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

Qing Zhao qing.zhao@oracle.com
Fri Aug 20 14:52:21 GMT 2021



> On Aug 19, 2021, at 8:54 AM, Qing Zhao via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> 
>>>>> 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.

However, there was two runtime errors with CPU2017 with this change.

I guess that there might be some issue with “expand_expr” + “make_tree” here. 

So, I changed the code to use the alternative solution that you suggested:

> 
>> 
>> 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).

This did resolve all the issues including CPU2017 runtime errors.
With this solution, all SSA_NAMEs will be expanded through “expand_assignment” even though they are in stack.
the generated code will be correct. But the performance might be a little bit different than through memset. 

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



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