[stdlib] Mark assume as has_side_effect=False

[soraros]

Also mark assume as side-effect free.

[soraros]

Actually, will assume get DCE'd if we set has_side_effect to False?

[JoeLoser]

@jackos do you mind taking a look at this? This is modeled after https://en.cppreference.com/w/cpp/language/attributes/assume.

[JoeLoser]

!sync

[soraros]

Also mark assume as side-effect free.

@JoeLoser I now think this is the wrong thing to do. llvm.assume returns void, so if it's marked as pure, and nobody is using the result, it won't pass DCE. Nothing shows in the IR generated by ff.

fn ff(n: Int) -> Int:
  llvm_intrinsic["llvm.assume", NoneType, has_side_effect=False](n == 1)
  return 666 if n == 1 else 777

fn gg(n: Int) -> Int:
  llvm_intrinsic["llvm.assume", NoneType](n == 1)
  return 666 if n == 1 else 777

Maybe we still want to put assume in debug_assert, but we will have to wait for #3662.

[jackos]

Hi @soraros I've updated externally to keep the has_side_effect=False for assume as that's the right thing to do, but removed assume from debug_assert as it's always DCE'd.

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