[PATCH] D63044: [LangRef] Clarify poison semantics

[Juneyoung Lee via Phabricator via llvm-commits]

aqjune added inline comments.


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Comment at: llvm/docs/LangRef.rst:3274
+-  The divisor operand of a ``udiv``, ``sdiv``, ``urem`` or ``srem``
+   instruction.
+
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How about mentioning right-shift operations on poison (e.g. `ashr x, poison`) as well?
Its semantics is equivalent to `x / 2^y` when x and y are non-poison(and non-undef), but LLVM seems to hoist `ashr x, poison` during optimization, implying that it cannot be UB : https://godbolt.org/z/2LG0xX .


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Comment at: llvm/docs/LangRef.rst:3276
+
+Additionally, undefined behavior occurs if a side effect depends on poison.
 
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jdoerfert wrote:
> Now, side effects, what do we count and why do we have it explicitly.
> 
> Also, should we explicitly mention control flow or is it sufficiently covered by the "*depends*" definition above?
I also think that control flow on poison is an important issue.

https://github.com/llvm-mirror/llvm/blob/master/lib/Transforms/Scalar/LoopUnswitch.cpp#L585 also mentions that there's discrepancy between semantics of branch on undef/poison.
My suggestion is to mention the discrepancy explicitly here so people can acknowledge the issue while reading LangRef.


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