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0 pointsrom-antics3y ago0 comments

"Worthless" is a bit strong. The Rust devs want to add TCO/TCE, but there are still problems left to solve. The "become" keyword is already reserved in anticipation of guaranteed tail calls being added.

https://github.com/rust-lang/rfcs/issues/2691

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masklinn3y ago

> "Worthless" is a bit strong.

No, TCO is worthless, it’s an unreliable optimisation.

rom-anticsOP3y ago

Compiler optimizations are best-effort. -O3 gets you 90% of the optimization for 10% of the work of writing hand-rolled assembly. Individual optimizations are not guaranteed, and sometimes even regress in newer compiler versions, but everyone uses them because overall programs run much faster with optimizations than without. I don't see how TCO is any different. Would you say all optimizations are worthless because they're not guaranteed?

masklinn3y ago

> Would you say all optimizations are worthless because they're not guaranteed?

Context is relevant, the context here is language semantics, not things going faster.

TCO is fine to make things go faster, like inlining, unrolling, LICM, ....

TCO is hot garbage when you rely on it for program correctness.

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anon2913y ago

If given explicit support in a language then no it's not. Especially with the keyword approach rust is taking, the compiler will guarantee the semantics.

Do you believe the exec system call is similarly unreliable at replacing your process image? Amazing that unix systems work at all

masklinn3y ago

> If given explicit support in a language then no it's not. Especially with the keyword approach rust is taking, the compiler will guarantee the semantics.

Then it's TCE not TCO.

> Do you believe the exec system call is similarly unreliable at replacing your process image?

Exec is entirely unlike TCO. If exec was a syscall which sometimes did and sometimes did not replace your process image, then it'd be like TCO, and it'd be similarly unreliable.

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masklinn3y ago

> "Worthless" is a bit strong.

No, TCO is worthless, it’s an unreliable optimisation.

rom-anticsOP3y ago

masklinn3y ago

> Would you say all optimizations are worthless because they're not guaranteed?

Context is relevant, the context here is language semantics, not things going faster.

TCO is fine to make things go faster, like inlining, unrolling, LICM, ....

TCO is hot garbage when you rely on it for program correctness.

1 more reply

anon2913y ago

If given explicit support in a language then no it's not. Especially with the keyword approach rust is taking, the compiler will guarantee the semantics.

Do you believe the exec system call is similarly unreliable at replacing your process image? Amazing that unix systems work at all

masklinn3y ago

> If given explicit support in a language then no it's not. Especially with the keyword approach rust is taking, the compiler will guarantee the semantics.

Then it's TCE not TCO.

> Do you believe the exec system call is similarly unreliable at replacing your process image?

Exec is entirely unlike TCO. If exec was a syscall which sometimes did and sometimes did not replace your process image, then it'd be like TCO, and it'd be similarly unreliable.

1 more reply

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