On AIX, the first page is readable (but not writable). Therefore, implicit null checks are not possible for loads.

That causes the IR test TestImplicitNullChecks.testLoad to fail. A zeroCheckP_reg_imm0 gets emitted which is not expected by the test:
Failed IR Rules (1) of Methods (1)
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1) Method "static java.lang.Object compiler.gcbarriers.TestImplicitNullChecks.testLoad(compiler.gcbarriers.TestImplicitNullChecks$Outer)" - [Failed IR rules: 1]:
   * @IR rule 1: "@compiler.lib.ir_framework.IR(phase={FINAL_CODE}, applyIfPlatformAnd={}, applyIfCPUFeatureOr={}, counts={"_#NULL_CHECK#_", "1"}, failOn={}, applyIfPlatform={}, applyIfPlatformOr={}, applyIfOr={"UseZGC", "true", "UseG1GC", "true"}, applyIfCPUFeatureAnd={}, applyIf={}, applyIfCPUFeature={}, applyIfAnd={}, applyIfNot={})"
     > Phase "Final Code":
       - counts: Graph contains wrong number of nodes:
         * Constraint 1: "(\\d+(\\s){2}(NullCheck.*)+(\\s){2}===.*)"
           - Failed comparison: [found] 0 = 1 [given]
           - No nodes matched!

The IR rule should be switched off for AIX.