Predictive commoning leads to register to register moves through memory.

[Jeff Law]

On 09/18/2015 03:13 AM, Richard Biener wrote:
> On Thu, Sep 17, 2015 at 5:58 PM, Simon Dardis <Simon.Dardis@imgtec.com> wrote:
>> I've since taken another look at this recently and I've tracked the issue down to
>> tree-predcom.c, specifically ref_at_iteration almost always generating MEM_REFs.
>> With MEM_REFs, GCC's RTL GCSE cannot compare them as equal and hence
>> remove them. A previous version of the code did generate ARRAY_REFs
>> (pre 204458), but that was changed to generate MEM_REFs for pr/58653.
>
> Ok, so the issue is the full redundancy in
>
>    <bb 5>:
>    # count_28 = PHI <0(4), count_23(7)>
>    if (pretmp_42 > 1.0e+0)
>      goto <bb 6>;
>    else
>      goto <bb 9>;
>
>    <bb 6>:
>    _8 = 1.0e+0 / pretmp_42;
>    _12 = _8 * _8;
>    poly[1] = _12;
>
>    <bb 7>:
>    # prephitmp_30 = PHI <_12(6), _36(9)>
>    # T_lsm.8_22 = PHI <_8(6), pretmp_42(9)>
>    poly_I_lsm0.10_38 = MEM[(double *)&poly + 8B];
> ...
>    if (count_23 != iterations_6(D))
>      goto <bb 5>;
>    else
>      goto <bb 8>;
>
>    <bb 9>:
>    _36 = pretmp_42 * pretmp_42;
>    poly[1] = _36;
>    goto <bb 7>;
>
> ?
>
> On x86_64 I have only one load from poly[1] left in cprop1, the one
> using the MEM_REF.  It's
>
> (insn 31 30 32 6 (set (reg:DF 93 [ poly_I_lsm0.10 ])
>          (mem/c:DF (const:DI (plus:DI (symbol_ref:DI ("poly") [flags
> 0x2]  <var_decl 0x7f6eab532e10 poly>)
>                      (const_int 8 [0x8]))) [1 MEM[(double *)&poly +
> 8B]+0 S8 A64])) 126 {*movdf_internal}
>       (nil))
>
> vs.
>
> (insn 67 65 5 8 (set (mem/c:DF (const:DI (plus:DI (symbol_ref:DI
> ("poly") [flags 0x2]  <var_decl 0x7f6eab532e10 poly>)
>                      (const_int 8 [0x8]))) [1 poly+8 S8 A64])
>          (reg:DF 90)) t.c:22 126 {*movdf_internal}
>
> for the partial redundancy across the backedge.  I don't see why GCSE
> should even care for the MEM_EXPR
> here!
>
>> Would something like:
>> --- a/gcc/tree-predcom.c
>> +++ b/gcc/tree-predcom.c
>> @@ -1409,7 +1409,21 @@ ref_at_iteration (data_reference_p dr, int iter, gimple_seq *stmts)
>>                               addr, alias_ptr),
>>                       DECL_SIZE (field), bitsize_zero_node);
>>       }
>> -  else
>> +  /* Generate an ARRAY_REF for array references when all details are INTEGER_CST
>> +     rather than a MEM_REF so that CSE passes can potientially optimize them.  */
>> +  else if (TREE_CODE (DR_REF (dr)) == ARRAY_REF
>> +          && TREE_CODE (DR_STEP (dr)) == INTEGER_CST
>> +          && TREE_CODE (DR_INIT (dr)) == INTEGER_CST
>> +          && TREE_CODE (DR_OFFSET (dr)) == INTEGER_CST)
>> +    {
>> +       /* Reverse engineer the element from memory offset.  */
>> +       tree offset = size_binop (MINUS_EXPR, coff, off);
>> +       tree sizdiv = TYPE_SIZE (TREE_TYPE (TREE_TYPE (DR_BASE_OBJECT (dr))));
>> +       sizdiv = div_if_zero_remainder (EXACT_DIV_EXPR, sizdiv, ssize_int (BITS_PER_UNIT));
>> +       tree element = div_if_zero_remainder (EXACT_DIV_EXPR, offset, sizdiv);
>> +       if (element != NULL_TREE)
>> +         return build4 (ARRAY_REF, TREE_TYPE (DR_REF (dr)), DR_BASE_OBJECT (dr),
>> +                        element, NULL_TREE, NULL_TREE);
>> +    }
>>       return fold_build2 (MEM_REF, TREE_TYPE (DR_REF (dr)), addr, alias_ptr);
>>
>> be an appropriate start to fixing this? That fix appears to work in in my testing.
>
> No, that seems to be the wrong fix and in the light of wrong-code
> issues we had with the middle-end
> generating ARRAY_REFs even more so.
>
> What we can do and what would maybe help this case is in
> ref_at_iteration detect the DR_STEP == 0 case
> and simply return unshare_expr (DR_REF (dr)) in that case.  Well, iff
> you don't want to explore the
> GCSE issue...
>
> Index: gcc/tree-predcom.c
> ===================================================================
> --- gcc/tree-predcom.c  (revision 227899)
> +++ gcc/tree-predcom.c  (working copy)
> @@ -1386,6 +1386,10 @@ replace_ref_with (gimple stmt, tree new_
>   static tree
>   ref_at_iteration (data_reference_p dr, int iter, gimple_seq *stmts)
>   {
> +  /* If the reference doesn't change just return it.  */
> +  if (integer_zerop (DR_STEP (dr)))
> +    return unshare_expr (DR_REF (dr));
> +
>     tree off = DR_OFFSET (dr);
>     tree coff = DR_INIT (dr);
>     if (iter == 0)
>
> hmm, no, then we end up with
>
>    # prephitmp_30 = PHI <_12(6), _36(9)>
>    # T_lsm.8_22 = PHI <_8(6), pretmp_42(9)>
>    poly_I_lsm0.10_38 = poly[1];
>    _24 = MEM[(double *)&poly + 8B];
>
> beause DOM has a similar issue preventing it from CSEing those two.
Presumably you're referring to the last two statements?  If so, yes, I 
wouldn't expect DOM to handle that.  I don't think its hasher is 
prepared to try and hash those two RHS expressions to the same value.

jeff