Drop poison implying metadata #65216
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…ttrsAndMetadata`. Hoisting a poison is safe only when it doesn't introduce more UB.
nikic
requested changes
Sep 3, 2023
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Fixes #65195.
Hoisting a poison is safe only when it doesn't introduce more UB.
Currently, I'm just preventing preserving metadata like range.
I'm going to backport to LLVM.
17, so I don't want to introduce too many changes.
I think there are two approaches to restoring this range:
a. Do a may analysis using dataflow. However, in the issue, we have to consider both the result of the instruction and the operation value propagation. This can make the analysis complicated or useless.
b. Introduces a new valid scope for the metadata record
range. This way we can reasonably determine whether UB can be triggered.I'm not sure which is better. But I tend to favor method B. From what I understand, we use UB for optimization. I think a poison that can't trigger a UB makes no sense.