Re: [PATCH] RTL: Bugfix for wrong code with v16hi compare & mask

[Hongtao Liu <crazylht@gmail.com>]

On Sun, Mar 26, 2023 at 3:01 AM Jeff Law via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
>
>
> On 3/24/23 08:11, pan2.li--- via Gcc-patches wrote:
> > From: Pan Li <pan2.li@intel.com>
> >
> > Fix the bug of the incorrect code generation for the
> > below code sample.
> >
> > typedef unsigned short __attribute__((__vector_size__ (32))) V;
> > typedef unsigned short u16;
> >
> > void
> > foo (V m, u16 *ret)
> > {
> >    V v = 6 > ((V) { 2049, 8 } & m);
> >    *ret = v[0]; // + a + b + c + d;
> > }
> >
> > Before this patch.
> > addi    sp,sp,-64
> > ld      a5,0(a0)
> > li      a4,528384
> > addi    a4,a4,-2047
> > and     a5,a5,a4
> > // slli    a5,a5,48 <- eliminated by mistake
> > // srli    a5,a5,48 <- eliminated by mistake
> > sltiu   a5,a5,6
> > negw    a5,a5
> > sh      a5,0(a1)
> >
> > After this patch.
> > addi    sp,sp,-64
> > ld      a5,0(a0)
> > li      a4,528384
> > addi    a4,a4,-2047
> > and     a5,a5,a4
> > slli    a5,a5,48
> > srli    a5,a5,48
> > sltiu   a5,a5,6
> > negw    a5,a5
> > sh      a5,0(a1)
> >
> > The simplify_comparation for the AND operation will
> > try to simplify below RTL code from:
> > (and:DI (subreg:DI (reg:HI 154) 0) (const_int 0x801))
> > to:
> > (subreg:DI (and (reg:HI 154) (const_int 0x801)) 0)
> These look equivalent to me -- assuming they're used as rvalues.
They're equivalent only when WORD_REGISTER_OPERATIONS, orelse the
upper bits of latter is UD, but the former is 0.

 (and (reg:HI 154) (const_int 0x801)) is simplified to (reg:HI 154)
since nonzero_bits (reg:154, HImode) is exactly same as 0x801.

These two optimizations are fine on their own, but if they are put
together, there are problems. The first optimization relies on the
WORD_REGISTER_OPERATIONS, but the second optimize the operation off
which make upper bits of (subreg:DI (reg:HI 154) 0) UD, but originally
it should be 0 after AND (const_int 0x801).
>
>
> >
> > If reg:HI 154 is 0x801 and reg:DI 154 is 0x80801, the RTL will
> > be simplified continuely to:
> That statement has no meaning.  Each pseudo has one and only one native
> mode and you can only refer to it in that mode.  ie reg:HI 154.  reg:DI
> 154 has no meaning.  You might say that (subreg:DI (reg:HI 154) 0) has
> the value 0x80801, but that's OK.  The subreg says those bits outside
> HImode simply don't matter -- you can not depend on them having any
> particular value.
>
> > (subreg:DI (reg:HI 154) 0)
> I think that's equivalent to (subreg:DI (and:HI (reg:HI 154) (const_int
> 0x801)) 0) when used as an rvalue.
>
> I suspect your problem is elsewhere.
>
> jeff
>


-- 
BR,
Hongtao