Both the concurrent GC and the concurrent JIT rely on concurrency support in fundamental JSC runtime components like JSValue. So, the thing that guards it should be a "feature" called CONCURRENT_JS not CONCURRENT_JIT.
Created attachment 294879 [details] the patch
Attachment 294879 [details] did not pass style-queue: ERROR: Source/JavaScriptCore/bytecode/LazyOperandValueProfile.h:158: The parameter name "key" adds no information, so it should be removed. [readability/parameter_name] [5] ERROR: Source/JavaScriptCore/bytecode/LazyOperandValueProfile.h:172: The parameter name "holder" adds no information, so it should be removed. [readability/parameter_name] [5] ERROR: Source/JavaScriptCore/bytecode/LazyOperandValueProfile.h:178: The parameter name "key" adds no information, so it should be removed. [readability/parameter_name] [5] ERROR: Source/JavaScriptCore/runtime/JSObject.h:761: The parameter name "structure" adds no information, so it should be removed. [readability/parameter_name] [5] Total errors found: 4 in 74 files If any of these errors are false positives, please file a bug against check-webkit-style.
Comment on attachment 294879 [details] the patch rs=me
Landed in https://trac.webkit.org/changeset/208761
Fixed a regression in https://trac.webkit.org/changeset/208777