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

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

> On Aug 17, 2021, at 9:45 AM, Richard Biener <rguenther@suse.de> wrote:
> 
> On Tue, 17 Aug 2021, Qing Zhao wrote:
> 
>> 
>> 
>>> On Aug 17, 2021, at 3:43 AM, Richard Biener <rguenther@suse.de> wrote:
>>> 
>>> On Mon, 16 Aug 2021, Qing Zhao wrote:
>>> 
>>>> 
>>>> 
>>>>> On Aug 16, 2021, at 2:40 AM, Richard Biener <rguenther@suse.de> wrote:
>>>>> 
>>>>> On Thu, 12 Aug 2021, Qing Zhao wrote:
>>>>> 
>>>>>> Hi, Richard,
>>>>>> 
>>>>>> For RTL expansion of call to .DEFERRED_INIT, I changed my code per your suggestions like following:
>>>>>> 
>>>>>> ======================
>>>>>> #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 || (!can_native_interpret_type_p (var_type)))
>>>>>>  {
>>>>>>  /* If this is a VLA or the type of the variable cannot be natively
>>>>>>     interpreted, expand to a memset to initialize it.  */
>>>>>>    if (TREE_CODE (lhs) == SSA_NAME)
>>>>>>      lhs = SSA_NAME_VAR (lhs);
>>>>>>    tree var_addr = NULL_TREE;
>>>>>>    if (is_vla)
>>>>>>      var_addr = TREE_OPERAND (lhs, 0);
>>>>>>    else
>>>>>>      {
>>>>>>       TREE_ADDRESSABLE (lhs) = 1;
>>>>>>       var_addr = build_fold_addr_expr (lhs);
>>>>>>      }
>>>>>>    tree value = (init_type == AUTO_INIT_PATTERN) ?
>>>>>>                  build_int_cst (unsigned_char_type_node,
>>>>>>                                 INIT_PATTERN_VALUE) :
>>>>>>                  build_zero_cst (unsigned_char_type_node);
>>>>>>    tree m_call = build_call_expr (builtin_decl_implicit (BUILT_IN_MEMSET),
>>>>>>                                   3, var_addr, value, var_size);
>>>>>>    /* Expand this memset call.  */
>>>>>>    expand_builtin_memset (m_call, NULL_RTX, TYPE_MODE (var_type));
>>>>>>  }
>>>>>> else
>>>>>>  {
>>>>>>  /* If this is not a VLA and the type of the variable can be natively 
>>>>>>     interpreted, expand to assignment to generate better code.  */
>>>>>>    tree pattern = NULL_TREE;
>>>>>>    unsigned HOST_WIDE_INT total_bytes
>>>>>>      = tree_to_uhwi (TYPE_SIZE_UNIT (var_type));
>>>>>> 
>>>>>>    if (init_type == AUTO_INIT_PATTERN)
>>>>>>      {
>>>>>>        unsigned char *buf = (unsigned char *) xmalloc (total_bytes);
>>>>>>        memset (buf, INIT_PATTERN_VALUE, total_bytes);
>>>>>>        pattern = native_interpret_expr (var_type, buf, total_bytes);
>>>>>>        gcc_assert (pattern);
>>>>>>      }
>>>>>> 
>>>>>>    tree init = (init_type == AUTO_INIT_PATTERN) ?
>>>>>>                 pattern :
>>>>>>                 build_zero_cst (var_type);
>>>>>>    expand_assignment (lhs, init, false);
>>>>>>  }
>>>>>> }
>>>>>> ===========================
>>>>>> 
>>>>>> Now, I used “can_native_interpret_type_p (var_type)” instead of “use_register_for_decl (lhs)” to decide 
>>>>>> whether to use “memset” or use “assign” to expand this function.
>>>>>> 
>>>>>> However, this exposed an bug that is very hard to be addressed:
>>>>>> 
>>>>>> *******For the testing case: test suite/gcc.dg/uninit-I.c:
>>>>>> 
>>>>>> /* { dg-do compile } */
>>>>>> /* { dg-options "-O2 -Wuninitialized" } */
>>>>>> 
>>>>>> int sys_msgctl (void)
>>>>>> {
>>>>>> struct { int mode; } setbuf;
>>>>>> return setbuf.mode;  /* { dg-warning "'setbuf\.mode' is used" } */
>>>>>> ==
>>>>>> 
>>>>>> ******the above auto var “setbuf” has “struct” type, which “can_native_interpret_type_p(var_type)” is false, therefore, 
>>>>>> Expanding this .DEFERRED_INIT call went down the “memset” expansion route. 
>>>>>> 
>>>>>> However, this structure type can be fitted into a register, therefore cannot be taken address anymore at this stage, even though I tried:
>>>>>> 
>>>>>>       TREE_ADDRESSABLE (lhs) = 1;
>>>>>>       var_addr = build_fold_addr_expr (lhs);
>>>>>> 
>>>>>> To create an address variable for it, the expansion still failed at expr.c: line 8412:
>>>>>> during RTL pass: expand
>>>>>> /home/opc/Work/GCC/latest-gcc/gcc/testsuite/gcc.dg/auto-init-uninit-I.c:6:24: internal compiler error: in expand_expr_addr_expr_1, at expr.c:8412
>>>>>> 0xd04104 expand_expr_addr_expr_1
>>>>>> 	../../latest-gcc/gcc/expr.c:8412
>>>>>> 0xd04a95 expand_expr_addr_expr
>>>>>> 	../../latest-gcc/gcc/expr.c:8525
>>>>>> 0xd13592 expand_expr_real_1(tree_node*, rtx_def*, machine_mode, expand_modifier, rtx_def**, bool)
>>>>>> 	../../latest-gcc/gcc/expr.c:11741
>>>>>> 0xd05142 expand_expr_real(tree_node*, rtx_def*, machine_mode, expand_modifier, rtx_def**, bool)
>>>>>> 	../../latest-gcc/gcc/expr.c:8713
>>>>>> 0xaed1d3 expand_expr
>>>>>> 	../../latest-gcc/gcc/expr.h:301
>>>>>> 0xaf0d89 get_memory_rtx
>>>>>> 	../../latest-gcc/gcc/builtins.c:1370
>>>>>> 0xafb4fb expand_builtin_memset_args
>>>>>> 	../../latest-gcc/gcc/builtins.c:4102
>>>>>> 0xafacde expand_builtin_memset(tree_node*, rtx_def*, machine_mode)
>>>>>> 	../../latest-gcc/gcc/builtins.c:3886
>>>>>> 0xe97fb3 expand_DEFERRED_INIT
>>>>>> 
>>>>>> ******That’s the major reason why I chose “use_register_for_decl(lhs)” to decide “memset” expansion or “assign” expansion, “memset” expansion
>>>>>> needs to take address of the variable, if the variable has been decided to fit into a register, then its address cannot taken anymore at this stage.
>>>>>> 
>>>>>> ******using “can_native_interpret_type_p” did make the “pattern” generation part much  cleaner and simpler, however, looks like it didn’t work correctly.
>>>>>> 
>>>>>> Based on this, I’d like to keep my previous implementation by using “use_register_for_decl” to decide whether to take “memset” expansion or “assign” expansion.
>>>>>> Therefore, I might still need to keep the “UGLY”  implementation of generatting “pattern” constant for different types?
>>>>>> 
>>>>>> Let me know your opinion on this.
>>>>> 
>>>>> Hmm, I think you can use use_register_for_decl(lhs) to decide to use an
>>>>> alternate type to generate the pattern (and feed to 
>>>>> can_native_interpret_type_p) by using
>>>>> lang_hooks.type_for_mode (TYPE_MODE (TREE_TYPE (lhs))).
>>>> 
>>>> Do you mean that the TYPE returned by “lang_hooks.type_for_mode(TYPE_MODE (TREE_TYPE (lhs))” will always satisfy “can_native_interpret_type_p”? 
>>>> Even for big structure types?
>>> 
>>> I meant that for use_register_for_decl (lhs) the structures will be
>>> always small and the structure type will have a mode that is not BLKmode
>>> (but for example DImode for struct { int i; int j; }).
>> 
>> Oh, I see.
>> 
>>> 
>>>> i.e, 
>>>> 
>>>> tree var_type = TREE_TYPE(lhs);
>>>> tree pattern = NULL_TREE;
>>>> unsigned HOST_WIDE_INT total_bytes
>>>>       = tree_to_uhwi (TYPE_SIZE_UNIT (var_type));
>>>> 
>>>> If (use_register_for_decl(lhs)==false)
>>>> {
>>>>   tree alt_type = lang_hooks.type_for_mode(TYPE_MODE(var_type), TYPE_UNSIGNED(var_type);
>>>>   If (can_native_interpret_type_p (alt_type))
>>>>     {
>>>> 	unsigned char *buf = (unsigned char *) xmalloc (total_bytes); 
>>>> 	memset (buf, INIT_PATTERN_VALUE, total_bytes); 
>>>> 	pattern = native_interpret_expr (alt_type, buf, total_bytes); 
>>>> 	gcc_assert (pattern); 
>>>>     }
>>>>   else
>>>>     gcc_unreachable ();
>>>> }
>>>> 
>>>> ?
>>>> Don’t quite understand here. Please clarify.
>>> 
>>> For !use_register_for_decl you use memset already, but for
>>> use_register_for_decl not all types satisfy can_native_interpret_type_p
>>> (in particular all struct and union types).  But when we use a
>>> register for the decl then we can of course directly initialize the
>>> register.
>> 
>> So, you mean the following:
>> 
>> If (is_vla || (!use_register_for_decl (lhs)))
>> {
>>  /* expand as memset that is done currently.  */
>> }
>> else 
>> {
>>   tree alt_type = lang_hooks.type_for_mode(TYPE_MODE(var_type), TYPE_UNSIGNED(var_type);
>>   If (can_native_interpret_type_p (alt_type))
> 
> you can use var_type when it satisfies can_native_interpret_type_p

Yes, then under that situation, no VIEW_CONVERT is needed.

> 
>>     {
>> 	unsigned char *buf = (unsigned char *) xmalloc (total_bytes); 
>> 	memset (buf, INIT_PATTERN_VALUE, total_bytes); 
>> 	pattern = native_interpret_expr (alt_type, buf, total_bytes); 
>> 	gcc_assert (pattern); 
>>     }
>>   else
>>     gcc_unreachable ();
>> 
>>     tree init = (init_type == AUTO_INIT_PATTERN) ?
>>                   pattern :
>>                   build_zero_cst (alt_type);
>> 
>>    /* here build VIEW_CONVERT <alt_type> (lhs) = init;
>>       And then expand it.  */
>> }
>> 
>> ?
>> 
>>> As said, it would be much cleaner (and maybe also easier)
>>> to then simply expand the RTL directly rather than going through
>>> expand_assignment.
>> 
>> Dump questions here:  
>> 
>> 1. When building VIEW_CONVERT  <alt_type> (lhs) = init and expand it:
>> 
>> Is the following correct:
>> 
>> lhs = build1 (VIEW_CONVERT_EXPR, alt_type, lhs);
>> expand_assignment (lhs, init);
> 
> yes, I think so.

Okay.
> 
>> 
>> which utility routines should be used to building the assignment and then expand it?
>> 
>> Are the above utility routines correct?
> 
> yes.
> 
>> 
>>> It's also easier to directly see whether the
>>> LHS is a MEM_P or a REG_P but then for the MEM_P case it's a bit
>>> more "awkward" to use the easy way of the generic expand code
>>> (esp. if we eventually want to emit actual calls to memset - do we?)
>> 
>> If all these are guarded by “use_register_for_decl” already, the lhs should be fit into registers, 
>> Under such situation, I don’t think that we want to emit actual calls to memset. That’s too expensive.
>> 
>>> 
>>>>> You can then
>>>>> build the assignment from the pattern as
>>>>> 
>>>>> VIEW_CONVERT <reg-type> (lhs) = pattern_cst;
>>>> 
>>>> What’s the <reg_type> in the above? The type “alt_type” returned by “lang_hooks.type_for_mode(TREE_TYPE(lhs))?
>>>> Do I need to build “MODIFY_EXPR” for the above? 
>>> 
>>> Yes, the lang_hook.type_for_mode result and no, you could go through
>>> expand_assignment.
>> 
>> Okay. So, still use “expand_assigment” to expand it?
> 
> yes.

Thanks a lot.

Now, I am clean on this part -:)

Qing
> 
>>> 
>>>> lhs =  build1 (VIEW_CONVERT_EXPR, alt_type, lhs);
>>>> tree  final = build2 (MODIFY_EXPR, TREE_TYPE (alt_type), lhs, pattern);
>>>> 
>>>> Then how to expand this”final"? 
>>>>> 
>>>>> note that more RTL-expand-ish would be to simply expand 'lhs' and
>>>>> see whether it's a REG_P or a MEM_P and decide based on that.
>>>> 
>>>> You mean that the current RTL expansion will automatically expand LHS to memset route or assignment route based on whether
>>>> LHS is a REG_P or MEM_P? I don’t need to explicitly code for “expand_builtin_memset” or “expand_assign”?
>>> 
>>> No.  But you are inside the expander for the internal function call
>>> and this is expected to generate RTL.  You can simply generate
>>> RTL directly without "faking" new GENERIC calls or assignments and
>>> expanding those.
>>> 
>>> But lets not go there for now.
>> 
>> Okay. I see now.
>> 
>> Qing
>>> 
>>> Richard.
>> 
>> 
> 
> -- 
> Richard Biener <rguenther@suse.de>
> SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
> Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)