[Vm-dev] [OpenSmalltalk/opensmalltalk-vm] Armv8 crash in MVC (Issue #617)

Nicolas Cellier notifications at github.com
Wed Mar 9 21:06:54 UTC 2022

fastPathBottomToTop does not do much... It just reverse the line order.

I foresee two cases where it could fail:
- the fast operation by itself has a BUG (whatether BottomToTop or not)
- some integer operations are carried on a larger integer type, transforming negative pitch into a large positive int

The later is possible because `srcPitch` and `dstPitch` are declared usqInt in `operation_t`

`usqInt` is 64bits on ARM64, so far so good, unfortunately those variables are copied to 32 bits local temps in 

I think that it is the mistake, because pointer arithmetic in 64 bits will be performed with 64 bits size_t, and the uint32_t will just move the pointer a large amount forward (2^32-pitch) instead of backward (-pitch).

So I think that declaring the local `srcPitch` and `dstPitch` as `usqInt` instead of `uint32_t` might fix the bug.

I have no ARM64 at hand to play with, so up to you to verify the hypothesis.

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