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8352426: RelocIterator should correctly handle nullptr address of relocation data

Reviewed-by: dlong, vlivanov, kvn

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At the time when this comment was updated there had been 122 new commits pushed to the master branch:

• 2483340: 8352579: Refactor CDS legacy optimization for lambda proxy classes
• 1397ee5: 8334322: Misleading values of keys in jpackage resource bundle
• 441bd12: 8352812: remove useless class and function parameter in SuspendThread impl
• e83cccf: 8352948: Remove leftover runtime_x86_32.cpp after 32-bit x86 removal
• 5672a93: 8348400: GenShen: assert(ShenandoahHeap::heap()->is_full_gc_in_progress() || (used_regions_size() <= _max_capacity)) failed: Cannot use more than capacity #
• c2a4fed: 8348907: Stress times out when is executed with ZGC
• 5392674: 8352766: Problemlist hotspot tier1 tests requiring tools that are not included in static JDK
• 1d205f5: 8352716: (tz) Update Timezone Data to 2025b
• a2a64da: 8352588: GenShen: Enabling JFR asserts when getting GCId
• 79bffe2: 8349361: C2: RShiftL should support all applicable transformations that RShiftI does
• ... and 112 more: https://git.openjdk.org/jdk/compare/b025d8c2e062210b6148da43f11517666b0b4932...master

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Webrevs

• 00: Full (7c1bd828)

This looks OK, but it is still a work-around because RelocIterator forces us to write loops like this:

   while (iter.next()) {
     iter.reloc()->some_operation();

instead of the more modern range-for loop:
https://en.cppreference.com/w/cpp/language/range-for

instead of the more modern range-for loop

@dean-long
Let's look at this code. Would it be better with a for loop?

    RelocIterator iter(nm, instruction_address(), next_instruction_address());
    while (iter.next()) {
      if (iter.type() == relocInfo::oop_type) {
        oop* oop_addr = iter.oop_reloc()->oop_addr();
        *oop_addr = cast_to_oop(x);
        break;
      } else if (iter.type() == relocInfo::metadata_type) {
        Metadata** metadata_addr = iter.metadata_reloc()->metadata_addr();
        *metadata_addr = (Metadata*)x;
        break;
      }
    }

This change addresses the bug reported in JDK-8352426 by ensuring that a nullptr relocation table does not result in undefined behavior.

RelocIterator isn’t a simple iterator - it encapsulates a variety of functions beyond just iteration. Reworking the API to support a range-based for loop would require a significant redesign of its interface and behavior. In my opinion, such a rework is beyond the scope of JDK-8352426. If the goal is to modernize the RelocIterator API, including support for range-based for loops, we should either explicitly reformulate JDK-8352426 to include that broader scope, or better yet, create a separate JBS issue to track that as an independent refactoring. What do you think?

I agree with @bulasevich that rewriting RelocIterator is out of scope of this RFE. And I am not sure that we should rewrite it at all.

I submitted out testing.

Good. Thanks for testing. And thanks all for the review!

/integrate

Going to push as commit 0bfa636.
Since your change was applied there have been 129 commits pushed to the master branch:

• b7ffd22: 8352980: Purge infrastructure for FP-to-bits interpreter intrinsics after 32-bit x86 removal
• 9a87e21: 8352800: [PPC] OpenJDK fails to build on PPC after JDK-8350106
• 4100dc9: 8350801: Add a code signing hook to the JDK build system
• 8a40498: 8352678: Opensource few JMenuItem tests
• 66b5dba: 8350988: Consolidate Identity of self-inverse operations
• 1007811: 8352897: RISC-V: Change default value for UseConservativeFence
• 7853415: 8352218: RISC-V: Zvfh requires RVV
• 2483340: 8352579: Refactor CDS legacy optimization for lambda proxy classes
• 1397ee5: 8334322: Misleading values of keys in jpackage resource bundle
• 441bd12: 8352812: remove useless class and function parameter in SuspendThread impl
• ... and 119 more: https://git.openjdk.org/jdk/compare/b025d8c2e062210b6148da43f11517666b0b4932...master

Your commit was automatically rebased without conflicts.

@bulasevich Pushed as commit 0bfa636.

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