[llvm-branch-commits] [llvm] release/18.x: [DAGCombiner] Fix miscompile bug in combineShiftOfShiftedLogic (#89616) (PR #89766)

[Björn Pettersson via llvm-branch-commits]

bjope wrote:

> @bjope What do you think about backporting this?

Thanks! That looks/sounds good to me.

(The fault was found when using "-fsanitize=undefined -O0" so it was not as if we were using fuzzy pass pipelines etc to trigger the bug. I guess the special thing was the use of larger BitInt types being used in the test,. Combining two 128-bit shifts would't result in a combined shift amount larger than 255, so larger types than 128 bits would be needed to expose the problem.)

https://github.com/llvm/llvm-project/pull/89766