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

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi!

On Wed, Jun 14, 2023 at 07:59:04AM +0000, Richard Biener wrote:
> On Wed, 14 Jun 2023, Jiufu Guo wrote:
> > 3. "set (mem/c:DI (reg/f:DI 1 1) unspec:DI (const_int 0 [0])
> > UNSPEC_TIE".
> >    This avoids using BLK on unspec, but using DI.
> 
> That gives the MEM a size which means we can interpret the (set ..)
> as killing a specific area of memory, enabling DSE of earlier
> stores.

Or DSE can delete this tie even, if it can see some later store to the
same location without anything in between that can read what the tie
stores.

BLKmode avoids all of this.  You can call that elegant, you can call it
cheating, you can call it many things -- but it *works*.

> AFAIU this special instruction is only supposed to prevent
> code motion (of stack memory accesses?) across this instruction?

Form rs6000.md:
; This is to explain that changes to the stack pointer should
; not be moved over loads from or stores to stack memory.
(define_insn "stack_tie"

and from rs6000-logue.cc:
/* This ties together stack memory (MEM with an alias set of frame_alias_set)
   and the change to the stack pointer.  */
static void
rs6000_emit_stack_tie (rtx fp, bool hard_frame_needed)

A big reason this is needed is because of all the hard frame pointer
stuff, which the generic parts of GCC require, but there is no register
for that in the Power architecture.  Nothing is an issue here in most
cases, but sometimes we need to do unusual things to the stack, say for
alloca.

> I'd say a
> 
>   (may_clobber (mem:BLK (reg:DI 1 1)))

"clobber" always means "may clobber".  (clobber X) means X is written
with some unspecified value, which may well be whatever value it
currently holds.  Via some magical means or whatever, there is no
mechanism specified, just the effects :-)

> might be more to the point?  I've used "may_clobber" which doesn't
> exist since I'm not sure whether a clobber is considered a kill.
> The docs say "Represents the storing or possible storing of an 
> unpredictable..." - what is it?  Storing or possible storing?

It is the same thing.  "clobber" means the same thing as "set", except
the value that is written is not specified.

> I suppose stack_tie should be less strict than the documented
> (clobber (mem:BLK (const_int 0))) (clobber all memory).

"clobber" is nicer than the set to (const_int 0).  Does it work though?
All this code is always fragile :-/  I'm all for this change, don't get
me wrong, but preferably things stay in working order.

We use "stack_tie" as a last resort heavy hammer anyway, in all normal
cases we explain the actual data flow explicitly and correctly, also
between the various registers used in the *logues.


Segher