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

[Qing Zhao <qing.zhao@oracle.com>]

> On Aug 16, 2021, at 2:12 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> 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.

Can you explain what’s the major issue for this solution? 

To me,  treating “call to .DEFERRED_INIT” as “is_gimple_val” is reasonable since this call actually is a constant.

Thanks.

Qing
> 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)