Compound Assignment when ref temps are required crashes the compiler #36443
Comments
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Compiling this sort of scenario seems to work fine from the command-line, With both the native compiler and Roslyn, the write to the field involves an additional dereference after the await completes (ie. we couldn't save a ref to the field). // produced by native compiler, 4.7.3190.0
private void MoveNext()
{
try
{
bool flag = true;
object item;
int num;
TaskAwaiter<int> awaiter;
if (<>1__state != 0)
{
item = <>4__this;
num = ((C)item).field;
awaiter = <>4__this.M2().GetAwaiter();
if (!awaiter.IsCompleted)
{
Tuple<C, int> tuple = (Tuple<C, int>)(<>t__stack = new Tuple<C, int>((C)item, num));
<>1__state = 0;
<>u__$awaiter1 = awaiter;
<>t__builder.AwaitUnsafeOnCompleted(ref awaiter, ref this);
flag = false;
return;
}
}
else
{
Tuple<C, int> tuple = (Tuple<C, int>)<>t__stack;
item = tuple.Item1;
num = tuple.Item2;
<>t__stack = null;
awaiter = <>u__$awaiter1;
<>u__$awaiter1 = default(TaskAwaiter<int>);
<>1__state = -1;
}
int result = awaiter.GetResult();
awaiter = default(TaskAwaiter<int>);
((C)item).field = num + result; // assign to the field
Console.Write(<>4__this.field);
}
catch (Exception exception)
{
<>1__state = -2;
<>t__builder.SetException(exception);
return;
}
<>1__state = -2;
<>t__builder.SetResult();
}// produced by 3.2 compiler
private void MoveNext()
{
int num = <>1__state;
try
{
TaskAwaiter<int> awaiter;
if (num != 0)
{
<>s__1 = <>4__this.field;
awaiter = <>4__this.M2().GetAwaiter();
if (!awaiter.IsCompleted)
{
num = (<>1__state = 0);
<>u__1 = awaiter;
<M>d__1 stateMachine = this;
<>t__builder.AwaitUnsafeOnCompleted(ref awaiter, ref stateMachine);
return;
}
}
else
{
awaiter = <>u__1;
<>u__1 = default(TaskAwaiter<int>);
num = (<>1__state = -1);
}
<>s__2 = awaiter.GetResult();
<>4__this.field = <>s__1 + <>s__2; // assign to the field
Console.Write(<>4__this.field);
}
catch (Exception exception)
{
<>1__state = -2;
<>t__builder.SetException(exception);
return;
}
<>1__state = -2;
<>t__builder.SetResult();
}// original source
class C
{
int field = 1;
async System.Threading.Tasks.Task M()
{
this.field += await M2();
System.Console.Write(this.field);
}
async System.Threading.Tasks.Task<int> M2()
{
await System.Threading.Tasks.Task.Yield();
return 42;
}
} |
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The code sample in OP doesn't crash the latest compiler (tested in command-line, unittest and sharplab). Let's confirm and narrow down the repro scenario. Correction: it does crash in unittest with Update: repros with
if (compilation.Options.OptimizationLevel != OptimizationLevel.Release)
{
Debug.Assert(variablesToHoist.Count == 0);
// In debug build we hoist all locals and parameters:
foreach (var v in allVariables)
{
var symbol = v.Symbol;
if ((object)symbol != null && HoistInDebugBuild(symbol))
{
variablesToHoist.Add(symbol);
}
}
} |
gafter
added
4 - In Review
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The analysis fails to determine that the local |
gafter
added
4 - In Review
This code crashes the compiler with the following stack:
The issue here is that
TransformCompoundAssignmentLHScan return areflocal for the following case:roslyn/src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_CompoundAssignmentOperator.cs
Lines 490 to 493 in d5b8d64
The fix is likely to simply issue an understandable error here, instead of throwing an UnreachableCode exception. FYI @jcouv @agocke.
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