Re: Handle GIMPLE NOPs in is_maybe_undefined (PR, tree-optimization/79529).

[Martin Liška <mliska at suse dot cz>]

On 02/15/2017 05:06 PM, Aldy Hernandez wrote:
> On 02/15/2017 09:49 AM, Martin Liška wrote:
>> Hi.
>>
>> As mentioned in the PR, gimple nops are wrongly handled in is_maybe_undefined function.
>> Patch can bootstrap on ppc64le-redhat-linux and survives regression tests.
> 
>>
>>       gimple *def = SSA_NAME_DEF_STMT (t);
>>
>> +      if (!def || gimple_nop_p (def))
>> +    return true;
>> +
> 
> 
> Out of curiosity, what is T?

It's a SSA name which belongs to a result_decl:

(gdb) p debug_tree(t)
 <ssa_name 0x7fffdaf3b1f8
    type <reference_type 0x7ffff122c9d8
        type <record_type 0x7ffff0664888 UnicodeString addressable needs-constructing BLK
            size <integer_cst 0x7ffff6892030 constant 512>
            unit size <integer_cst 0x7ffff6931c00 constant 64>
            align 64 symtab 0 alias set -1 canonical type 0x7ffff66b0e70 fields <field_decl 0x7ffff0669390 D.165781> context <namespace_decl 0x7ffff66954c0 icu_58>
            pointer_to_this <pointer_type 0x7ffff06791f8> reference_to_this <reference_type 0x7fffea593e70> chain <type_decl 0x7ffff0669428 UnicodeString>>
        public unsigned DI
        size <integer_cst 0x7ffff6876be8 constant 64>
        unit size <integer_cst 0x7ffff6876c00 constant 8>
        align 64 symtab 0 alias set -1 structural equality>
    var <result_decl 0x7fffdaf1f528 resultPattern>
    def_stmt GIMPLE_NOP
    version 1
    ptr-info 0x7fffcc191000>

> 
> Because we early bail out if ssa_undefined_value_p() right before calling SSA_NAME_DEF_STMT, and ssa_undefined_value_p() will already bail if gimple_nop_p.
> 
> So T must be one of:
> 
> 
>   /* Parameters get their initial value from the function entry.  */
>   else if (TREE_CODE (var) == PARM_DECL)
>     return false;
>   /* When returning by reference the return address is actually a hidden
>      parameter.  */
>   else if (TREE_CODE (var) == RESULT_DECL && DECL_BY_REFERENCE (var))
>     return false;

This return statement is taken.

>   /* Hard register variables get their initial value from the ether.  */
>   else if (VAR_P (var) && DECL_HARD_REGISTER (var))
>     return false;
> 
> which I wonder if we should special case in is_maybe_undefined.

Do we need a special case in the function?

M.

> 
> Aldy