On 21 March 2012 21:48, Ian Lance Taylor <iant@xxxxxxxxxx> wrote: > Zhenqiang Chen <zhenqiang.chen@xxxxxxxxxx> writes: > >>>> 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 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. Here is the test case: gcc/testsuite/gcc.dg/pr30951.c. -Zhenqiang
