Which pass optimizes if (x != x + 10) to if (1)?

[Zhenqiang Chen]

>> Which pass optimizes if (x != x + 10) to if (1)? Why is it not applied to -Os?
>
> It's not a separate pass.  It's code in fold-const.c.
>
> I don't know why -Os makes a difference here.  It does seem odd.  I
> encourage you to investigate what is happening.
>

Thank you for the comment. I found the root cause. In function
tree_swap_operands_p (fold-const.c), there is a check

if (optimize_function_for_size_p (cfun))
    return 0;

which blocks to swap (x != x + 10) to (x + 10 != x). And the following
optimization can only handle (x + 10 != x).

In most cases, constant-folding will benefit for code size. Any reason
to add this check?

Thanks!
-Zhenqiang