Hi,
Segher Boessenkool <seg...@kernel.crashing.org> writes:
> On Thu, Jun 15, 2023 at 03:00:40PM +0800, Jiufu Guo wrote:
>> >> This is the existing pattern. It may be read as an action
>> >> to clean an unknown-size memory block.
>> >
>> > Including a size zero memory block, yes. BLKmode was originally to do
>> > things like bcopy (before modern names like memcpy were more usually
>> > used), and those very much need size zero as well.h
>>
>> The size is possible to be zero. No asm code needs to
>> be generated for "set 'const_int 0' to zero size memory"".
>> stack_tie does not generate any real code. It seems ok :)
>>
>> While, it may not be zero size mem. This may be a concern.
>> This is one reason that I would like to have an unspec_tie.
>
> It very much *can* be a zero size mem, that is perfectly find for
> mem:BLK.
There is still one concern: how to distinguish stack_tie
from other insn.
For example, below fake pattern:
(define_insn "xx_cleanmem"
[(parallel: [(set (mem:BLK (xxx)) (const_int 0))
(XXX/use "const_int_operand" "n")])]...
To avoid this pattern to be recognized as 'stack_tie',
'unspec_tie' was came to mind.
>
>> Another reason is unspec:blk is used but various ports :)
>
> unspec:BLK is undefined. BLKmode is allowed on mem only.
>
>> >> 2. "set (mem/c:BLK (reg/f:DI 1 1) unspec:blk (const_int 0 [0])
>> >> UNSPEC_TIE".
>> >> Current patch is using this one.
>> >
>> > What would be the semantics of that? Just the same as the current stuff
>> > I'd say, or less? It cannot be more!
>>
>> The semantic that I trying to achieve is "this is a special
>> insn, not only a normal set to unknown size mem".
>
> What does that *mean*? "Special instruction"? What would what code do
> for that? What would the RTL mean?
>
>> As you explained before on 'unspec:DI', the unspec would
>> just decorate the set_src part: something DI value with
>> machine-specific operation.
>
> An unspec is an operation on its operands, giving some (in this case)
> DImode value. There is nothing special about that operation, it can be
> optimised like any other, it's just not specified what exactly that
> value is (to the generic compiler, the backend itself can very much
> optimise stuff with it).
>
>> But, since 'tie_operand' is checked for this insn.
>> If 'tie_operand' checks UNPSEC_TIE, then the insn
>> with UNPSEC_TIE is 'a special insn'. Or interpret
>> the semantic of this insn as: this insn stack_ite
>> indicates "set/operate a zero size block".
>
> tie_operand is a predicate. The predicate of an insn has to return 1,
> or the insn is not recognised. You can do the same in insn conditions
> always (in principle anyway).
Thank you very much for your detailed and patient explanation!
BR,
Jeff (Jiufu Guo)
>
>
> Segher
