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

[Richard Biener <rguenther@suse.de>]

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.

>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. 

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. 

>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?
>
>Thanks a lot.
>
>Qing
>
>
>> On Aug 11, 2021, at 8:58 AM, Richard Biener <rguenther@suse.de> wrote:
>> 
>> On Wed, 11 Aug 2021, Qing Zhao wrote:
>> 
>>> 
>>> 
>>>> On Aug 11, 2021, at 8:37 AM, Richard Biener <rguenther@suse.de> wrote:
>>>> 
>>>> 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);
>>> 
>>> Which utility routine is used to build an Internal generic call?
>>> Currently, I used “gimple_build_call_internal” to build this internal gimple call.
>>> 
>>> For the generic call, shall I use “build_call_expr_loc” ? 
>> 
>> For example look at build_asan_poison_call_expr which does such thing
>> for ASAN poison internal function call insertion at gimplification time.
>> 
>> Richard.
>> 
>>> Qing
>>> 
>>>> 
>>>> 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)
>>> 
>>> 
>> 
>> -- 
>> Richard Biener <rguenther@suse.de>
>> SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
>> Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)
>