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

[Richard Biener <rguenther@suse.de>]

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

>      {
> 	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.

> 
> 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.

> > 
> >> 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>
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