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FunC codegen error for obvious cases of zero division in statements #716

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anton-trunov opened this issue Aug 19, 2024 · 2 comments · May be fixed by #852
Open

FunC codegen error for obvious cases of zero division in statements #716

anton-trunov opened this issue Aug 19, 2024 · 2 comments · May be fixed by #852
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@jeshecdom
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bug Something isn't working or isn't right func scope: const-eval The constant and partial evaluation optimization mechanisms
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v1.6.0

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@anton-trunov
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contract Test {

    a: Int = 0;

    init() {
        self.a / self.a;
        // self. a / 0 would fail too
    }

    get fun foo(): Int {
        let x = 42 - 42;
        return 42 / x;   // this also fails
    }
}

Results in the following error message from FunC:

Func compilation error: cannot generate code for function `$Test$_contract_init`:
/dist/tact_Test.code.fc:15:14: error: division by zero
($self'a / $self'a)

As far as I understand this happens during code generation when going from FunC to Fift-asm.

To resolve this, we need to track variables whose values can be determined at compile-time.
@anton-trunov anton-trunov added bug Something isn't working or isn't right scope: const-eval The constant and partial evaluation optimization mechanisms labels Aug 19, 2024
@jeshecdom
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Mmmm... the main problem I see here is that const-eval (or the interpreter) is only called to simplify expressions. For example, inside function foo, it is called to simplify the argument of the return. const-eval will not execute function foo because there is no expression calling function foo. As such, it will treat variable x in the return statement as uninterpreted.

To solve this issue, we would need some form of code analysis outside of const-eval. Another possibility would be to attempt calling functions that do not have parameters, just to check if its execution generates an error.
Probably, we will need an analysis/optimization pass after the typecheck pass, because currently, optimization and interpretation is only carried out on pieces of the code (in expressions, to be exact).

@anton-trunov
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To solve this issue, we would need some form of code analysis outside of const-eval.

To solve this issue you need to partially evaluate contracts (I almost always mean partial evaluation or constant evaluation by "const-eval", sorry if that's confusing)

@anton-trunov anton-trunov added this to the v1.5.0 milestone Sep 5, 2024
@anton-trunov anton-trunov modified the milestones: v1.5.0, v1.6.0 Sep 15, 2024
@jeshecdom jeshecdom linked a pull request Sep 19, 2024 that will close this issue
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bug Something isn't working or isn't right func scope: const-eval The constant and partial evaluation optimization mechanisms
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Simple constant propagation AST-based analysis tact-lang/tact
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@jeshecdom @anton-trunov