Re: shift/extract SHIFT_COUNT_TRUNCATED combine bug

[Richard Biener <richard dot guenther at gmail dot com>]

On Mon, Jan 12, 2015 at 11:12 PM, Jeff Law <law@redhat.com> wrote:
> On 04/08/14 14:07, Mike Stump wrote:
>>
>> Something broke in the compiler to cause combine to incorrectly optimize:
>>
>> (insn 12 11 13 3 (set (reg:SI 604 [ D.6102 ])
>>          (lshiftrt:SI (subreg/s/u:SI (reg/v:DI 601 [ x ]) 0)
>>              (reg:SI 602 [ D.6103 ]))) t.c:47 4436 {lshrsi3}
>>       (expr_list:REG_DEAD (reg:SI 602 [ D.6103 ])
>>          (nil)))
>> (insn 13 12 14 3 (set (reg:SI 605)
>>          (and:SI (reg:SI 604 [ D.6102 ])
>>              (const_int 1 [0x1]))) t.c:47 3658 {andsi3}
>>       (expr_list:REG_DEAD (reg:SI 604 [ D.6102 ])
>>          (nil)))
>> (insn 14 13 15 3 (set (reg:DI 599 [ D.6102 ])
>>          (zero_extend:DI (reg:SI 605))) t.c:47 4616 {zero_extendsidi2}
>>       (expr_list:REG_DEAD (reg:SI 605)
>>          (nil)))
>>
>> into:
>>
>> (insn 11 10 12 3 (set (reg:SI 602 [ D.6103 ])
>>          (not:SI (subreg:SI (reg:DI 595 [ D.6102 ]) 0))) t.c:47 3732
>> {one_cmplsi2}
>>       (expr_list:REG_DEAD (reg:DI 595 [ D.6102 ])
>>          (nil)))
>> (note 12 11 13 3 NOTE_INSN_DELETED)
>> (note 13 12 14 3 NOTE_INSN_DELETED)
>> (insn 14 13 15 3 (set (reg:DI 599 [ D.6102 ])
>>          (zero_extract:DI (reg/v:DI 601 [ x ])
>>              (const_int 1 [0x1])
>>              (reg:SI 602 [ D.6103 ]))) t.c:47 4668 {c2_extzvdi}
>>       (expr_list:REG_DEAD (reg:SI 602 [ D.6103 ])
>>          (nil)))
>>
>> This shows up in:
>>
>>    FAIL: gcc.c-torture/execute/builtin-bitops-1.c execution,  -Og -g
>>
>> for me.
>>
>> diff --git a/gcc/combine.c b/gcc/combine.c
>> index 708691f..c1f50ff 100644
>> --- a/gcc/combine.c
>> +++ b/gcc/combine.c
>> @@ -7245,6 +7245,18 @@ make_extraction (enum machine_mode mode, rtx inner,
>> HOST_WIDE_INT pos,
>>         extraction_mode = insn.field_mode;
>>       }
>>
>> +  /* On a SHIFT_COUNT_TRUNCATED machine, we can't promote the mode of
>> +     the extract to a larger size on a variable extract, as previously
>> +     the position might have been optimized to change a bit of the
>> +     index of the starting bit that would have been ignored before,
>> +     but, with a larger mode, will then not be.  If we wanted to do
>> +     this, we'd have to mask out those bits or prove that those bits
>> +     are 0.  */
>> +  if (SHIFT_COUNT_TRUNCATED
>> +      && pos_rtx
>> +      && GET_MODE_BITSIZE (extraction_mode) > GET_MODE_BITSIZE (mode))
>> +    extraction_mode = mode;
>> +
>>     /* Never narrow an object, since that might not be safe.  */
>>
>>     if (mode != VOIDmode
>>
>> is sufficient to never widen variable extracts on SHIFT_COUNT_TRUNCATED
>> machines.  So, the question is, how did people expect this to work?  I
>> didnât spot what changed recently to cause the bad code-gen.  The
>> optimization of sub into not is ok, despite how funny it looks, because is
>> feeds into extract which we know by SHIFT_COUNT_TRUNCATED is safe.
>>
>> Is the patch a reasonable way to fix this?
>
> On a SHIFT_COUNT_TRUNCATED target, I don't think it's ever OK to widen a
> shift, variable or constant.
>
> In the case of a variable shift, we could easily have eliminated the masking
> code before or during combine.  For a constant shift amount we could have
> adjusted the constant (see SHIFT_COUNT_TRUNCATED in cse.c)
>
> I think it's just an oversight and it has simply never bit us before.

IMHO SHIFT_COUNT_TRUNCATED should be removed and instead
backends should provide shift patterns with a (and:QI ...) for the
shift amount which simply will omit that operation if suitable.

Richard.

> jeff