Re: Combine patch ping

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

On Wed, Apr 10, 2024 at 08:32:39PM +0200, Uros Bizjak wrote:
> On Wed, Apr 10, 2024 at 7:56 PM Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> > This is never okay.  You cannot commit a patch without approval, *ever*.

This is the biggest issue, to start with.  It is fundamental.

> > That patch is also obvious -- obviously *wrong*, that is.  There are
> > big assumptions everywhere in the compiler how a CC reg can be used.
> > This violates that, as explained elsewhere.
> 
> Can you please elaborate what is wrong with this concrete patch.

The explanation of the patch is contradictory to how RTL works at all,
so it is just wrong.  It might even do something sane, but I didn't get
that far at all!

Write good email explanations, and a good proposed commit message.
Please.  It is the only one people can judge a patch.  Well, apart
from doing everything myself from first principles, ignoring everything
you said, just looking at the patch itself, but that is a hundred times
more work.  I don't do that.

> The
> part that the patch touches has several wrong assumptions, and the
> fixed "???" comment just emphasizes that. I don't see what is wrong
> with:
> 
> (define_insn "@pushfl<mode>2"
>   [(set (match_operand:W 0 "push_operand" "=<")
>     (unspec:W [(match_operand 1 "flags_reg_operand")]
>           UNSPEC_PUSHFL))]
>   "GET_MODE_CLASS (GET_MODE (operands[1])) == MODE_CC"
>   "pushf{<imodesuffix>}"
>   [(set_attr "type" "push")
>    (set_attr "mode" "<MODE>")])

What does it even mean?  What is a flags:CC?  You always always always
need to say what is *in* the flags, if you want to use it as input
(which is what unspec does).  CC is weird like this.  Most targets do
not have distinct physical flags for every condition, only a few
conditions are "alive" at any point in the program!

> it is just a push of the flags reg to the stack. If the push can't be
> described in this way, then it is the middle end at fault, we can't
> just change modes at will.

But that is not what this describes: it operates on the flags register
in some unspecified way, and pushes the result of *that* to the stack.

(Stack pointer modification is not described here btw, should it be?  Is
that magically implemented by the backend some way, via type=push
perhaps?)


Segher