fix hash(::BigInt) on 32 bit systems

[oscardssmith]

Theoretically fixes #50075 (CI will let us know for sure).

[LilithHafner]

Wow you're fast!

[oscardssmith]

you're assuming that the fix is actually correct :) I am mostly just guessing.

[LilithHafner]

Hmm, yeah. On 32-bit systems the pointer in a BigInt is a UInt32 pointer, so creating a UInt64 pointer and unsafe_loading from it may be broken when x.alloc == 1.

[oscardssmith]

Yeah. I don't think this is workable. Let's eval it out then.

[oscardssmith]

grr. The collision test fails. Any thoughts?

[LilithHafner]

The (length(types) - 1) ^ 2 upper bound in the collision test is cute, but afaict, the comment explaining why it works is wrong (or maybe its too clever for me cc @vtjnash #38031). My unmerged approach when adding BigFloat tests is to replace it with a hardcoded bound, which is also not good.

The best approach would be to adjust BigInt and BigFloat hashing to reduce collisions, but we shouldn't hold these bugfixes for that.

[rfourquet]

Was this broken only since 1.9? Do we know what is broken in the current code for 32 bits systems?

[LilithHafner]

The following is merely speculation:

Was this broken only since 1.9?

Unlikely

Do we know what is broken in the current code for 32 bits systems?

I believe Int64 gets hashed in one go while BigInt gets hashed one UInt32 chunk at a time, so for numbers in the range typemax(Int32) < x < typemax(Int64), BigInts and Int64s are hashed differently.

[vtjnash]

The hash range used to fold back on itself for certain types, so it is not a comment about the required guarantee but simply a statement about why it was not zero anymore and why the resulting collisions are still rare enough (and not e.g. clustered)

[oscardssmith]

This has been broken since at least #38031 (it goes further back, but git blame is hard to track then).

[jmkuhn]

This has been broken since at least #38031 (it goes further back, but git blame is hard to track then).

Testing a few old 32 bit arm builds that I have, it worked in v1.4.2 and was broken by v1.5.3.