[PATCH] Optimize (ne:SI (subreg:QI (ashift:SI x 7) 0) 0) as (and:SI x 1).

[Michael Matz]

On Tue, 10 Oct 2023, Roger Sayle wrote:

> 
> This patch is the middle-end piece of an improvement to PRs 101955 and
> 106245, that adds a missing simplification to the RTL optimizers.
> This transformation is to simplify (char)(x << 7) != 0 as x & 1.

Random observation:

So, why restrict to shifts of LEN-1 and mask 1?  It's always the case that
(type-of-LEN)(x << S)) != 0  ===  (x & ((1 << (LEN - S)) - 1)) != 0.

E.g. (char)(x << 5) != 0  ===  (x & 7) != 0.

(Eventually the mask will be a constant that's too costly to compute if S 
is target-dependendly too small, but all else being equal avoiding shifts 
seems sensible)


Ciao,
Michael.