Re: [PATCH] rs6000: replace '(const_int 0)' to 'unspec:BLK [(const_int 0)]' for stack_tie

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi,

Segher Boessenkool <segher@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