Re: [PATCH] match.pd: Add bitwise and pattern [PR106243]

[Jeff Law <jeffreyalaw@gmail.com>]

On 8/3/2022 2:44 PM, Prathamesh Kulkarni via Gcc-patches wrote:
> On Thu, 4 Aug 2022 at 00:41, Sam Feifer via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>> This patch adds a new optimization to match.pd. The pattern, -x & 1,
>> now gets simplified to x & 1, reducing the number of instructions
>> produced.
> Hi Sam,
> No comments on patch, but wondering if we can similarly add another pattern to
> simplify abs(x) & 1 -> x & 1 ?
> Currently we don't appear to do it on GIMPLE:
>
> __attribute__((noipa))
> int f1 (int x)
> {
>    return __builtin_abs (x) & 1;
> }
>
> .optimized dump shows:
>    _1 = ABS_EXPR <x_2(D)>;
>    _3 = _1 & 1;
>    return _3;
>
> altho combine simplifies it to x & 1 on RTL, resulting in code-gen:
> f1:
>          and     w0, w0, 1
>          ret
Doesn't the abs(x) & mask simplify to x & mask for any mask where the 
sign bit of x is off -- including cases where mask isn't necessarily a 
compile-time constant, but we have range data which allows us to know 
that x's sign bit is off in mask.

jeff