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Re: Inconsistent initialization for pic_offset_table_rtx?


2016-02-09 19:24 GMT+03:00 Jeff Law <law@redhat.com>:
> On 02/09/2016 07:27 AM, Bin.Cheng wrote:
>>
>> On Fri, Feb 5, 2016 at 10:32 AM, Ilya Enkovich <enkovich.gnu@gmail.com>
>> wrote:
>>>
>>> 2016-02-04 19:16 GMT+03:00 Bin.Cheng <amker.cheng@gmail.com>:
>>>>
>>>> On Thu, Feb 4, 2016 at 3:18 PM, Ilya Enkovich <enkovich.gnu@gmail.com>
>>>> wrote:
>>>>>
>>>>> 2016-02-04 17:12 GMT+03:00 Bin.Cheng <amker.cheng@gmail.com>:
>>>>>>
>>>>>> Hi,
>>>>>> I noticed that pic_offset_table_rtx is initialized twice in GCC.  Take
>>>>>> x86_32 as an example.
>>>>>> The first initialization is done in emit_init_regs, with below code:
>>>>>>
>>>>>>    pic_offset_table_rtx = NULL_RTX;
>>>>>>    if ((unsigned) PIC_OFFSET_TABLE_REGNUM != INVALID_REGNUM)
>>>>>>      pic_offset_table_rtx = gen_raw_REG (Pmode,
>>>>>> PIC_OFFSET_TABLE_REGNUM);
>>>>>>
>>>>>> On x86_32 with pic, we have:
>>>>>>
>>>>>> (gdb) call debug_rtx(this_target_rtl->x_pic_offset_table_rtx)
>>>>>> (reg:SI 3 bx)
>>>>>>
>>>>>> The second initialization is in expand_used_vars, with below code:
>>>>>>
>>>>>>    if (targetm.use_pseudo_pic_reg ())
>>>>>>      pic_offset_table_rtx = gen_reg_rtx (Pmode);
>>>>>>
>>>>>> On x86_32 with pic, we have:
>>>>>>
>>>>>> (gdb) call debug_rtx(this_target_rtl->x_pic_offset_table_rtx)
>>>>>> (reg:SI 87)
>>>>>>
>>>>>> So basically after expanding the first function, pic_offset_table_rtx
>>>>>> is set to a pseudo register, rather than the one initialized in
>>>>>> emit_init_regs.
>>>>>>
>>>>>> Also this causes inconsistent compilation for the first/rest functions
>>>>>> in one compilation unit.
>>>>>>
>>>>>> A bug?
>>>>>
>>>>>
>>>>> For i386 target PIC_OFFSET_TABLE_REGNUM actually checks
>>>>> ix86_use_pseudo_pic_reg and is supposed to return INVALID_REGNUM
>>>>> in case we use pseudo register for PIC. BUT we hit a case when PIC
>>>>> code is generated for cost estimation via target hooks while performing
>>>>> some GIMPLE pass. In this case we need to return some register to
>>>>
>>>> Thanks IIya.  This is exact the case I ran into.  See PR69042.
>>>>
>>>>> generate PIC usage but we don't have any allocated. In this case we
>>>>> return a hard register. We detect such situation by checking
>>>>> pic_offset_table_rtx.
>>>>>
>>>>> Thus if we use pseudo PIC register but pic_offset_table_rtx is not
>>>>> initialized yet,
>>>>> then PIC_OFFSET_TABLE_REGNUM returns a hard register.
>>>>>
>>>>> So I suppose we may consider the first assignment as a bug.
>>>>
>>>>
>>>> But I don't quite follow.  So hard register is returned so that gimple
>>>> passes can construct PIC related addresses?  If this is the case, the
>>>> first initialization is necessary.
>>>
>>>
>>> Right, we need some initialization but current way leads to inconsistent
>>> value and we may 'randomly' get hard or pseudo register for different
>>> functions
>>> which possibly affects some optimizations. We probably need a better
>>> way to check if we should return a hard reg for PIC register. Or maybe
>>> reset ix86_use_pseudo_pic_reg to NULL when function is finalized or
>>> get rid of hard reg at all and always use a pseudo register.
>>>
>>>> Another question is about address cost:
>>>>
>>>>    if (parts.index
>>>>        && (!REG_P (parts.index) || REGNO (parts.index) >=
>>>> FIRST_PSEUDO_REGISTER)
>>>>        && (current_pass->type == GIMPLE_PASS
>>>>        || !pic_offset_table_rtx
>>>>        || !REG_P (parts.index)
>>>>        || REGNO (pic_offset_table_rtx) != REGNO (parts.index)))
>>>>      cost++;
>>>> Is it a bug in the second sub condition?  Considering
>>>> "current_pass->type == GIMPLE_PASS" in the third sub condition, can I
>>>> assume the second is for non-GIMPLE passes only?
>>>
>>>
>>> There is just a code duplicated for parts.base and parts.index
>>> registers. We use same
>>> conditions for both of them because you can easily swap them if scale is
>>> 1.
>>>
>>> I don't know why we don't try to recognize PIC registers when in GIMPLE
>>> pass.
>>
>>
>> Could we define PIC_OFFSET_TABLE_REGNUM to a pseudo register when
>> ix86_use_pseudo_pic_reg returns true, as in this case?  Current logic
>> doesn't look consistent to me.  Of course, I know little about x86.
>
> I think defining it as a hard reg was a stopgap.  I don't think we've got
> the level of consistency we want for PIC register as a pseudo throughout the
> generic code or hte x86 backend.
>
> My recollection is that it doesn't appear in the IL as a hard register
> anymore.

Unfortunately we still use it as EBX.  It happens when we try to
compute address cost and call target hook to legitimize address for
that.  It involves pic_offset_table_rtx usage and would ICE if we
don't initialize it.  Since pic_offset_table_rtx initialization in
init_emit_regs happens only if PIC_OFFSET_TABLE_REGNUM holds some hard
reg, we use EBX_REG value as workaround.

When some function is expanded pic_offset_table_rtx gets a pseudo
register which is used for address legitimization during next function
optimization.  Thus we have a kind of inconsistency using either hard
or pseudo register for address cost estimation depending on function
compilation order.

Thanks,
Ilya

> jeff
>


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