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

[Xi Ruoyao <xry111@xry111.site>]

On Wed, 2023-06-14 at 09:55 +0800, Jiufu Guo wrote:
> 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.)

The generic issue here is to fix (not "papering over") the signed
overflow, we need to perform the addition in a target machine mode.  We
may always use Pmode (IIRC const_anchor was introduced for optimizing
some constant addresses), but can we do better?

Should we try addition in both DImode and SImode for a 64-bit capable
machine?

Or should we even try more operations than addition (for eg bit
operations like xor or shift)?  Doing so will need to create a new
target hook for const anchoring, this is the "complete rework" I meant.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University