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

[Ian Lance Taylor <iant at google dot com>]

Zhenqiang Chen <zhenqiang.chen@linaro.org> writes:

>>> Which pass optimizes if (x != x + 10) to if (1)? Why is it not applied to -Os?
>>
>> It's not a separate pass. ÂIt's code in fold-const.c.
>>
>> I don't know why -Os makes a difference here. ÂIt does seem odd. ÂI
>> 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 for looking into this.  The test was added here:

http://gcc.gnu.org/ml/gcc-patches/2003-10/msg01208.html

When I tried your test case, I agree that the test was not optimized out
initially, but it was optimized out in the generated code.  Do you have
a test case in which the final generated code is worse?  If you do,
please open a bug report.

Ian