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

[Jeff Law <jeffreyalaw@gmail.com>]

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.


> 
> 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