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:

> Hi, 
> 
> I finally decided to take another approach to resolve this issue, it resolved all the potential issues with the “address taken” auto variable.
> 
> The basic idea is to avoid generating the temporary variable in the beginning. 
> As you mentioned, "The reason is that alt_reloc is memory (because it is address taken) and that GIMPLE says that register typed stores 
> need to use a is_gimple_val RHS which the call is not.”
> In order to avoid generating the temporary variable for “address taken” auto variable, I updated the utility routine “is_gimple_val” as following:
> 
> diff --git a/gcc/gimple-expr.c b/gcc/gimple-expr.c
> index a2563a45c37d..d5ef1aef8cea 100644
> --- a/gcc/gimple-expr.c
> +++ b/gcc/gimple-expr.c
> @@ -787,8 +787,20 @@ is_gimple_reg (tree t)
>    return !DECL_NOT_GIMPLE_REG_P (t);
>  }
>  
> +/* Return true if T is a call to .DEFERRED_INIT internal function.  */ 
> +static bool
> +is_deferred_init_call (tree t)
> +{
> +  if (TREE_CODE (t) == CALL_EXPR
> +      &&  CALL_EXPR_IFN (t) == IFN_DEFERRED_INIT)
> +    return true;
> +  return false;
> +}
> +
>  
> -/* Return true if T is a GIMPLE rvalue, i.e. an identifier or a constant.  */
> +/* Return true if T is a GIMPLE rvalue, i.e. an identifier or a constant,
> +   or a call to .DEFERRED_INIT internal function because the call to
> +   .DEFERRED_INIT will eventually be expanded as a constant.  */
>  
>  bool
>  is_gimple_val (tree t)
> @@ -799,7 +811,8 @@ is_gimple_val (tree t)
>        && !is_gimple_reg (t))
>      return false;
>  
> -  return (is_gimple_variable (t) || is_gimple_min_invariant (t));
> +  return (is_gimple_variable (t) || is_gimple_min_invariant (t)
> +         || is_deferred_init_call (t));
>  }
>  
> With this change, the temporary variable will not be created for “address taken” auto variable, and uninitialized analysis does not need any change. 
> Everything works well.
> 
> And I believe that treating “call to .DEFERRED_INIT” as “is_gimple_val” is reasonable since this call actually is a constant.
>
> Let me know if you have any objection on this solution.

Yeah, I object to this solution.

Richard.

> thanks.
> 
> Qing
> 
> > On Aug 11, 2021, at 3:30 PM, Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> > 
> > Hi, 
> > 
> > I met another issue for “address taken” auto variable, see below for details:
> > 
> > **** the testing case: (gcc/testsuite/gcc.dg/uninit-16.c)
> > 
> > int foo, bar;
> > 
> > static
> > void decode_reloc(int reloc, int *is_alt)
> > {
> >  if (reloc >= 20)
> >      *is_alt = 1;
> >  else if (reloc >= 10)
> >      *is_alt = 0;
> > }
> > 
> > void testfunc()
> > {
> >  int alt_reloc;
> > 
> >  decode_reloc(foo, &alt_reloc);
> > 
> >  if (alt_reloc) /* { dg-warning "may be used uninitialized" } */
> >    bar = 42;
> > }
> > 
> > ****When compiled with -ftrivial-auto-var-init=zero -O2 -Wuninitialized -fdump-tree-all:
> > 
> > .*************gimple dump:
> > 
> > void testfunc ()
> > { 
> >  int alt_reloc;
> > 
> >  try
> >    {
> >      _1 = .DEFERRED_INIT (4, 2, 0);
> >      alt_reloc = _1;
> >      foo.0_2 = foo;
> >      decode_reloc (foo.0_2, &alt_reloc);
> >      alt_reloc.1_3 = alt_reloc;
> >      if (alt_reloc.1_3 != 0) goto <D.1952>; else goto <D.1953>;
> >      <D.1952>:
> >      bar = 42;
> >      <D.1953>:
> >    }
> >  finally
> >    {
> >      alt_reloc = {CLOBBER};
> >    }
> > }
> > 
> > **************fre1 dump:
> > 
> > void testfunc ()
> > {
> >  int alt_reloc;
> >  int _1;
> >  int foo.0_2;
> > 
> >  <bb 2> :
> >  _1 = .DEFERRED_INIT (4, 2, 0);
> >  foo.0_2 = foo;
> >  if (foo.0_2 > 19)
> >    goto <bb 3>; [50.00%]
> >  else
> >    goto <bb 4>; [50.00%]
> > 
> >  <bb 3> :
> >  goto <bb 7>; [100.00%]
> > 
> >  <bb 4> :
> >  if (foo.0_2 > 9)
> >    goto <bb 5>; [50.00%]
> >  else
> >    goto <bb 6>; [50.00%]
> > 
> >  <bb 5> :
> >  goto <bb 8>; [100.00%]
> > 
> >  <bb 6> :
> >  if (_1 != 0)
> >    goto <bb 7>; [INV]
> >  else
> >    goto <bb 8>; [INV]
> > 
> >  <bb 7> :
> >  bar = 42;
> > 
> >  <bb 8> :
> >  return;
> > 
> > }
> > 
> > From the above IR file after “FRE”, we can see that the major issue with this IR is:
> > 
> > The address taken auto variable “alt_reloc” has been completely replaced by the temporary variable “_1” in all
> > the uses of the original “alt_reloc”. 
> > 
> > The major problem with such IR is,  during uninitialized analysis phase, the original use of “alt_reloc” disappeared completely.
> > So, the warning cannot be reported.
> > 
> > 
> > My questions:
> > 
> > 1. Is it possible to get the original “alt_reloc” through the temporary variable “_1” with some available information recorded in the IR?
> > 2. If not, then we have to record the relationship between “alt_reloc” and “_1” when the original “alt_reloc” is replaced by “_1” and get such relationship during
> >    Uninitialized analysis phase.  Is this doable?
> > 3. Looks like that for “address taken” auto variable, if we have to introduce a new temporary variable and split the call to .DEFERRED_INIT into two:
> > 
> >      temp = .DEFERRED_INIT (4, 2, 0);
> >      alt_reloc = temp;
> > 
> >   More issues might possible.
> > 
> > Any comments and suggestions on this issue?
> > 
> > Qing
> > 
> > j
> >> On Aug 11, 2021, at 11:55 AM, Richard Biener <rguenther@suse.de> wrote:
> >> 
> >> On August 11, 2021 6:22:00 PM GMT+02:00, Qing Zhao <qing.zhao@oracle.com> wrote:
> >>> 
> >>> 
> >>>> On Aug 11, 2021, at 10:53 AM, Richard Biener <rguenther@suse.de> wrote:
> >>>> 
> >>>> On August 11, 2021 5:30:40 PM GMT+02:00, Qing Zhao <qing.zhao@oracle.com> wrote:
> >>>>> I modified the routine “gimple_add_init_for_auto_var” as the following:
> >>>>> ====
> >>>>> /* Generate initialization to automatic variable DECL based on INIT_TYPE.
> >>>>> Build a call to internal const function DEFERRED_INIT:
> >>>>> 1st argument: SIZE of the DECL;
> >>>>> 2nd argument: INIT_TYPE;
> >>>>> 3rd argument: IS_VLA, 0 NO, 1 YES;
> >>>>> 
> >>>>> as DEFERRED_INIT (SIZE of the DECL, INIT_TYPE, IS_VLA).  */
> >>>>> static void
> >>>>> gimple_add_init_for_auto_var (tree decl,
> >>>>>                           enum auto_init_type init_type,
> >>>>>                           bool is_vla,
> >>>>>                           gimple_seq *seq_p)
> >>>>> {
> >>>>> gcc_assert (VAR_P (decl) && !DECL_EXTERNAL (decl) && !TREE_STATIC (decl));
> >>>>> gcc_assert (init_type > AUTO_INIT_UNINITIALIZED);
> >>>>> tree decl_size = TYPE_SIZE_UNIT (TREE_TYPE (decl));
> >>>>> 
> >>>>> tree init_type_node
> >>>>> = build_int_cst (integer_type_node, (int) init_type);
> >>>>> tree is_vla_node
> >>>>> = build_int_cst (integer_type_node, (int) is_vla);
> >>>>> 
> >>>>> tree call = build_call_expr_internal_loc (UNKNOWN_LOCATION, IFN_DEFERRED_INIT,
> >>>>>                                         TREE_TYPE (decl), 3,
> >>>>>                                         decl_size, init_type_node,
> >>>>>                                         is_vla_node);
> >>>>> 
> >>>>> /* If this DECL is a VLA, a temporary address variable for it has been
> >>>>>  created, the replacement for DECL is recorded in DECL_VALUE_EXPR (decl),
> >>>>>  we should use it as the LHS of the call.  */
> >>>>> 
> >>>>> tree lhs_call
> >>>>> = is_vla ? DECL_VALUE_EXPR (decl) : decl;
> >>>>> gimplify_assign (lhs_call, call, seq_p);
> >>>>> }
> >>>>> 
> >>>>> With this change, the current issue is resolved, the gimple dump now is:
> >>>>> 
> >>>>> (*arr.1) = .DEFERRED_INIT (D.1952, 2, 1);
> >>>>> 
> >>>>> However, there is another new issue:
> >>>>> 
> >>>>> For the following testing case:
> >>>>> 
> >>>>> ======
> >>>>> [opc@qinzhao-ol8u3-x86 gcc]$ cat t.c
> >>>>> int bar;
> >>>>> 
> >>>>> extern void decode_reloc(int *);
> >>>>> 
> >>>>> void testfunc()
> >>>>> {
> >>>>> int alt_reloc;
> >>>>> 
> >>>>> decode_reloc(&alt_reloc);
> >>>>> 
> >>>>> if (alt_reloc) /* { dg-warning "may be used uninitialized" } */
> >>>>> bar = 42; 
> >>>>> }
> >>>>> =====
> >>>>> 
> >>>>> In the above, the auto var “alt_reloc” is address taken, then the gimple dump for it when compiled with -ftrivial-auto-var-init=zero is:
> >>>>> 
> >>>>> void testfunc ()
> >>>>> {
> >>>>> int alt_reloc;
> >>>>> 
> >>>>> try
> >>>>> {
> >>>>>   _1 = .DEFERRED_INIT (4, 2, 0);
> >>>>>   alt_reloc = _1;
> >>>>>   decode_reloc (&alt_reloc);
> >>>>>   alt_reloc.0_2 = alt_reloc;
> >>>>>   if (alt_reloc.0_2 != 0) goto <D.1949>; else goto <D.1950>;
> >>>>>   <D.1949>:
> >>>>>   bar = 42;
> >>>>>   <D.1950>:
> >>>>> }
> >>>>> finally
> >>>>> {
> >>>>>   alt_reloc = {CLOBBER};
> >>>>> }
> >>>>> }
> >>>>> 
> >>>>> I.e, instead of the expected IR:
> >>>>> 
> >>>>> alt_reloc = .DEFERRED_INIT (4, 2, 0);
> >>>>> 
> >>>>> We got the following:
> >>>>> 
> >>>>> _1 = .DEFERRED_INIT (4, 2, 0);
> >>>>>   alt_reloc = _1;
> >>>>> 
> >>>>> I guess the temp “_1” is created because “alt_reloc” is address taken. 
> >>>> 
> >>>> Yes and no. The reason is that alt_reloc is memory (because it is address taken) and that GIMPLE says that register typed stores need to use a is_gimple_val RHS which the call is not.
> >>> 
> >>> Okay.
> >>>> 
> >>>>> My questions:
> >>>>> 
> >>>>> Shall we accept such IR for .DEFERRED_INIT purpose when the auto var is address taken? 
> >>>> 
> >>>> I think so. Note it doesn't necessarily need address taking but any other reason that prevents SSA rewriting the variable suffices. 
> >>> 
> >>> You mean, in addition to “address taken”, there are other situations that will introduce such IR:
> >>> 
> >>> temp = .DEFERRED_INIT();
> >>> auto_var = temp;
> >>> 
> >>> So, such IR is unavoidable and we have to handle it?
> >> 
> >> Yes. 
> >> 
> >>> If we have to handle it,  what’ the best way to do it?
> >>> 
> >>> The solution in my mind is:
> >>> 1. During uninitialized analysis phase, following the data flow to connect .DEFERRED_INIT to “auto_var”, and then decide that “auto_var” is uninitialized.
> >> 
> >> Yes. Basically if there's an artificial variable auto initialized you have to look at its uses. 
> >> 
> >>> 2. During RTL expansion, following the data flow to connect .DEFERRED_INIT to “auto_var”, and then delete “temp”, and then expand .DEFERRED_INIT to auto_var.
> >> 
> >> That shouldn't be necessary. You'd initialize a temporary register which is then copied to the real variable. That's good enough and should be optimized by the RTL pipeline. 
> >> 
> >>> Let me know your comments and suggestions on this.
> >>> 
> >>> 
> >>>> 
> >>>> The only other option is to force. DEFERED_INIT making the LHS address taken which I think could be achieved by passing it the address as argument instead of having a LHS. But let's not go down this route - it will have quite bad behavior on alias analysis and optimization. 
> >>> 
> >>> Okay.
> >>> 
> >>> Qing
> >>>> 
> >>>>> If so, “uninitialized analysis” phase need to be further adjusted to specially handle such IR. 
> >>>>> 
> >>>>> If not, what should we do when the auto var is address taken?
> 
> 

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