Follow-up to https://bugs.webkit.org/show_bug.cgi?id=160922 The above patch added value profiling to the two opcodes, they got faster, but the new tests have odd behavior: $ ./Tools/Scripts/run-jsc-benchmarks old:./before/bin/jsc new:./current/bin/jsc --benchmarks "super-get-by-" --microbenchmarks --outer 400 old new super-get-by-id-with-this-monomorphic 29.3852+-0.2030 ^ 27.1366+-0.1896 ^ definitely 1.0829x faster super-get-by-id-with-this-polymorphic 27.0021+-0.2115 ^ 25.6205+-0.1965 ^ definitely 1.0539x faster super-get-by-val-with-this-monomorphic 29.8851+-0.1339 ^ 28.9029+-0.1612 ^ definitely 1.0340x faster super-get-by-val-with-this-polymorphic 27.6204+-0.1616 ? 27.8924+-0.2388 ? <geometric> 28.4117+-0.0838 ^ 27.3187+-0.0971 ^ definitely 1.0400x faster I'd expect monomorphic to be faster than polymorphic, for both test, before as well as after the patch. The patch isn't a regression (poly was faster than mono before it), but we should investigate why this is happening. The difference seems to be that there were many more exits in mono than poly as shown by the profile from that patch: https://bugs.webkit.org/attachment.cgi?id=287636