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8361140: Missing OptimizePtrCompare check in ConnectionGraph::reduce_phi_on_cmp #26125

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8361140: Missing OptimizePtrCompare check in ConnectionGraph::reduce_phi_on_cmp #26125

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f118400
8361140: Missing OptimizePtrCompare check in ConnectionGraph::reduce_…
hgqxjj Jul 4, 2025
787eb0c
Merge remote-tracking branch 'upstream/master' into 8361140
hgqxjj Jul 10, 2025
2feca6a
update modification and add regression test
hgqxjj Jul 10, 2025
6dda1e9
Merge remote-tracking branch 'upstream/master' into 8361140
hgqxjj Jul 10, 2025
fd6f90f
update regression test
hgqxjj Jul 10, 2025
4ef7ed3
Merge remote-tracking branch 'upstream/master' into 8361140
hgqxjj Jul 11, 2025
0e9aa95
Remove the unused variable
hgqxjj Jul 11, 2025
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3 changes: 3 additions & 0 deletions src/hotspot/share/opto/escape.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -976,6 +976,9 @@ void ConnectionGraph::reduce_phi_on_castpp_field_load(Node* curr_castpp, Growabl
//
//
void ConnectionGraph::reduce_phi_on_cmp(Node* cmp) {
if (!OptimizePtrCompare) {
return;
}
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Thanks for working on this! IIUC, having the bailout here will fail to reduce the phi which could be unexpected. Shouldn't we just return UNKNOWN from within ConnectionGraph::optimize_ptr_compare() when we run without OptimizePtrCompare?

On a separate note, can you also add a regression test? Maybe you can also just add a run with -XX:-OptimizePtrCompare - maybe together with -XX:+VerifyReduceAllocationMerges for more verification - to compiler/c2/irTests/scalarReplacement/AllocationMergesTests.java.

@JohnTortugo you might also want to have a look at this.

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Thanks a lot for your suggestion!
I took a closer look at the code, and I now fully agree that your approach is the better one. Returning UNKNOWN from optimize_ptr_compare() when OptimizePtrCompare is disabled makes the behavior more consistent and avoids skipping reduce_phi_on_cmp() entirely, which could lead to unexpected results or missed optimization opportunities. I appreciate your feedback and will move forward with this approach. Thanks again!

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Thanks for the ping @chhagedorn and I fully agree with your comment. Actually, that's the correct way to do this.
Thank you for fixing this @hgqxjj .

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hi @chhagedorn , I already update PR and add regression test. Please take another look when you have time .
Thanks a lot.

Node* ophi = cmp->in(1)->is_Con() ? cmp->in(2) : cmp->in(1);
assert(ophi->is_Phi(), "Expected this to be a Phi node.");

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