JSPI asynchronous calls into WASM are much slower than Asyncify

I think this is more likely to be a Chromium bug and I have [filed a bug](https://bugs.chromium.org/p/chromium/issues/detail?id=1518374&q=&can=4) there, but I don't understand the details well enough to know for sure.

Asynchronous calls from JavaScript into WASM are 2+ orders of magnitude slower with JSPI over Asyncify. I'm using Chrome 122.0.6248.0 (Official Build) canary (arm64) on an M2 Mac with chrome://flags/#enable-experimental-webassembly-stack-switching enabled.

I have a [test page here](https://rhashimoto.github.io/browser-test-cases/jspi/) ([source](https://github.com/rhashimoto/browser-test-cases/tree/master/jspi)) that times 10,000 calls to a C function that calls an asynchronous JavaScript function (use Chrome Canary as some older versions just crash the page). It loops over the calls once from C and once from Javascript, for both Asyncify and JavaScript builds. The output of the page looks like this:

```
asyncify 10000 iterations in C 0.01 seconds
asyncify 10000 iterations in JS 0.01 seconds
jspi 10000 iterations in C 0.003 seconds
jspi 10000 iterations in JS 3.554 seconds
```

This shows that with Asyncify, calling the JS function from C and calling it from JavaScript to C to JavaScript, is the same 0.01 seconds. JSPI is faster than Asyncify at calling the JS function from C, but is 350 times slower at calling from JS.

Emscripten output

**Version of emscripten/emsdk:**
$ emcc -v
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.47 (431685f05c67f0424c11473cc16798b9587bb536)
clang version 18.0.0 (https://github.com/llvm/llvm-project 21030b9ab4487d845e29792063f5666d8c4b8e09)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/dev/emsdk/upstream/bin

**Full link command and output with `-v` appended:**
$ make (both Asyncify and JSPI builds shown here)
emcc -O2 -I. -s ALLOW_MEMORY_GROWTH=1 -s ASYNCIFY_EXPORTS='["cfunc", "cloop"]' -s ASYNCIFY_IMPORTS='["jfunc"]' -s ENVIRONMENT="web,worker" -s EXPORTED_RUNTIME_METHODS='["ccall", "cwrap"]' -s INVOKE_RUN -s WASM=1 -v -s ASYNCIFY -o cfunc-asyncify.mjs cfunc.c --js-library libfoo.js
"/home/dev/emsdk/upstream/bin/clang" --version
"/home/dev/emsdk/upstream/bin/clang" -target wasm32-unknown-emscripten -fignore-exceptions -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -DEMSCRIPTEN -Werror=implicit-function-declaration --sysroot=/var/tmp/emsdk/upstream/emscripten/cache/sysroot -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -O2 -I. -v cfunc.c -c -o /tmp/emscripten_temp_8v67r6_8/cfunc_0.o
clang version 18.0.0 (https://github.com/llvm/llvm-project 21030b9ab4487d845e29792063f5666d8c4b8e09)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/dev/emsdk/upstream/bin
(in-process)
"/var/tmp/emsdk/upstream/bin/clang-18" -cc1 -triple wasm32-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name cfunc.c -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home/dev/browser-test-cases/jspi -v -fcoverage-compilation-dir=/home/dev/browser-test-cases/jspi -resource-dir /var/tmp/emsdk/upstream/lib/clang/18 -D EMSCRIPTEN -I . -isysroot /var/tmp/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /var/tmp/emsdk/upstream/lib/clang/18/include -internal-isystem /var/tmp/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /var/tmp/emsdk/upstream/emscripten/cache/sysroot/include -O2 -Werror=implicit-function-declaration -ferror-limit 19 -fvisibility=default -fgnuc-version=4.2.1 -fignore-exceptions -fcolor-diagnostics -vectorize-loops -vectorize-slp -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /tmp/emscripten_temp_8v67r6_8/cfunc_0.o -x c cfunc.c
clang -cc1 version 18.0.0 based upon LLVM 18.0.0git default target aarch64-unknown-linux-gnu
ignoring nonexistent directory "/var/tmp/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
.
/var/tmp/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
/var/tmp/emsdk/upstream/emscripten/cache/sysroot/include/compat
/var/tmp/emsdk/upstream/lib/clang/18/include
/var/tmp/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
"/home/dev/emsdk/upstream/bin/wasm-ld" -o cfunc-asyncify.wasm /tmp/emscripten_temp_8v67r6_8/cfunc_0.o -L/var/tmp/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten -lGL -lal -lhtml5 -lstubs -lnoexit -lc -ldlmalloc -lcompiler_rt -lc++-noexcept -lc++abi-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr /tmp/tmpnogg_6b0libemscripten_js_symbols.so --export-if-defined=main --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=__main_argc_argv --export=stackSave --export=stackRestore --export=stackAlloc --export=__errno_location --export=__get_temp_ret --export=__set_temp_ret --export=malloc --export=free --export=__wasm_call_ctors --export-table -z stack-size=65536 --initial-memory=16777216 --max-memory=2147483648 --no-entry --table-base=1 --global-base=1024
"/home/dev/emsdk/upstream/bin/llvm-objcopy" cfunc-asyncify.wasm cfunc-asyncify.wasm --remove-section=.debug* --remove-section=producers
"/home/dev/emsdk/upstream/bin/wasm-emscripten-finalize" --pass-arg=legalize-js-interface-exported-helpers cfunc-asyncify.wasm -o cfunc-asyncify.wasm --detect-features
"/home/dev/emsdk/node/16.20.0_64bit/bin/node" /var/tmp/emsdk/upstream/emscripten/src/compiler.js /tmp/tmp_6pmyt7_.json
"/home/dev/emsdk/upstream/bin/wasm-opt" --post-emscripten -O2 --low-memory-unused --asyncify --pass-arg=asyncify-imports@env.jfunc,env.invoke_*,env.__asyncjs__*,*.fd_sync,*.emscripten_promise_await,*.emscripten_idb_load,*.emscripten_idb_store,*.emscripten_idb_delete,*.emscripten_idb_exists,*.emscripten_idb_clear,*.emscripten_idb_load_blob,*.emscripten_idb_store_blob,*.emscripten_sleep,*.emscripten_wget_data,*.emscripten_scan_registers,*.emscripten_lazy_load_code,*._load_secondary_module,*.emscripten_fiber_swap,*.SDL_Delay,*.jfunc --zero-filled-memory --pass-arg=directize-initial-contents-immutable cfunc-asyncify.wasm -o cfunc-asyncify.wasm --mvp-features --enable-mutable-globals --enable-sign-ext
"/home/dev/emsdk/node/16.20.0_64bit/bin/node" /var/tmp/emsdk/upstream/emscripten/tools/acorn-optimizer.js /tmp/emscripten_temp_8v67r6_8/cfunc-asyncify.js JSDCE minifyWhitespace --exportES6 -o /tmp/emscripten_temp_8v67r6_8/cfunc-asyncify.jso1.js
emcc -O2 -I. -s ALLOW_MEMORY_GROWTH=1 -s ASYNCIFY_EXPORTS='["cfunc", "cloop"]' -s ASYNCIFY_IMPORTS='["jfunc"]' -s ENVIRONMENT="web,worker" -s EXPORTED_RUNTIME_METHODS='["ccall", "cwrap"]' -s INVOKE_RUN -s WASM=1 -v -s ASYNCIFY=2 -o cfunc-jspi.mjs cfunc.c --js-library libfoo.js
"/home/dev/emsdk/upstream/bin/clang" --version
"/home/dev/emsdk/upstream/bin/clang" -target wasm32-unknown-emscripten -fignore-exceptions -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -DEMSCRIPTEN -Werror=implicit-function-declaration --sysroot=/var/tmp/emsdk/upstream/emscripten/cache/sysroot -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -O2 -I. -v cfunc.c -c -o /tmp/emscripten_temp_jls2o26n/cfunc_0.o
clang version 18.0.0 (https://github.com/llvm/llvm-project 21030b9ab4487d845e29792063f5666d8c4b8e09)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /home/dev/emsdk/upstream/bin
(in-process)
"/var/tmp/emsdk/upstream/bin/clang-18" -cc1 -triple wasm32-unknown-emscripten -emit-obj -disable-free -clear-ast-before-backend -disable-llvm-verifier -discard-value-names -main-file-name cfunc.c -mrelocation-model static -mframe-pointer=none -ffp-contract=on -fno-rounding-math -mconstructor-aliases -target-cpu generic -debugger-tuning=gdb -fdebug-compilation-dir=/home/dev/browser-test-cases/jspi -v -fcoverage-compilation-dir=/home/dev/browser-test-cases/jspi -resource-dir /var/tmp/emsdk/upstream/lib/clang/18 -D EMSCRIPTEN -I . -isysroot /var/tmp/emsdk/upstream/emscripten/cache/sysroot -internal-isystem /var/tmp/emsdk/upstream/lib/clang/18/include -internal-isystem /var/tmp/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten -internal-isystem /var/tmp/emsdk/upstream/emscripten/cache/sysroot/include -O2 -Werror=implicit-function-declaration -ferror-limit 19 -fvisibility=default -fgnuc-version=4.2.1 -fignore-exceptions -fcolor-diagnostics -vectorize-loops -vectorize-slp -iwithsysroot/include/fakesdl -iwithsysroot/include/compat -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr -o /tmp/emscripten_temp_jls2o26n/cfunc_0.o -x c cfunc.c
clang -cc1 version 18.0.0 based upon LLVM 18.0.0git default target aarch64-unknown-linux-gnu
ignoring nonexistent directory "/var/tmp/emsdk/upstream/emscripten/cache/sysroot/include/wasm32-emscripten"
#include "..." search starts here:
#include <...> search starts here:
.
/var/tmp/emsdk/upstream/emscripten/cache/sysroot/include/fakesdl
/var/tmp/emsdk/upstream/emscripten/cache/sysroot/include/compat
/var/tmp/emsdk/upstream/lib/clang/18/include
/var/tmp/emsdk/upstream/emscripten/cache/sysroot/include
End of search list.
emcc: warning: -sASYNCIFY=2 (JSPI) is still experimental [-Wexperimental]
"/home/dev/emsdk/upstream/bin/wasm-ld" -o cfunc-jspi.wasm /tmp/emscripten_temp_jls2o26n/cfunc_0.o -L/var/tmp/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten -lGL -lal -lhtml5 -lstubs -lnoexit -lc -ldlmalloc -lcompiler_rt -lc++-noexcept -lc++abi-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr /tmp/tmpddcpx76vlibemscripten_js_symbols.so --export-if-defined=main --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=__main_argc_argv --export=stackSave --export=stackRestore --export=stackAlloc --export=__errno_location --export=__get_temp_ret --export=__set_temp_ret --export=malloc --export=free --export=__wasm_call_ctors --export-table -z stack-size=65536 --initial-memory=16777216 --max-memory=2147483648 --no-entry --table-base=1 --global-base=1024
"/home/dev/emsdk/upstream/bin/llvm-objcopy" cfunc-jspi.wasm cfunc-jspi.wasm --remove-section=.debug* --remove-section=producers
"/home/dev/emsdk/upstream/bin/wasm-emscripten-finalize" --dyncalls-i64 --pass-arg=legalize-js-interface-exported-helpers cfunc-jspi.wasm -o cfunc-jspi.wasm --detect-features
"/home/dev/emsdk/node/16.20.0_64bit/bin/node" /var/tmp/emsdk/upstream/emscripten/src/compiler.js /tmp/tmprhq8bvm5.json
"/home/dev/emsdk/upstream/bin/wasm-opt" --post-emscripten -O2 --low-memory-unused --jspi --pass-arg=jspi-imports@env.jfunc,env.invoke_*,env.__asyncjs__*,*.fd_sync,*.emscripten_promise_await,*.emscripten_idb_load,*.emscripten_idb_store,*.emscripten_idb_delete,*.emscripten_idb_exists,*.emscripten_idb_clear,*.emscripten_idb_load_blob,*.emscripten_idb_store_blob,*.emscripten_sleep,*.emscripten_wget_data,*.emscripten_scan_registers,*.emscripten_lazy_load_code,*._load_secondary_module,*.emscripten_fiber_swap,*.SDL_Delay,*.jfunc --pass-arg=jspi-exports@cfunc,cloop,main,__main_argc_argv,_ZN10emscripten8internal5async* --zero-filled-memory --pass-arg=directize-initial-contents-immutable cfunc-jspi.wasm -o cfunc-jspi.wasm --mvp-features --enable-mutable-globals --enable-sign-ext --enable-reference-types
"/home/dev/emsdk/node/16.20.0_64bit/bin/node" /var/tmp/emsdk/upstream/emscripten/tools/acorn-optimizer.js /tmp/emscripten_temp_jls2o26n/cfunc-jspi.js JSDCE minifyWhitespace --exportES6 -o /tmp/emscripten_temp_jls2o26n/cfunc-jspi.jso1.js