On Thu, 21 Jan 2021 16:54:36 GMT, Paul Sandoz <psan...@openjdk.org> wrote:
> The intrinsic enables C2 to more reliably elide bounds checks. I don't know > the exact details but at a high-level it transforms signed values into > unsigned values (and therefore the signed comparisons become unsigned > comparisons), which helps C2 remove checks when there is a dominating check > of, for example, an upper loop bound. > > You say "the intrinsified Objects.checkIndex seems to generate more code than > inlined if-statements". Can you present some examples of Java code and the > corresponding C2 generated assembler where this happens? Hi @PaulSandoz , I agree with you that let's keep the code as it is for the sake of performance. I spent some time looking into the assembly code generated by Objects.checkIndex and inlined if-statements. Here are the test program [1], running script [2] and diff [3]. - For testSimple [4] that I checked last week, inlined if-statements [5] is better than Objects.checkIndex [6]. - However, for testLoop [7], Objects.checkIndex [8] is better than inlined if-statements [9]. (I'm sorry I didn't check loop methods last week.) AFAICS, the inlined if-statements will generate two more instructions [10] to check wether idx >= 0 for each loop iteration. It seems that the intrinsified Objects.checkIndex will be able to optimize out the lower bound check for loops. So I also agree with you that an intrinsified method seems the right choice. Thanks. Best regards, Jie [1] https://github.com/DamonFool/experiment/blob/main/JDK-8259925/Bar.java [2] https://github.com/DamonFool/experiment/blob/main/JDK-8259925/1.sh [3] https://github.com/DamonFool/experiment/blob/main/JDK-8259925/1.diff [4] https://github.com/DamonFool/experiment/blob/main/JDK-8259925/Bar.java#L10 [5] https://github.com/DamonFool/experiment/blob/main/JDK-8259925/3-testSimple.log [6] https://github.com/DamonFool/experiment/blob/main/JDK-8259925/2-testSimple.log [7] https://github.com/DamonFool/experiment/blob/main/JDK-8259925/Bar.java#L15 [8] https://github.com/DamonFool/experiment/blob/main/JDK-8259925/2-testLoop.log [9] https://github.com/DamonFool/experiment/blob/main/JDK-8259925/3-testLoop.log [10] https://github.com/DamonFool/experiment/blob/main/JDK-8259925/3-testLoop.log#L135 ------------- PR: https://git.openjdk.java.net/jdk/pull/2127
