Novo wrote on Tue, 09 February 2021 00:00

mirek wrote on Mon, 08 February 2021 14:49

Novo wrote on Mon, 08 February 2021 19:35

AFAIK, WebAssembly requires all data to be aligned by at least the word size. This can be a problem ...

Even char? Like you cannot do char *s = ...; *s++?

Probably, you can do that. Last time I had to deal with compiling C++ to Web was, probably, ~5 years ago. And that was Emscripten.
The only thing I remember for sure I had to properly align unaligned data structures, otherwise we were getting runtime exceptions. Part of the code had to be disabled because of problems with data alignment.

This was ~FIVE years ago. Life has changed since that time. And we have WebAssembly in addition to Emscripten now.

Another thing: compilation with Emscripten wasn't a problem at all.

P.S. I personally would prefer to use Turtle because of security reasons. WebAssembly can be easily decompiled, and in case of Turtle the only thing people can steal is a picture in a Web-browser.

Hi,

Thanks for your response. Yes, I agree very much: Turtle is and remains the solution for code to be kept safe. However, to move the computational load to the client side requires different approach. (Maybe even a hybrid approach, where GUI runs on the client side and the critical algorithms on the server side out of reach.)

Anyway, I tried out the structure alignment and it seems to work just fine like in MSC/CLANG/GCC when using #pragma pack(push,1):

#include <stdio.h>

#pragma pack(push,1)

typedef struct{
        char    a;
        short   b;
        int     c;
}struct_t;

#pragma pack(pop)

int main(){
        char buffer[]={0,1,2,3,4,5,6,7,8,9,10,11};
        struct_t &s=*(struct_t*)buffer;
        printf("a = %XH\n",s.a);
        printf("b = %XH\n",s.b);
        printf("c = %XH\n",s.c);
        return 0;
}

The result is:

tom@TomVM:~/test$ emcc test.cpp 
tom@TomVM:~/test$ node a.out.js 
a = 0H
b = 201H
c = 6050403H

(When I tried it without #pragma pack(), the structure alignment was on 16 bit boundaries by default, just as you pointed out.)

This was using Emscripten producing WebAssembly+JS output files.

Well, I guess it is still a long way to a U++ based GUI app running on a browser as WebAssembly with WebGL graphics backend.

Best regards,

Tom