tianshilei1992 added a comment.
Another thing is how we deal with a corner case. Say the OpenMP code is written
in the following way:
#pragma omp atomic compare
x = e < x ? e : x;
That's how OpenMP spec defines the atomic operation. `x` is always in "else
statement" of a conditional statement.
Now we need to lower it to LLVM IR, which is `atomicrmw` operation. Based on
the LLVM IR reference, it only supports the atomic operations that `x` is in
the "then statement". For example: `x = x > e ? x : e`. See the `x` here is
before `:`. In order to lower the OpenMP statement, we have to do a
transformation. In order to swap `e` and `x`, we need to transform it to `x = e
>= x : x : e`, a.k.a. `x = x <= e : x : e`. However, we don't have an atomic
operation for `<=`. We only have `<`. So if `x != e`, the result is good.
The incorrectness happens if `x == e`. Recall at the OpenMP statement, when `x
== e`, the result should be `x = x`. But if we look at our lowered LLVM IR, `x
= x < e : x : e`, when `x == e`, it becomes `x = e`, which doesn't conform with
OpenMP spec.
What should we do here?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D102449/new/
https://reviews.llvm.org/D102449
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