RFR: 8299546: C2: MulLNode::mul_ring() wrongly returns bottom type due to casting errors with large numbers

[Christian Hagedorn]

On Tue, 10 Jan 2023 02:57:55 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

> I would suggest we do a 128-bit multiplication or a 64-bit high multiplication instead. This will help constant folding of `MulHiNode`s and also `MulLNode` even if the multiplication overflows. Also, a high multiplication is likely to be cheaper than 4 division.

Thanks for your suggestion. I'm not sure if I understand this correctly, though. How would this help for `MulHiNode`? And if we have an overflow, I'm not sure how we can get a better type then bottom for the `Mul` node since we cannot have multiple ranges for a type. Could you explain your idea in more detail?

But if we are just talking about the implementation, how could we do a 128-bit multiplication inside the VM? Using `__int128_t` (I haven't seen any usage of that though - I guess it is not supported)? However, I think we should not worry too much about the performance given that these are only 4 divisions and we are in the context of C2 compiling a method.

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PR: https://git.openjdk.org/jdk/pull/11907