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

[Richard Biener <rguenther@suse.de>]

On Thu, 19 Aug 2021, Qing Zhao wrote:

> 
> 
> > 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?

Because it's simply the wrong entity - I have no idea why that even
worked.  Ah, cfgexpand associates it with some DECL_RTL for the 
benefit of debug info.  But it's still wrong.

> I should use as following
> 
> 
>    If (TREE_CODE (lhs) == SSA_NAME) && SSA_NAME_VAR (lhs))
>       Lhs = SSA_NAME_VAR (lhs)
> 
> ?

No.  A SSA_NAME_VAR can have multiple SSA_NAMEs (obviously) and
they do not necessarily have to be allocated to the same variable
partition - that is, there's no 1:1 relationship between SSA_NAME
and stack slot or (pseudo) register.  You want to initialize the
storage associated with the SSA_NAME in the .DEFERRED_INIT call,
not some other storage.

> > 
> > 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? 

It depends.  In the end I'd consider it a missed-optimization bug on
the side that generates worse code - but I do expect cases will exist
for both.  Clearly memset will be worse when dealing with register
initialization (thus the !MEM_P check) and I expect memset to be OK
for stack where member-wise init esp. with non-zero might turn up
worse code.

Richard.

> 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)
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imendörffer; HRB 36809 (AG Nuernberg)