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

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

On Wed, Jun 14, 2023 at 06:25:10PM +0200, Richard Biener wrote:
> > 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"
> 
> That suggests it’s the hard register value that‘s protected, not the memory pointed to.  I suppose that means an unspec volatile with the reg as input would serve the same?

No?  It says what it says.  That is pretty vague language, of course,
not entirely by accident no doubt.

> Or maybe that’s not the whole story.
> 
> 
> > 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)
> 
> I cannot make sense of that comment, but not sure if I really want to know …

It really is the same thing: this is a bloody heavy hammer keeping the
change to the stack pointer (or "hard" frame pointer) in place wrt any
accesses to the stack memory.

If there was a nice portable way to avoid needing this we haven't found
it yet -- or a non-portable way even, and it doesn't have to be all that
nice either come to think of it :-)


Segher