Created attachment 18808 [details]
a small failing example

When -enable-emscripten-cxx-exceptions is used, the WebAssemblyLowerEmscriptenEHSjLj pass lowers invoke instructions to calls to calls to @__invoke_SIG functions that take the originally invoked function as a first argument and have the same return type. If that return type is a struct, the initial lowering step creates an sret parameter in the first argument position, bumping the function pointer to the second argument position, which causes a crash when the sret pointer is used as a function pointer at run time.

To reproduce:

llc exceptions.ll -enable-emscripten-cxx-exceptions -debug

Observe that 

  %ign = invoke { i32, i8* } @bar(i32 3)
            to label %invoke.cont unwind label %lpad

is lowered to

    ...
    %ign = call { i32, i8* } @"__invoke_{i32.i8*}_i32"({ i32, i8* } (i32)* @bar, i32 3)
    ...

and later lowered to

    ...
    CALL_VOID <ga:@"__invoke_{i32.i8*}_i32">, <fi#0>, %vreg2<kill>, %vreg1<kill>, %ARGUMENTS<imp-def,dead>, %SP32<imp-use>, %SP64<imp-use>; I32:%vreg2,%vreg1
    ...

which has the sret pointer, not the function pointer, as the first argument.