aaronpuchert added a comment.
@hokein Please have a look at https://reviews.llvm.org/D52888, maybe you can
try it out already. The problem was that `?:` expressions are considered a
branch point and when merging both branches the warning was emitted. Before
this change, we couldn't look into `__builtin_expect` and so we didn't see that
the result of `TryLock` is used to branch. Take this source:
void foo() {
if (mu.TryLock() ? 1 : 0)
mu.Unlock();
}
The CFG is:
void foo()
[B6 (ENTRY)]
Succs (1): B5
[B1]
1: mu
2: [B1.1].Unlock
3: [B1.2]()
Preds (1): B2
Succs (1): B0
[B2]
1: [B5.3] ? [B3.1] : [B4.1]
2: [B2.1] (ImplicitCastExpr, IntegralToBoolean, _Bool)
T: if [B2.2]
Preds (2): B3 B4
Succs (2): B1 B0
[B3]
1: 1
Preds (1): B5
Succs (1): B2
[B4]
1: 0
Preds (1): B5
Succs (1): B2
[B5]
1: mu
2: [B5.1].TryLock
3: [B5.2]()
T: [B5.3] ? ... : ...
Preds (1): B6
Succs (2): B3 B4
[B0 (EXIT)]
Preds (2): B1 B2
So we start in `B5`, then call `TryLock` and branch on `?:` into `B3` or `B4`.
We find that in `B3` the lock is held and in `B4` it isn't, so when both merge
into `B2` we complain that we don't know if the lock is held or not.
I think the proper solution is to not branch on conditionals, and rather
introspect them when a "real" branch happens. There is a slight possibility
that this might break other use cases, but I hope not.
Repository:
rL LLVM
https://reviews.llvm.org/D52398
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