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

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi,

Xi Ruoyao <xry111@xry111.site> writes:

> On Tue, 2023-06-13 at 20:23 +0800, Jiufu Guo via Gcc-patches wrote:
>
>> Compare with previous version, this addes ChangeLog and removes
>> const_anchor parts.
>> https://gcc.gnu.org/pipermail/gcc-patches/2023-June/621356.html.
>
> [Off topic]
>
> const_anchor is just broken now.  See
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104843 and the thread
> beginning at
> https://gcc.gnu.org/pipermail/gcc-patches/2022-March/591470.html.  If
> you want to use it for rs6000 I guess you need to fix it first...

Thanks so much for pointing out this.  It seems about supporting
negative value, right?

As you say: for 1. "g(0x8123ffff, 0x81240001)", it would be fine.

The generated insns are:
(insn 5 2 6 2 (set (reg:DI 117)
        (const_int -2128347135 [0xffffffff81240001])) "negative.c":5:3 681 {*movdi_internal64}
     (nil))
(insn 6 5 7 2 (set (reg:DI 118)
        (plus:DI (reg:DI 117)
            (const_int -2 [0xfffffffffffffffe]))) "negative.c":5:3 66 {*adddi3}
     (expr_list:REG_EQUAL (const_int -2128347137 [0xffffffff8123ffff])
        (nil)))

While for 2. "g (0x7fffffff, 0x80000001)", the generated rtl insns:
(insn 5 2 6 2 (set (reg:DI 117)
        (const_int -2147483647 [0xffffffff80000001])) "negative.c":5:3 681 {*movdi_internal64}
     (nil))
(insn 7 6 8 2 (set (reg:DI 3 3)
        (const_int 2147483647 [0x7fffffff])) "negative.c":5:3 681 {*movdi_internal64}
     (nil))

The current const_anchor does not generate sth like: "r3 = r117 - 2"
But I would lean to say it is the limitation of current implementation:
"0xffffffff80000001" and "0x7fffffff" hit different anchors(even these
two values are 'close' on some aspect.)

BR,
Jeff (Jiufu Guo)

>
> To me const_anchor needs a complete rework but I don't want to spend my
> time on it.