qing Zhao wrote:
> > 
> >> 
> >> 
> >>> On Aug 10, 2021, at 3:16 PM, Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> >>> 
> >>> Hi, Richard,
> >>> 
> >>>> On Aug 10, 2021, at 10:22 AM, Richard Biener <rguenther@suse.de> wrote:
> >>>>>> 
> >>>>>> Especially in the VLA case but likely also in general (though unlikely
> >>>>>> since usually the receiver of initializations are simple enough).  I'd
> >>>>>> expect the VLA case end up as
> >>>>>> 
> >>>>>> *ptr_to_decl = .DEFERRED_INIT (...);
> >>>>>> 
> >>>>>> where *ptr_to_decl is the DECL_VALUE_EXPR of the decl.
> >>>>> 
> >>>>> So, for the following small testing case:
> >>>>> 
> >>>>> ====
> >>>>> extern void bar (int);
> >>>>> 
> >>>>> void foo(int n)
> >>>>> {
> >>>>> int arr[n];
> >>>>> bar (arr[2]);
> >>>>> return;
> >>>>> }
> >>>>> =====
> >>>>> 
> >>>>> If I compile it with -ftrivial-auto-var-init=zero -fdump-tree-gimple -S -o auto-init-11.s -fdump-rtl-expand, the *.gimple dump is:
> >>>>> 
> >>>>> =====
> >>>>> void foo (int n)
> >>>>> {
> >>>>> int n.0;
> >>>>> sizetype D.1950;
> >>>>> bitsizetype D.1951;
> >>>>> sizetype D.1952;
> >>>>> bitsizetype D.1953;
> >>>>> sizetype D.1954;
> >>>>> int[0:D.1950] * arr.1;
> >>>>> void * saved_stack.2;
> >>>>> int arr[0:D.1950] [value-expr: *arr.1];
> >>>>> 
> >>>>> saved_stack.2 = __builtin_stack_save ();
> >>>>> try
> >>>>>  {
> >>>>>    n.0 = n;
> >>>>>    _1 = (long int) n.0;
> >>>>>    _2 = _1 + -1;
> >>>>>    _3 = (sizetype) _2;
> >>>>>    D.1950 = _3;
> >>>>>    _4 = (sizetype) n.0;
> >>>>>    _5 = (bitsizetype) _4;
> >>>>>    _6 = _5 * 32;
> >>>>>    D.1951 = _6;
> >>>>>    _7 = (sizetype) n.0;
> >>>>>    _8 = _7 * 4;
> >>>>>    D.1952 = _8;
> >>>>>    _9 = (sizetype) n.0;
> >>>>>    _10 = (bitsizetype) _9;
> >>>>>    _11 = _10 * 32;
> >>>>>    D.1953 = _11;
> >>>>>    _12 = (sizetype) n.0;
> >>>>>    _13 = _12 * 4;
> >>>>>    D.1954 = _13;
> >>>>>    arr.1 = __builtin_alloca_with_align (D.1954, 32);
> >>>>>    arr = .DEFERRED_INIT (D.1952, 2, 1);
> >>>>>    _14 = (*arr.1)[2];
> >>>>>    bar (_14);
> >>>>>    return;
> >>>>>  }
> >>>>> finally
> >>>>>  {
> >>>>>    __builtin_stack_restore (saved_stack.2);
> >>>>>  }
> >>>>> }
> >>>>> 
> >>>>> ====
> >>>>> 
> >>>>> You think that the above .DEFEERED_INIT is not correct?
> >>>>> It should be:
> >>>>> 
> >>>>> *arr.1 = .DEFERRED_INIT (D.1952. 2, 1);
> >>>>> 
> >>>>> ?
> >>>> 
> >>>> Yes.
> >>>> 
> >>> 
> >>> I updated gimplify.c for VLA and now it emits the call to .DEFERRED_INIT as:
> >>> 
> >>>     arr.1 = __builtin_alloca_with_align (D.1954, 32);
> >>>     *arr.1 = .DEFERRED_INIT (D.1952, 2, 1);
> >>> 
> >>> However, this call triggered the assertion failure in verify_gimple_call of tree-cfg.c because the LHS is not a valid LHS. 
> >>> Then I modify tree-cfg.c as:
> >>> 
> >>> diff --git a/gcc/tree-cfg.c b/gcc/tree-cfg.c
> >>> index 330eb7dd89bf..180d4f1f9e32 100644
> >>> --- a/gcc/tree-cfg.c
> >>> +++ b/gcc/tree-cfg.c
> >>> @@ -3375,7 +3375,11 @@ verify_gimple_call (gcall *stmt)
> >>>     }
> >>> 
> >>>  tree lhs = gimple_call_lhs (stmt);
> >>> +  /* For .DEFERRED_INIT call, the LHS might be an indirection of
> >>> +     a pointer for the VLA variable, which is not a valid LHS of
> >>> +     a gimple call, we ignore the asssertion on this.  */ 
> >>>  if (lhs
> >>> +      && (!gimple_call_internal_p (stmt, IFN_DEFERRED_INIT))
> >>>      && (!is_gimple_reg (lhs)
> >>>         && (!is_gimple_lvalue (lhs)
> >>>             || verify_types_in_gimple_reference
> >>> 
> >>> The assertion failure in tree-cfg.c got resolved, but I got another assertion failure in operands_scanner::get_expr_operands (tree *expr_p, int flags), line 945:
> >>> 
> >>> 939   /* If we get here, something has gone wrong.  */
> >>> 940   if (flag_checking)
> >>> 941     {
> >>> 942       fprintf (stderr, "unhandled expression in get_expr_operands():\n");
> >>> 943       debug_tree (expr);
> >>> 944       fputs ("\n", stderr);
> >>> 945       gcc_unreachable ();
> >>> 946     }
> >>> 
> >>> Looks like that  the gimple statement:
> >>>   *arr.1 = .DEFERRED_INIT (D.1952, 2, 1);
> >>> 
> >>> Is not valid.  i.e, the LHS should not be an indirection to a pointer. 
> >>> 
> >>> How to resolve this issue?
> > 
> > It sounds like the LHS is an INDIRECT_REF maybe?  That means it's
> > still not properly gimplified because it should end up as a MEM_REF
> > instead.
> > 
> > But I'm just guessing here ... if you are in a debugger then you can
> > invoke debug_tree (lhs) in the inferior to see what it exactly is
> > at the point of the failure.
> 
> Yes, it’s an INDIRECT_REF at the point of the failure even though I added a 
> 
> gimplify_var_or_parm_decl  (lhs) 

I think the easiest is to build the .DEFERRED_INIT as GENERIC
and use gimplify_assign () to gimplify and add the result
to the sequence.  Thus, build a GENERIC CALL_EXPR and then
gimplify_assign (lhs, call_expr, seq);

Richard.

> Qing
> 
> > 
> >> I came up with the following solution:
> >> 
> >> Define the IFN_DEFERRED_INIT function as:
> >> 
> >>   LHS = DEFERRED_INIT (SIZE of the DECL, INIT_TYPE, IS_VLA);
> >> 
> >>   if IS_VLA is false, the LHS is the DECL itself,
> >>   if IS_VLA is true, the LHS is the pointer to this DECL that created by
> >>   gimplify_vla_decl.
> >> 
> >> 
> >> The benefit of this solution are:
> >> 
> >> 1. Resolved the invalid IR issue;
> >> 2. The call stmt carries the address of the VLA natually;
> >> 
> >> The issue with this solution is:
> >> 
> >> For VLA and non-VLA, the LHS will be different, 
> >> 
> >> Do you see any other potential issues with this solution?
> >> 
> >> thanks.
> >> 
> >> Qing
> >> 
> >> 
> >> 
> >> 
> >> 
> > 
> > -- 
> > Richard Biener <rguenther@suse.de>
> > SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
> > Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)