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

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

> On Aug 19, 2021, at 4:00 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> On Wed, 18 Aug 2021, Qing Zhao wrote:
> 
>> 
>> 
>>> On Aug 18, 2021, at 2:15 AM, Richard Biener <rguenther@suse.de> wrote:
>>> 
>>> On Tue, 17 Aug 2021, Qing Zhao wrote:
>>> 
>>>> 
>>>> 
>>>>> On Aug 17, 2021, at 9:50 AM, Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>>>>> 
>>>>> 
>>>>> 
>>>>>> On Aug 17, 2021, at 3:29 AM, Richard Biener <rguenther@suse.de> wrote:
>>>>>> 
>>>>>> On Mon, 16 Aug 2021, Qing Zhao wrote:
>>>>>> 
>>>>>>> My current code for expand_DEFERRED_INIT is like the following, could you check and see whether there is any issue for it:
>>>>>>> 
>>>>>>> #define INIT_PATTERN_VALUE  0xFE
>>>>>>> static void
>>>>>>> expand_DEFERRED_INIT (internal_fn, gcall *stmt)
>>>>>>> {
>>>>>>> tree lhs = gimple_call_lhs (stmt);
>>>>>>> tree var_size = gimple_call_arg (stmt, 0);
>>>>>>> enum auto_init_type init_type
>>>>>>> = (enum auto_init_type) TREE_INT_CST_LOW (gimple_call_arg (stmt, 1));
>>>>>>> bool is_vla = (bool) TREE_INT_CST_LOW (gimple_call_arg (stmt, 2));
>>>>>>> 
>>>>>>> tree var_type = TREE_TYPE (lhs);
>>>>>>> gcc_assert (init_type > AUTO_INIT_UNINITIALIZED);
>>>>>>> 
>>>>>>> if (is_vla || (!use_register_for_decl (lhs)))
>>>>>>> {
>>>>>>>   if (TREE_CODE (lhs) == SSA_NAME)
>>>>>>>     lhs = SSA_NAME_VAR (lhs);
>>>>>> 
>>>>>> this should not be necessary (in fact you shouldn't see a SSA_NAME
>>>>>> here, if you do then using SSA_NAME_VAR is wrong)
>>>>> You mean during RTL expansion phase, all SSA_NAMEs are gone already?
>>>> 
>>>> Actually, the lhs could be SSA_NAME here, 
>>>> 
>>>> Breakpoint 1, expand_DEFERRED_INIT (stmt=0x7fffe96ae348) at ../../latest-gcc/gcc/internal-fn.c:3021
>>>> 3021	      mark_addressable (lhs);
>>>> (gdb) call debug_tree(lhs)
>>>> <ssa_name 0x7fffe9584e58
>>>>   type <real_type 0x7fffe959b2a0 float sizes-gimplified SF
>>>>       size <integer_cst 0x7fffe9579f48 constant 32>
>>>>       unit-size <integer_cst 0x7fffe9579f60 constant 4>
>>>>       align:32 warn_if_not_align:0 symtab:0 alias-set 2 canonical-type 0x7fffe959b2a0 precision:32
>>>>       pointer_to_this <pointer_type 0x7fffe959b7e0>>
>>>>   visited var <var_decl 0x7ffff7ff7bd0 temp1>
>>>>   def_stmt temp1_5 = .DEFERRED_INIT (4, 2, 0, &"temp1"[0]);
>>>>   version:5>
>>>> 
>>>> when I deleted:
>>>> 
>>>> if (TREE_CODE (lhs) == SSA_NAME
>>>>  lhs = SSA_NAME_VAR (lhs);
>>> 
>>> but then using SSA_NAME_VAR is broken.  I suspect use_register_for_decl
>>> isn't the correct thing to look at.  I think we need to look at what
>>> the LHS expanded to if it is a SSA_VAR_P (that includes SSA names
>>> but also plain DECLs but not what we get from VLAs where we'd see
>>> *ptr).  So sth like
>>> 
>>> bool reg_lhs;
>>> if (SSA_VAR_P (lhs))
>>>   {
>>>     rtx tem = expand_expr (lhs, NULL_RTX, VOIDmode, EXPAND_WRITE);
>>>     reg_lhs = !MEM_P (tem);
>>>     /* If not MEM_P reg_lhs should be REG_P or SUBREG_P (but maybe
>>>        also CONCAT or lowpart...?)  */
>>>   }
>>> else
>>>   {
>>>     gcc_assert (is_vla);
>>>     reg_lhs = false;
>>>   }
>>> 
>>> if (!reg_lhs)
>>>   memset path
>>> else
>>>   expand_assignment path
>> 
>> After making the following change:
>> 
>> +  bool reg_lhs = true;
>> 
>>   tree var_type = TREE_TYPE (lhs);
>>   gcc_assert (init_type > AUTO_INIT_UNINITIALIZED);
>> 
>> -  if (is_vla || (!use_register_for_decl (lhs)))
>> +  if (SSA_VAR_P (lhs))
>> +    {
>> +      rtx tem = expand_expr (lhs, NULL_RTX, VOIDmode, EXPAND_WRITE);
>> +      reg_lhs = !MEM_P (tem);
>> +    }
>> +  else
>> +    {
>> +      gcc_assert (is_vla);
>> +      reg_lhs = false;
>> +    }
>> +
>> +  if (!reg_lhs)
>>     {
>> 
>> I got exactly the same internal error that failed at expr.c:
>> 
>> 8436   /* We must have made progress.  */
>> 8437   gcc_assert (inner != exp);
>> 
>> 
>> Looks like for the following code:
>> 
>> 3026   if (!reg_lhs)
>> 3027     {
>> 3028     /* If this is a VLA or the variable is not in register,
>> 3029        expand to a memset to initialize it.  */
>> 3030       mark_addressable (lhs);
>> 3031       tree var_addr = build_fold_addr_expr (lhs);
>> 3032 
>> 3033       tree value = (init_type == AUTO_INIT_PATTERN) ?
>> 3034                     build_int_cst (integer_type_node,
>> 3035                                    INIT_PATTERN_VALUE) :
>> 3036                     integer_zero_node;
>> 3037       tree m_call = build_call_expr (builtin_decl_implicit (BUILT_IN_MEMSET),
>> 3038                                      3, var_addr, value, var_size);
>> 3039       /* Expand this memset call.  */
>> 3040       expand_builtin_memset (m_call, NULL_RTX, TYPE_MODE (var_type));
>> 3041     }
>> 
>> At line 3030, “lhs” could be a SSA_NAME.
>> 
>> My questions are:
>> 
>> 1. Could the routine “mark_addressable” and “build_fold_addr_expr” be applied on SSA_NAME?
> 
> No.
> 
>> 2. Could the routine “expand_builtin_memset” be applied on the memset call whose “DEST” is
>>    an address expression on SSA_NAME? 
> 
> No.
> 
>> 3. Within “expand_DEFERRED_INIT”, can I call “expand_builtin_memset” to expand .DEFERRED_INIT?
> 
> Well, not with "invalid" GENERIC I fear (address of a SSA name).
> 
>> I suspect that one of the above 3 might be the issue, but not sure which one?
> 
> All of the above ;)  So while reg_lhs is now precise as to how the
> variable will end up (the SSA name will end up as a stack variable in this
> case, for whatever reason), expansion via memcpy only works when
> working on the RTL representation.  The usual "workaround" (ugh)
> is to use make_tree (), so in the !reg_lhs path you'd do
> 
>  /* Get a new GENERIC representation for the RTL.  That's necesary
>     in case LHS is an SSA name.  */
>  lhs = make_tree (TREE_TYPE (lhs), tem);

This resolved the issue.

Another question,

Previously, I used

    if (TREE_CODE (lhs) == SSA_NAME)
       lhs = SSA_NAME_VAR (lhs);

To resolve this issue. The purpose looks like the same as “make_tree”, just get an generic tree for the LHS. 

Why you said using SSA_NAME_VAR is broken?  Is it because SSA_NAME_VAR will not always return a valid TREE?

I should use as following


   If (TREE_CODE (lhs) == SSA_NAME) && SSA_NAME_VAR (lhs))
      Lhs = SSA_NAME_VAR (lhs)

?

> 
> alternatively you could maybe do
> 
>  if (DECL_P (lhs))
>    {
> +      rtx tem = expand_expr (lhs, NULL_RTX, VOIDmode, EXPAND_WRITE);
> +      reg_lhs = !MEM_P (tem);
>    }
>  else if (TREE_CODE (lhs) == SSA_NAME)
>    reg_lhs = true;
>  else
>    reg_lhs = false;
> 
> thus treat SSA names as register storage always (even if it will end
> up on the stack).

My question here, for a complicate structure SSA_NAME, will expanding through memset better than expand_asssignment? 

Qing
> 
> Richard.
> 
>> Thanks a lot.
>> 
>> Qing
>> 
>> 
>> 
>>> bool reg_lhs;
>>> if (SSA_VAR_P (lhs))
>>>   {
>>>     rtx tem = expand_expr (lhs, NULL_RTX, VOIDmode, EXPAND_WRITE);
>>>     reg_lhs = !MEM_P (tem);
>>>     /* If not MEM_P reg_lhs should be REG_P or SUBREG_P (but maybe
>>>        also CONCAT or lowpart...?)  */
>>>   }
>>> else
>>>   {
>>>     gcc_assert (is_vla);
>>>     reg_lhs = false;
>>>   }
>> 
>> 
>>> 
>>>> Many testing cases failed with internal compiler error:
>>>> 
>>>> /home/opc/Work/GCC/latest-gcc/gcc/testsuite/c-c++-common/auto-init-3.c:9:9: internal compiler error: in expand_expr_addr_expr_1, at expr.c:8437
>>>> 0xe237aa expand_expr_addr_expr_1
>>>> 	../../latest-gcc/gcc/expr.c:8437
>>>> 0xe24059 expand_expr_addr_expr
>>>> 	../../latest-gcc/gcc/expr.c:8525
>>>> 0xe32b56 expand_expr_real_1(tree_node*, rtx_def*, machine_mode, expand_modifier, rtx_def**, bool)
>>>> 	../../latest-gcc/gcc/expr.c:11741
>>>> 0xe2da52 expand_expr_real_1(tree_node*, rtx_def*, machine_mode, expand_modifier, rtx_def**, bool)
>>>> 	../../latest-gcc/gcc/expr.c:10777
>>>> 0xe24706 expand_expr_real(tree_node*, rtx_def*, machine_mode, expand_modifier, rtx_def**, bool)
>>>> 	../../latest-gcc/gcc/expr.c:8713
>>>> 0xc13f15 expand_expr
>>>> 	../../latest-gcc/gcc/expr.h:301
>>>> 0xc17acb get_memory_rtx
>>>> 	../../latest-gcc/gcc/builtins.c:1370
>>>> 0xc2223d expand_builtin_memset_args
>>>> 	../../latest-gcc/gcc/builtins.c:4102
>>>> 0xc21a20 expand_builtin_memset(tree_node*, rtx_def*, machine_mode)
>>>> 	../../latest-gcc/gcc/builtins.c:3886
>>>> 0xfb5c85 expand_DEFERRED_INIT
>>>> 	../../latest-gcc/gcc/internal-fn.c:3031
>>>> 
>>>> 
>>>> So, did I do anything wrong?
>>>> 
>>>> 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)