[llvm-dev] Behaviour of APInt (original) (raw)

Tim Northover via llvm-dev llvm-dev at lists.llvm.org
Thu Jan 31 05:25:32 PST 2019

On Thu, 31 Jan 2019 at 12:56, Sam Parker via llvm-dev <llvm-dev at lists.llvm.org> wrote:

The APInt documentation states that it 'is a functional replacement for common case unsigned integer type', but I'm not seeing this because the internal logic is that the value is always treated as negative if the most significant bit is set.

I take that as saying it's a 2s-complement type rather than overflow being UB, but the statement may still be misleading.

I know there are operators for when the sign matters, but from my example, either my understanding or the functionality is broken.

It's definitely quirky that it's always printed as a signed integer. My guess would be it stems from a very early decision about the friendliest ways to print IR's iN types, which was probably its first use-case (i.e. most people would prefer to see i64 -1 over i64 18446744073709551616). But I haven't done the archaeology to confirm it.

If an abstract structure exists, why does the MSB still represent the sign? Especially when it's supposed to be an unsigned type!

I think it's be more correct to say it's an arbitrary precision type that could be either sign (again, much like LLVM's iN). There's a separate APSInt for a type that genuinely is either signed or unsigned in all cases.

Cheers.

Tim.

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