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

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

> 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. 
Okay. 

> 
>> 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. 

Okay, I see. 

I will try to update the code to see whether all the issues can be resolved.

Thanks a lot for your help.

Qing
> 
>> 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?
>>>> 
>>>> 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)
>>>> 
>>> 
>> 
>