speedup decoder by 0.2%-1.2% #198

Job	Run time
Build and test cmake:gcc9	1s
Build and test cmake:clang12	1m 3s
Build and test cmake:clang12:asan	1m 3s
Build and test cmake:clang12:tsan	1m 16s
Build and test cmake:clang12:ubsan	1m 9s
Build and test cmake:qemu-arm-neon-gcc	1m 34s
Build and test cmake-osx:clang	1m 2s
Build and test cmake-osx:gcc	45s
Build and test cmake-win64:msvc2019-rel	1m 27s
Build and test cmake-win64:msvc2019-dbg	1m 14s
Build and test autotools:gcc10	1m 21s
Build and test autotools:clang12	1m 27s
Build and test fuzz:clang12	28s
Build and test python27:clang9	1s
Build and test python39:clang12	1m 31s
Build and test python39-win	51s
Build and test maven	55s
Build and test bazel:root	50s
Build and test bazel:go	52s
Build and test bazel:java	1m 37s
Build and test bazel:js	1m 37s
Build and test bazel:research	36s
Build and test bazel-osx:root	1m 34s
Build and test bazel-osx:go	1m 36s
Build and test bazel-osx:java	1m 34s
Build and test bazel-osx:js	1m 36s
Build and test bazel-osx:research	1m 34s
Build and test bazel-win:root	44s
Build and test bazel-win:java	1m 35s
Build and test bazel-win:research	45s
Build and test make	43s
Build and test make-osx	41s
Build and test make-win	1m 18s
	36m 20s

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