CODEWITHSUNDEEP
emscriptenwebassembly
MB-F
MB-F

Reputation: 23637

Purpose of invoke_* functions generated by emscripten

When compiling to WebAssembly, emscripten generates functions like invoke_vii. What is their purpose?

Details:

WebAssembly modules compiled from C with emcc -s WASM=1 -g expect various functions to be provided by the host environment.

...
(import "env" "abortOnCannotGrowMemory" (func $import$9 (result i32)))
(import "env" "invoke_vii" (func $import$10 (param i32 i32 i32)))
(import "env" "___syscall221" (func $import$11 (param i32 i32) (result i32)))
...

I am working on a WebAssembly interpreter written in C. So I have to deal with providing these functions, but I am dumbfounded by functions like invoke_vii, invoke_iiii, and the like.

The postfix is obviously related to the call signature. For example, vii means return void and take some ints. The name invoke suggests the function is supposed to invoke something.

I could find no information about these functions other than their implementation in the Java Script code generated by emscripten:

function invoke_ii(index,a1) {
  # removed error checking for brevity
  return Module["dynCall_ii"](index,a1);
}

dynCall_ii exists in the WebAssembly module.

It seems all the invoke_* functions do is to instruct the interpreter to run the corresponding dynCall_* function. What is the purpose of this indirection? Why doesn't the WebAssembly code call dynCall_ directly?

Upvotes: 3

Views: 1077

Answers (1)

sbc100
sbc100

Reputation: 3022

These are used for handling of setjmp/longjmp and also C++ exceptions. In these mechanisms JavaScript requires the ability to call back into a WebAssembly and invoke functions in the WebAssembly indirect function table.

If you build without exceptions and with -s SUPPORT_LONGJMP=0 (not the default) you should not see any these functions at all.

Upvotes: 4

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