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

[Richard Biener <rguenther@suse.de>]

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);

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).

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)