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

[Richard Biener <rguenther@suse.de>]

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?
>>> 
>>> 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)
>>> 
>> 
>