On Mon, Jan 12, 2015 at 11:12 PM, Jeff Law <l...@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
>> didn’t 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
