RFR: 8261137: Optimization of Box nodes in uncommon_trap

[Eric Liu]

On Thu, 4 Feb 2021 08:43:35 GMT, Wang Huang <whuang at openjdk.org> wrote:

> JDK-8075052 has  removed useless autobox. However, in some cases, the box is still saved. For instance:
> @Benchmark
> public void testMethod(Blackhole bh) {
>   int sum = 0;
>   for (int i = 0; i < data.length; i++) {
>       Integer ii = Integer.valueOf(data[i]);
>       if (i < data.length) {
>           sum += ii.intValue();
>       }
>   }
>   bh.consume(sum);
> }
> Although the variable ii is only used at ii.intValue(), it cannot be eliminated as a result of being used by a hidden uncommon_trap.
> The uncommon_trap is generated by the optimized "if", because its condition is always true.
> 
> We can postpone box in uncommon_trap in this situation. We treat box as a scalarized object by adding a SafePointScalarObjectNode in the uncommon_trap node,
> and deleting the use of box:
> 
> There is no additional fail/error(s) of jtreg after this patch.

I was wandering if we can remove the useless `if` as it's always true in this case. Do you know why this kind of `if` haven't been eliminated by GVN phase?

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PR: https://git.openjdk.java.net/jdk/pull/2401