infinite loop optimized away unless function is exported

export fn square(num: u8) u8 {
             
             //while(true){} // would generate loop
             return eternity(123); 
}

pub fn eternity(bla:u8) u8 {
  while(true)// is optimized away completely
  {}
}

See asm output on goldbold.
As you said during live coding, it seems like the eternal loop is considered to have no side effects and is thus removed.

Expected behaviour would be to refuse compilation

as there is no return of an u8
as in case of eternity being declared as returning void, an implicit return; at the end of the function cannot be reached

Isn't this a long time LLVM bug present in other languages like C and C++ where an empty infinite loop results in no consequences and thus creates invalid programs? (for example there are many articles like https://stackoverflow.com/questions/59925618/how-do-i-make-an-infinite-empty-loop-that-wont-be-optimized-away). I guess the solution Zig would need for this specific case is adding in a nop op-code inside of the loop so LLVM says it "has an effect" and doesn't generate an invalid program/function.

 export fn square(num: u8) u8 {          
    //while(true){}
    return eternity(123); 
}

pub fn eternity(bla:u8) u8 {
    while(true){
        asm volatile("nop");
    }
}

This code generates as intended as we can see on GodBolt.

Related: rust-lang/rust#82884 - apparently this was fixed in LLVM 12.

export fn square(num: u8) u8 {
    _ = num;
    return eternity(123);
}

pub fn eternity(bla: u8) u8 {
    _ = bla;
    while (true) {}
}

export fn square2(num: u8) u8 {
    _ = num;
    while (true) {}
}

export fn square3() u8 {
    while (true) {}
}

This code, paired with zig build-obj on my master build of Zig (approx 1 week old) is producing while loops correctly in all three cases, in all four -O flags.

(pictured above: ReleaseSmall)

pub fn main() void {
    while (true) {}
}

I've also tested this executable with all four -O flags, and this works on all of them.

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infinite loop optimized away unless function is exported #1658

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infinite loop optimized away unless function is exported #1658

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IridescentRose commented on Oct 20, 2020

IridescentRose commented on Oct 20, 2020

Sobeston commented on Sep 22, 2021

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