13 ms·

It would be ideal if Go would add support for computed goto, so that we could build direct threaded interpreters.

• zabzonk 2y ago
  I don't think "ideal" would be the exact word - I used to shudder at it in FORTRAN.
• kiitos 2y ago
  hard no - goto is a feature for code generators, not anything meant for human use
  • greenavocado 2y ago
    I never understood this argument. Without RAII you can easily get screwed by resource leaks without goto when returning early. In this regard, using goto is expedient. How do C programmers avoid this problem without goto? bool function_with_cleanup(void) { int *buffer1 = NULL; int *buffer2 = NULL; FILE *file = NULL; bool success = false; // Allocate first resource buffer1 = malloc(sizeof(int) * 100); if (!buffer1) { goto cleanup; // Error, jump to cleanup } // Allocate second resource buffer2 = malloc(sizeof(int) * 200); if (!buffer2) { goto cleanup; // Error, jump to cleanup } // Open a file file = fopen("data.txt", "r"); if (!file) { goto cleanup; // Error, jump to cleanup } // Do work with all resources... success = true; // Only set to true if everything succeeded cleanup: // Free resources in reverse order of acquisition if (file) fclose(file); free(buffer2); // free() is safe on NULL pointers free(buffer1); return success; }
    • dwattttt 2y ago
      Open a scope when you check resource acquisition passed, rather than the opposite (jump to the end of the function if it failed). It can get quite hilly, which doesn't look great. It does have the advantage that each resource is explicitly only valid in a visible scope, and there's a marker at the end to denote the valid region of the resource is ending. EDIT: you mentioned early return, this style forbids early return (at least, any early return after the first resource acquisition)
    • gigatexal 2y ago
      In this example couldn’t the go to cleanup instead be return cleanup_func where the same cleanup code was executed?
    • pjmlp 2y ago
      Maybe that is exactly the problem, stop using a language designed in 1970's that ignored on purpose the ecosystem outside Bell Labs, unless where it is unavoidable. And in such case, the C compiler doesn't have a limit to write functions and better modularize their implementations. bool function_with_cleanup(void) { int *buffer1 = NULL; int *buffer2 = NULL; FILE *file = NULL; // Allocate first resource buffer1 = malloc(sizeof(int) * 100); if (!buffer1) { return false; } // Allocate second resource buffer2 = malloc(sizeof(int) * 200); if (!buffer2) { free(buffer1); return false; } // Open a file file = fopen("data.txt", "r"); if (!file) { free(buffer1); free(buffer1); return false; } // Do work with all resources... fclose(file); free(buffer1); free(buffer1); return true; } Ah, but all those free() calls get tedious can be forgotten and mistyped bool function_with_cleanup(void) { int *buffer1 = NULL; int *buffer2 = NULL; FILE *file = NULL; // Allocate first resource buffer1 = arena_alloc(&current_arena, sizeof(int) * 100); if (!buffer1) { return false; } // Allocate second resource buffer2 = arena_alloc(&current_arena, sizeof(int) * 200); if (!buffer2) { arena_reset(&current_arena); return false; } // Open a file file = fopen("data.txt", "r"); if (!file) { arena_reset(&current_arena); return false; } // Do work with all resources... fclose(file); arena_reset(&current_arena); return true; } Can still be improved with a mix of macros and varargs functions. Or if using language extensions is a thing, the various ways to do defer in C.
      • LtWorf 2y ago
        You understand go doesn't have exceptions right?
        • pjmlp 2y ago
          You understand that I wrote C code, and in what concerns Go, panic/recover are exceptions that don't want to assume themselves as such?
    • shakna 2y ago
      Attributes, mostly. Which have become so common that defer is very likely to be in the next C standard. [0] bool function_with_cleanup(void) { // Allocate first resource int* buffer1 __attribute__((__cleanup__(free))) = malloc(sizeof(int) * 100); if(!buffer1) { return false; } // Allocate second resource int* buffer2 __attribute__((__cleanup__(free))) = malloc(sizeof(int) * 200); if(!buffer2) { return false; } // Open a file FILE* file __attribute__((__cleanup__(fclose))) = fopen("data.txt", "r"); if (!file) { return false; } return true; } [0] https://thephd.dev/c2y-the-defer-technical-specification-its-time-go-go-go https://thephd.dev/c2y-the-defer-technical-specification-its...
    • mkl 2y ago
      Nested ifs are my preference: bool function_with_cleanup(void) { int *buffer1 = NULL; int *buffer2 = NULL; FILE *file = NULL; bool success = false; // Allocate first resource buffer1 = malloc(sizeof(int) * 100); if (buffer1) { // Allocate second resource buffer2 = malloc(sizeof(int) * 200); if (buffer2) { // Open a file file = fopen("data.txt", "r"); if (file) { // Do work with all resources... fclose(file); success = true; // Only set to true if everything succeeded } free(buffer2); } free(buffer1); } return success; } Much shorter and more straightforward. One-time loops with break also work if you're not doing the resource allocation in another loop: bool function_with_cleanup(void) { int *buffer1 = NULL; int *buffer2 = NULL; FILE *file = NULL; bool success = false; do { // One-time loop to break out of on error // Allocate first resource buffer1 = malloc(sizeof(int) * 100); if (!buffer1) { break; // Error, jump to cleanup } // Allocate second resource buffer2 = malloc(sizeof(int) * 200); if (!buffer2) { break; // Error, jump to cleanup } // Open a file file = fopen("data.txt", "r"); if (!file) { break; // Error, jump to cleanup } // Do work with all resources... success = true; // Only set to true if everything succeeded } while(false); // Free resources in reverse order of acquisition if (file) fclose(file); free(buffer2); // free() is safe on NULL pointers free(buffer1); return success; } Still simpler to follow than goto IMHO. Both these patterns work in other languages without goto too, e.g. Python.
      • gpderetta 2y ago
        Nested if, aside from being awful, doesn't scale. And break is extremely thin sugar on top of go-to.
  • jandrewrogers 2y ago
    There are situations in practical code where goto is the only reasonable choice if you want to avoid spaghetti code. Absolutes have no place in software, you can find scenarios where almost every typically bad idea is actually a good idea. It is a tool, not a religion.
    • 9rx 2y ago
      Even Dijkstra was clear that he was only referring to unbridled gotos.
• 9dev 2y ago
  Go to goto, go!
• pklausler 2y ago
  Did you mean "assigned GOTO", not computed GOTO? Because that's just a switch list.
  • knome 2y ago
    https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html 'computed goto' is used in gcc to mean the same as assigned goto in Fortran. The Fortran variation appears to have more restrictions than the gnuc one.
    • johnisgood 2y ago
      I may be dumb, but what is "goto *ptr;" useful for? Or &&foo.
      • withinboredom 2y ago
        error conditions when you don't have exceptions: goto *errOrSuccess; is a pretty normal one. This basically allows you to emulate a "finally". State machines are another big one, which look much nicer with computed goto than a switch statement.
      • trealira 2y ago
        It's used in VMs, emulators, and other interpreters for making a dispatch loop with less branching than a standard loop with a switch inside it. https://eli.thegreenplace.net/2012/07/12/computed-goto-for-efficient-dispatch-tables https://eli.thegreenplace.net/2012/07/12/computed-goto-for-e...
        • rurban 2y ago
          No, it has the same number of branches as a switch inside. The only difference is that computed goto switches miss the initial range check. And proper switch implementations by a better compiler would be more optimizable. But we are waiting more than 20 years for that already. Sparse switch dispatch tables are also compiled horribly.
• nemo1618 2y ago
  Is computed goto used for anything other than interpreter loops? Because if not, I would rather have a special "it looks like you're trying to implement an interpreter loop" case in the compiler than add new syntax.
  • grandempire 2y ago
    Yes. Goto is useful for efficient control flow in many stateful algorithms.
    • dwattttt 2y ago
      I don't know that _efficient_ is the best word. If you use goto to force a particular control flow rather than a more constrained form of control flow (e.g. if), you make it harder for the optimiser to work; it can only make "as-if" changes, ones that mean the code that executes looks as if it's what you wrote. The most efficient control flow is one that describes only what your algorithm needs, coupled with an optimiser that can exploit the particular flow you're describing. Among the many things discovered by the author of https://blog.nelhage.com/post/cpython-tail-call/ https://blog.nelhage.com/post/cpython-tail-call/, Clang/LLVM was able to optimise the standard switch based interpreter loop as if it had been written with computed gotos.
      • grandempire 2y ago
        > The most efficient control flow is one that describes only what your algorithm needs, Yes. I’m not saying goto is just faster in general. But some algorithms are difficult to describe with while and if (bunch of examples in Knuth). > Clang/LLVM was able to optimise Because it’s implicit, this is the kind of optimization that’s easy to silently regress a few weeks before ship by accidentally violating a rule. I think unrestricted goto is not good. But an alternative is to make the semantics and scope stricter, something like a Common Lisp prog/tag body block.