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Re: CSE not combining equivalent expressions.
> First, it seems to me that in your case:
>
> (1) a = a | 1 /* a |= 1 */
> (2) a = a | 1 /* a |= 1 */
>
> the expressions "a | 1" in (1) and (2) are different as the "a"
> is not the same. So there is nothing to do for CSE.
It's not a CSE issue, but after (1), you know that the low-order bit of
"a" is a one, so that (2) is a no-op. Note that the similar
a &= ~1;
a &= ~1;
we do catch in combine.
It could also be caught by converting
a = ((a | 1) | 1);
into
a = (a | (1 | 1));