wasm2c: fix not all control paths return a value warning

[marler8997]

I got the following warning when compiling zig-wasm2c:

C:\git\zig\stage1\wasm.h(55) : warning C4715: 'WasmMut_toC': not all control paths return a value

This is because the C compiler doesn't understand that the panic function from panic.h is noreturn. I addressed this copying the body of the panic function into the call-site directly which is just a printf and abort. Now the compiler knows the codepath afterwards is unreachable because abort has the proper function decorators to mark it noreturn.

[alexrp]

I think adding a return NULL to WasmMut_toC, as in WasmValType_toC, is better than introducing compiler/target checks here.

[marler8997]

Well, adding "noreturn" to the panic declaration not only fixes this warning, but, also improves all the other places panic is called by letting the compiler know it doesn't return, which gives the compiler/optimize more accurate information. That seems like a win/win to me? I'm not seeing a downside?

[alexrp]

We have a policy of keeping the stage1/ C code as compiler/target-agnostic as possible. The only exception to this rule is zig.h which comes from lib/.

[marler8997]

I see. I think I can work with that. I just inlined the body of the custom panic function with the call to fprintf/abort. No more warning since the abort function has whatever noreturn decorator the toolchain requires.

[marler8997]

Ooo nevermind, I think I came up with something better. I changed panic to a macro so the call to abort is inlined at EVERY call site :)

[alexrp]

But you're still relying on the compiler being able to figure out that abort doesn't return. A dead return is nice because it's fully portable.

[marler8997]

We're not relying on the compiler to know that abort is noreturn for the program to behave correctly. It's an extra piece of information that the compiler/optimizer can use to eliminate dead branches (and erroneous warnings).

[jacobly0]

An optimizer that understands noreturn already understands that a function that calls noreturn functions on every path is also noreturn. The attribute exists so that the optimizer knows whether external functions that it can't analyze are noreturn. Also this program compiles and runs in 0m0.725s, it does not need optimizations at this time.