Hi Richard and Segher
I don't know if I exactly understood your discussion.
If I misunderstood, please let me know.
I am trying to test these two cases.
Case 1. keep the TYPE attribute unchanged, add new attributes
It works well as below.
(define_attr "shift_imm_value"
"shift_imm_none,shift_imm_0,shift_imm_1to4,shift_imm_ge5" (const_string
"shift_imm_none"))
(define_insn "*add_<shift>_<mode>"
[(set (match_operand:GPI 0 "register_operand" "=r")
(plus:GPI (ASHIFT:GPI (match_operand:GPI 1 "register_operand" "r")
(match_operand:QI 2
"aarch64_shift_imm_<mode>" "n"))
(match_operand:GPI 3 "register_operand" "r")))]
""
"add\\t%<w>0, %<w>3, %<w>1, <shift> %2"
[(set_attr "type" "alu_shift_imm")
(set (attr "shift_imm_value")
(cond [(eq (symbol_ref "XINT(operands[2],0)") (const_int 0))
(const_string "shift_imm_0")
(and (ge (symbol_ref "XINT(operands[2],0)") (const_int 1))
(le (symbol_ref "XINT(operands[2],0)") (const_int 4))) (const_string
"shift_imm_1to4")]
(const_string "shift_imm_ge5")))]
)
(define_insn_reservation "a64fx_alu_shift1to4" 2
(and (eq_attr "tune" "a64fx")
(eq_attr "type" "alu_shift_imm")
(eq_attr "shift_imm_value" "shift_imm_1to4"))
"(a64fx_exa,a64fx_exa)|(a64fx_exb,a64fx_exb)")
Case2. expand the TYPE attribute
I replaced the alu_shift_imm to alu_shift_imm, alu_shift_imm1to4, and I got
the error message " bad number of entries in SET_ATTR_ALTERNATIVE, was 2
expected 1"
(define_attr "type"
...
alu_shift_imm,\
alu_shift_imm1to4,\
...
(define_insn "*add_<shift>_<mode>"
[(set (match_operand:GPI 0 "register_operand" "=r")
(plus:GPI (ASHIFT:GPI (match_operand:GPI 1 "register_operand" "r")
(match_operand:QI 2
"aarch64_shift_imm_<mode>" "n"))
(match_operand:GPI 3 "register_operand" "r")))]
""
"add\\t%<w>0, %<w>3, %<w>1, <shift> %2"
[(set_attr "type" "alu_shift_imm,alu_shift_imm1to4")]
)
It means that one "type" value should be matched by one operand constraint
pattern. So this will raise two problems.
1. One instruction should be classified to multiple patterns
- define multiple operand constraints, such as "(match_operand:QI 2
"aarch64_shift_imm_<mode>" "0, 1to4, ...")"
- or, use "cond" at set_attr just like Case1.
2. Whatever you do with the first problem, the type attribute cannot be
set with "alu_shift_imm"
and "alu_shift_imm1to4" at the same time by one operand constraint
pattern.
- If we cannot resolve it, the existing CPUs' descriptions need to
be changed. This is not what I expected.
- If we want to add new attribute to resolve this problem, why not
use the Case1 directly?
> It is very much not what I am saying. I *am* saying that if people trying to
> improve one CPU's modelling have to edit over 20 models for CPUs that they do
> not care about, mistakes happen.
That's what I'm worried about.
Regards
Qian
> -----Original Message-----
> From: Segher Boessenkool <seg...@kernel.crashing.org>
> Sent: Friday, September 11, 2020 9:59 PM
> To: Qian, Jianhua/钱 建华 <qia...@cn.fujitsu.com>; gcc@gcc.gnu.org;
> richard.sandif...@arm.com
> Subject: Re: A problem with one instruction multiple latencies and pipelines
>
> Hi!
>
> On Fri, Sep 11, 2020 at 08:44:54AM +0100, Richard Sandiford wrote:
> > Segher Boessenkool <seg...@kernel.crashing.org> writes:
> > > Consider cores that do not care about the "subtype" at all: when
> > > using just "type", all cores have to test for "foo,foo_subtype",
> > > while with a separate attribute they can just test for "foo".
> >
> > Right. But that was exactly the case I was considering with the sed
> > comment above :-)
> >
> > I don't see adding a new attribute value to a set_attr as extra
> > complication. It's just adding a value to a set. To me extra
> > complication is adding (ior …)s, (and …)s, etc.
>
> Combinatorial explosion is real.
>
> (ior)s etc. are easier to read, and importantly it is much more obvious if
> any case
> is missed with them.
>
> Here is the very first diff in d839f53b7dfd, where I started this for
> rs6000:
>
> (define_insn_reservation "ppc403-load" 2
> - (and (eq_attr "type"
> "load,load_ext,load_ext_u,load_ext_ux,load_ux,load_u,\
> - load_l,store_c,sync")
> + (and (eq_attr "type" "load,load_l,store_c,sync")
> (eq_attr "cpu" "ppc403,ppc405"))
> "iu_40x")
>
> We started off with 77 insn types. Now we have 63 (it was 57 before, we added
> some more).
>
> > > Yeas, exactly. And for rs6000 we *did* have many more types before,
> > > and very frequently one was missed (in a scheduling description usually).
> > > Especially common was when some new type attribute was added, not
> > > all existing cpu descriptions were updated correctly. Maybe this is
> > > the strongest argument "for" actually :-)
> >
> > The update shouldn't be done by hand.
>
> No, it very much had to, because the existing code missed many cases and was
> inconsistent in others.
>
> > What I'm really wary of with taking the line above is that it feeds
> > into the attitude that existing scheduling descriptions should become
> > fossilised at the point they're added: noone working on a different
> > core should change the declarations in the file later in case they
> > miss something. I don't think that's what you're saying, but it could
> > feed into that general attitude.
>
> It is very much not what I am saying. I *am* saying that if people trying to
> improve one CPU's modelling have to edit over 20 models for CPUs that they do
> not care about, mistakes happen. Such churn is conceptually wrong as well,
> of course (the new types are just the same as the old types on most older
> CPUs!)
>
> > > Yes.
> > >
> > > One example of what we do:
> > >
> > > ;; Is this instruction using operands[2] as shift amount, and can
> > > that be a ;; register?
> > > ;; This is used for shift insns.
> > > (define_attr "maybe_var_shift" "no,yes" (const_string "no"))
> > >
> > > ;; Is this instruction using a shift amount from a register?
> > > ;; This is used for shift insns.
> > > (define_attr "var_shift" "no,yes"
> > > (if_then_else (and (eq_attr "type" "shift")
> > > (eq_attr "maybe_var_shift" "yes"))
> > > (if_then_else (match_operand 2 "gpc_reg_operand")
> > > (const_string "yes")
> > > (const_string "no"))
> > > (const_string "no")))
> > >
> > > define_insns only use maybe_var_shift. Only the power6 and e*500*
> > > cpu descriptions ever look at var_shift. (Directly. There is some
> > > other scheduling code that looks at it too -- and all but the power6
> > > ones seem to be incorrect! That is all a direct translation of
> > > "type-only" code there was before...)
> >
> > Right. This is similar to the:
> >
> > (define_attr "shift_imm_type" "none,alu_shift_op2")
> >
> > thing I suggested upthread. I think it's better for the define_insn
> > attribute to specify the operand number directly, since there might
> > well be cases where the shift isn't operand 2.
>
> Not in this case; all such insns always have it as op 2 in the machine
> instruction, and RTL almost always uses the same order.
>
> > Anyway, I think we're in violent agreement on how this works, we're
> > just taking different conclusions from it.
>
> Yeah :-) And not even so different: I think you agree it is a trade-off, you
> just
> see it tilt more to one side, while I see it go to the other side often.
>
> We still have over fifty instruction types. We (hopefully :-) ) use the
> "extra
> attribute" method only where that helps more than it hurts. It is just
> another
> tool in the toolbox.
>
>
> Segher
>
