RFR: 8265767: compiler/eliminateAutobox/TestIntBoxing.java crashes on arm32 after 8264649 in debug VMs

[Vladimir Kozlov]

On Fri, 30 Apr 2021 14:17:23 GMT, Hui Shi <hshi at openjdk.org> wrote:

> This patch fix failure exposed by JDK-8264649.
> 
> compiler/eliminateAutobox/TestIntBoxing.java crashes on arm32 in Compile::check_no_dead_use assertion.
> In LoadNode::eliminate_autobox, early "result" is dead after line 1450 but not added into PhaseGVN worklist for optimization.
> Its out_cnt is 0. If it isn't removed, will trigger assertion in Compile::check_no_dead_use.
> 
> 
> 1443             } else if (result->is_Add() && result->in(2)->is_Con() &&
> 1444                        result->in(1)->Opcode() == Op_LShiftX &&
> 1445                        result->in(1)->in(2) == phase->intcon(shift)) {
> 1446               // We can't do general optimization: ((X<<Z) + Y) >> Z ==> X + (Y>>Z)
> 1447               // but for boxing cache access we know that X<<Z will not overflow
> 1448               // (there is range check) so we do this optimizatrion by hand here.
> 1449               Node* add_con = new RShiftXNode(result->in(2), phase->intcon(shift));
>                        --- result before is  dead and might not removed
> 1450               result = new AddXNode(result->in(1)->in(1), phase->transform(add_con)); 
> 1451             } else 
> 
> 
> Detail analysis is in https://bugs.openjdk.java.net/browse/JDK-8265767 
> 
> @mychris  I have verified compiler/eliminateAutobox/TestIntBoxing.java on qemu, it failed with same assertion and now passes with this fix. Would you please help verify it on arm32 machine?
> 
> Testing:
> - Passed Tier1-3 on Linux x86_64, release and fastdebug build,  default option and -XX:-TieredCompilation.
> - compiler/eliminateAutobox/TestIntBoxing.java on arm32 release/fastdebug/slowdebug

The fix is reasonable.
Add comment `// remove dead node later`
Pass `PhaseIterGVN* igvn` instead of `eliminate_autobox(PhaseGVN* phase)` to simplify code. Note, `can_reshape == true` only for PhaseIterGVN.

-------------

Changes requested by kvn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3818