Re: [PATCH] Fix PR54733 Optimize endian independent load/store

[Andrew Pinski <pinskia@gmail.com>]

On Fri, May 16, 2014 at 9:58 AM,  <pinskia@gmail.com> wrote:
>
>
>> On May 16, 2014, at 4:13 AM, Richard Biener <richard.guenther@gmail.com> wrote:
>>
>> On Fri, May 16, 2014 at 1:03 PM, Richard Biener
>> <richard.guenther@gmail.com> wrote:
>>> On Fri, May 16, 2014 at 12:56 PM,  <pinskia@gmail.com> wrote:
>>>>
>>>>
>>>>> On May 16, 2014, at 3:48 AM, Richard Biener <richard.guenther@gmail.com> wrote:
>>>>>
>>>>> On Fri, May 16, 2014 at 12:07 PM, Thomas Preud'homme
>>>>> <thomas.preudhomme@arm.com> wrote:
>>>>>> Ping?
>>>>>
>>>>> Sorry ...
>>>>>
>>>>>> Best regards,
>>>>>>
>>>>>> Thomas Preud'homme
>>>>>>
>>>>>>> -----Original Message-----
>>>>>>> From: gcc-patches-owner@gcc.gnu.org [mailto:gcc-patches-
>>>>>>> owner@gcc.gnu.org] On Behalf Of Thomas Preud'homme
>>>>>>> Sent: Friday, May 09, 2014 6:26 PM
>>>>>>> To: GCC Patches
>>>>>>> Subject: RE: [PATCH] Fix PR54733 Optimize endian independent load/store
>>>>>>>
>>>>>>> Sorry, took longer than expected as I got distracted by some other patch.
>>>>>>> I merged the whole patchset in a single patch as I was told the current setup
>>>>>>> is actually more difficult to read.
>>>>>>>
>>>>>>> Here are the updated ChangeLogs:
>>>>>>>
>>>>>>> *** gcc/ChangeLog ***
>>>>>>>
>>>>>>> 2014-05-09  Thomas Preud'homme  <thomas.preudhomme@arm.com>
>>>>>>>
>>>>>>>     PR tree-optimization/54733
>>>>>>>     * expr.c (get_inner_reference): Add a parameter to control whether
>>>>>>> a
>>>>>>>     MEM_REF should be split into base + offset.
>>>>>>>     * tree.h (get_inner_reference): Default new parameter to false.
>>>>>>>     * tree-ssa-math-opts.c (nop_stats): New "bswap_stats" structure.
>>>>>>>     (CMPNOP): Define.
>>>>>>>     (find_bswap_or_nop_load): New.
>>>>>>>     (find_bswap_1): Renamed to ...
>>>>>>>     (find_bswap_or_nop_1): This. Also add support for memory source.
>>>>>>>     (find_bswap): Renamed to ...
>>>>>>>     (find_bswap_or_nop): This. Also add support for memory source and
>>>>>>>     detection of bitwise operations equivalent to load in host endianness.
>>>>>>>     (execute_optimize_bswap): Likewise. Also move its leading
>>>>>>> comment back
>>>>>>>     in place and split statement transformation into ...
>>>>>>>     (bswap_replace): This. Add assert when updating bswap_stats.
>>>>>>>
>>>>>>> *** gcc/testsuite/ChangeLog ***
>>>>>>>
>>>>>>> 2014-05-09  Thomas Preud'homme  <thomas.preudhomme@arm.com>
>>>>>>>
>>>>>>>     PR tree-optimization/54733
>>>>>>>     * gcc.dg/optimize-bswapdi-3.c: New test to check extension of
>>>>>>> bswap
>>>>>>>     optimization to support memory sources and bitwise operations
>>>>>>>     equivalent to load in host endianness.
>>>>>>>     * gcc.dg/optimize-bswaphi-1.c: Likewise.
>>>>>>>     * gcc.dg/optimize-bswapsi-2.c: Likewise.
>>>>>>>     * gcc.c-torture/execute/bswap-2.c: Likewise.
>>>>>>>
>>>>>>> Ok for trunk?
>>>>>
>>>>> Ok, I now decided otherwise and dislike the new parameter to
>>>>> get_inner_reference.  Can you please revert that part and just
>>>>> deal with a MEM_REF result in your only caller?
>>>>>
>>>>> And (of course) I found another possible issue.  The way you
>>>>> compute load_type and use it here:
>>>>>
>>>>> +      /* Perform the load.  */
>>>>> +      load_offset_ptr = build_int_cst (n->alias_set, 0);
>>>>> +      val_expr = fold_build2 (MEM_REF, load_type, addr_tmp,
>>>>> +                             load_offset_ptr);
>>>>>
>>>>> makes the load always appear aligned according to the mode of
>>>>> load_type.  On strict-alignment targets this may cause faults.
>>>>>
>>>>> So what you have to do is either (simpler)
>>>>>
>>>>>  unsigned int align = get_pointer_alignment (addr_tmp);
>>>>>  tree al_load_type = load_type;
>>>>>  if (align < TYPE_ALIGN (load_type))
>>>>>    al_load_type = build_aligned_type (load_type, align);
>>>>> ...
>>>>>   val_expr = fold_build2 (MEM_REF, al_load_type, addr_tmp,
>>>>>                                    load_offset_ptr);
>>>>>
>>>>> or keep track of the "first" actual load and use
>>>>>
>>>>>  unsigned int align = get_object_alignment (that_first_load);
>>>>>
>>>>> "first" in the one that corresponds to addr_tmp.  From that there
>>>>> is a much better chance to derive good alignment values.
>>>>>
>>>>> Of course on STRICT_ALIGNMENT targets a not aligned load
>>>>> will be decomposed again, so eventually doing the transformation
>>>>> may no longer be profitable(?).
>>>>
>>>> Not always decomposed. On MIPS, it should using the load/store left/right instructions for unaligned load/stores which is normally better than decomposed load/stores. So having a cost model would be nice.
>>>
>>> Agreed, but I am happy with doing that as a followup.  Btw,
>>> a very simple one would be to reject unaligned
>>> SLOW_UNALIGNED_ACCESS (TYPE_MODE (load_type), align).
>>> [of course that may be true on MIPS even for the cases where
>>> a "reasonable" fast unalgined variant exists - nearly no target
>>> defines that macro in a too fancy way]
>>
>> Oh, and what happens for
>>
>> unsigned foo (unsigned char *x)
>> {
>>  return x[0] << 24 | x[2] << 8 | x[3];
>> }
>>
>> ?  We could do an unsigned int load from x and zero byte 3
>> with an AND.  Enhancement for a followup, similar to also
>> considering vector types for the load (also I'm not sure
>> that uint64_type_node always has non-BLKmode for all
>> targets).
>
> No we cannot if x[4] is on a different page which is not mapped in, we get a fault. Not something we want.

I was reading that code wrong.  I trying to say if we don't load from
x[3] then we can't do it.  But with the example above we can.

Thanks,
Andrew Pinski


>
> Thanks,
> Andrew
>
>
>>
>> Richard.
>>
>>> Richard.
>>>
>>>> Thanks,
>>>> Andrew
>>>>
>>>>>
>>>>> Thanks and sorry again for the delay.
>>>>>
>>>>> Otherwise the patch looks good to me.
>>>>>
>>>>> Richard.
>>>>>
>>>>>>> Best regards,
>>>>>>>
>>>>>>> Thomas
>>>>>>>
>>>>>>>> -----Original Message-----
>>>>>>>> From: gcc-patches-owner@gcc.gnu.org [mailto:gcc-patches-
>>>>>>>> owner@gcc.gnu.org] On Behalf Of Thomas Preud'homme
>>>>>>>> Sent: Monday, May 05, 2014 7:30 PM
>>>>>>>> To: GCC Patches
>>>>>>>> Subject: RE: [PATCH][2/3] Fix PR54733 Optimize endian independent
>>>>>>>> load/store
>>>>>>>>
>>>>>>>> I found a way to improve the function find_bswap/find_bswap_or_nop
>>>>>>>> and reduce its size. Please hold for the review, I will post an updated
>>>>>>>> version as soon as I finish testing.
>>>>>>>>
>>>>>>>> Best regards,
>>>>>>>>
>>>>>>>> Thomas Preud'homme
>>>>>>
>>>>>>