Re: Combine patch ping

[Uros Bizjak <ubizjak@gmail.com>]

On Thu, Apr 11, 2024 at 4:02 PM Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> 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!

The commit message explains the problem, the solution is explained in
the last couple of lines. Please see [1] for a more thorough
explanation of the problem.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112560#c13

> 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!

From our previous discussion, we concluded that "use" means
cc-compared-to-0, but we also need a "copy" operation, to be able to
move CC reg around as a physical register (e.g. sahf, lahf, pushfl,
popfl instructions). This is a register that contains the state of the
CPU, described in [1] , not some RTL concept. The register is even
listed in i386.md:

(FLAGS_REG                   17)

with the "mode" that defines the value in the register more precisely.

[1] https://en.wikipedia.org/wiki/FLAGS_register

>
> > 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.

No, the "use" is defined as cc-compared-to-0. The above is a "copy"
operation, the register that holds the state of the CPU is pushed on
the stack (and can be later popped from the stack to reload the saved
state). The pushfl instruction does not use the register in the sense
that it examines its contents.

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

Please see gen_pushfl() in i386.cc that emits the pattern:

#(insn:TI 5 2 6 2 (set (mem:DI (pre_dec:DI (reg/f:DI 7 sp)) [0  S8 A8])
#        (unspec:DI [
#                (reg:CC 17 flags)
#            ] UNSPEC_PUSHFL)) "flags.c":3:10 70 {pushfldi2}
#     (expr_list:REG_DEAD (reg:CC 17 flags)
#        (nil)))
       pushfq          # 5     [c=4 l=1]  pushfldi2

Uros.