Build the cloop as follows: $ ./Tools/Scripts/build-jsc --release --cloop Run the JSC tests as follows: $ ./Tools/Scripts/run-javascriptcore-tests --release --cloop --no-build --no-jit-stress-tests You'll see these 26 failures: ** The following JSC stress test failures have been introduced: ChakraCore.yaml/ChakraCore/test/TaggedIntegers/predecrement.js.default ChakraCore.yaml/ChakraCore/test/TaggedIntegers/preincrement.js.default es6.yaml/es6/typed_arrays_Int16Array.js.default es6.yaml/es6/typed_arrays_Int8Array.js.default jsc-layout-tests.yaml/js/script-tests/dfg-int16array.js.layout jsc-layout-tests.yaml/js/script-tests/dfg-int8array.js.layout jsc-layout-tests.yaml/js/script-tests/regress-139548.js.layout jsc-layout-tests.yaml/js/script-tests/regress-150434.js.layout stress/arith-abs-on-various-types.js.default stress/arith-abs-to-arith-negate-range-optimizaton.js.default stress/arith-abs-with-bitwise-or-zero.js.default stress/arith-ceil-on-various-types.js.default stress/arith-clz32-on-various-types.js.default stress/arith-cos-on-various-types.js.default stress/arith-floor-on-various-types.js.default stress/arith-fround-on-various-types.js.default stress/arith-log-on-various-types.js.default stress/arith-negate-on-various-types.js.default stress/arith-round-on-various-types.js.default stress/arith-sin-on-various-types.js.default stress/arith-sqrt-on-various-types.js.default stress/arith-tan-on-various-types.js.default stress/arith-trunc-on-various-types.js.default stress/check-string-ident.js.default stress/compare-strict-eq-on-various-types.js.default stress/redundant-add-overflow-checks.js.default Results for JSC stress tests: 26 failures found.