Le 01/09/2022 à 09:37, Gabriel Paubert a écrit :
> On Thu, Sep 01, 2022 at 05:22:32AM +0000, Christophe Leroy wrote:
>>
>>
>> Le 01/09/2022 à 00:45, Segher Boessenkool a écrit :
>>> Hi!
>>>
>>> On Tue, Aug 30, 2022 at 09:10:02AM +0000, Christophe Leroy wrote:
>>>> Le 30/08/2022 à 11:01, Nicholas Piggin a écrit :
>>>>> On Tue Aug 30, 2022 at 3:24 PM AEST, Christophe Leroy wrote:
>>>>>>> This is still slightly concerning to me. Is there any guarantee that the
>>>>>>> compiler would not use a different sequence for the address here?
>>>>>>>
>>>>>>> Maybe explicit r13 is required.
>>>>>>>
>>>>>>
>>>>>> local_paca is defined as:
>>>>>>
>>>>>> register struct paca_struct *local_paca asm("r13");
>>>
>>> And this is in global scope, making it a global register variable.
>>>
>>>>>> Why would the compiler use another register ?
>>>>>
>>>>> Hopefully it doesn't. Is it guaranteed that it won't?
>>>
>>> Yes, this is guaranteed.
>>>
>>> For a local register variable this is guaranteed only for operands to an
>>> extended inline asm; any other access to the variable does not have to
>>> put it in the specified register.
>>>
>>> But this is a global register variable. The only thing that would make
>>> this crash and burn is if *any* translation unit did not see this
>>> declaration: it could then use r13 (if that was allowed by the ABI in
>>> effect, heh).
>>>
>>>>> I'm sure Segher will be delighted with the creative asm in __do_IRQ
>>>>> and call_do_irq :) *Grabs popcorn*
>>>
>>> All that %% is blinding, yes.
>>>
>>> Inline tabs are bad taste.
>>>
>>> Operand names instead of numbers are great for obfuscation, and nothing
>>> else -- unless you have more than four or five operands, in which case
>>> you have bigger problems already.
>>>
>>> Oh, this function is a good example of proper use of local register asm,
>>> btw.
>>>
>>> Comments like "// Inputs" are just harmful. As is the "creative"
>>> indentation here. Both harm readability and do not help understanding
>>> in any other way either.
>>>
>>> The thing about inline asm is the smallest details change meaning of the
>>> whole, it is a very harsh environment, you are programming both in C and
>>> directly assembler code as well, and things have to be valid for both,
>>> although on the other hand there is almost no error checking. Keeping
>>> it small, simple, readable is paramount.
>>>
>>> The rules for using inline asm:
>>>
>>> 0) Do no use inline asm.
>>> 1) Use extended asm, unless you know all differences with basic asm, and
>>> you know you want that. And if you answer "yes I do" to the latter,
>>> you answered wrong to the former.
>>> 2) Do not use toplevel asm.
>>> 3) Do no use inline asm.
>>> 4) Do no use inline asm.
>>> 5) Do no use inline asm.
>>>
>>> Inline asm is a very powerful escape hatch. Like all emergency exits,
>>> you should not use them if you do not need them! :-)
>>>
>>> But, you are talking about the function calling and the frame change I
>>> bet :-) Both of these are only okay because everything is back as it
>>> was when this (single!) asm is done, and the state created is perfectly
>>> fine (this is very dependent on exact ABI used, etc.)
>>>
>>> I would have used real assembler code here (in a .s file). But there
>>> probably are reasons to do things this way, performance probably?
>>
>> We changed it to inline asm in order to ... inline it in the caller.
>
> And there is a single caller it seems. Typically GCC inlines function
> with a single call site, but it may be confused by asm statements.
Regardless, it is tagged __always_inline.
>
>>
>> I also find that those operand names make it awull more difficult to
>> read that traditional numbering. I really dislike that new trend.
>> And same with those // comments, better use meaningfull C variable names.
>
> Agree, but there is one thing which escapes me: why is r3 listed in the
> outputs section (actually as a read write operand with the "+"
> constraint modifier) but is not used after the asm which is the last
> statement of function returning void?
>
> Do I miss something?
As far as I remember, that's to tell GCC that r3 register is modified by
the callee. As it is an input, it couldn't be listed in the clobber list.
Christophe
