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Dead code elimination for if (typeof(T).IsValueType) #97080

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Jan 18, 2024
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Dead code elimination for if (typeof(T).IsValueType) #97080

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Dead code elimination for `if (typeof(T).IsValueType)`
MichalStrehovsky Jan 17, 2024
9ad4047
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MichalStrehovsky Jan 17, 2024
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85 changes: 63 additions & 22 deletions src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/FeatureSwitchManager.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -591,6 +591,13 @@ private bool TryGetConstantArgument(MethodIL methodIL, byte[] body, OpcodeFlags[
{
return true;
}
else if (method.IsIntrinsic && method.Name is "get_IsValueType"
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method.IsIntrinsic can be de-duplicated with the existing case above.

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We need this to land in the else branch all the way below if it's an intrinsic but not the one we recognize.

&& method.OwningType is MetadataType mdt
&& mdt.Name == "Type" && mdt.Namespace == "System" && mdt.Module == mdt.Context.SystemModule
&& TryExpandTypeIsValueType(methodIL, body, flags, currentOffset, out constant))
{
return true;
}
else
{
constant = 0;
Expand Down Expand Up @@ -745,6 +752,40 @@ private string GetResourceStringForAccessor(EcmaMethod method)
return null;
}

private static bool TryExpandTypeIsValueType(MethodIL methodIL, byte[] body, OpcodeFlags[] flags, int offset, out int constant)
{
// We expect to see a sequence:
// ldtoken Foo
// call GetTypeFromHandle
// -> offset points here
constant = 0;
const int SequenceLength = 10;
if (offset < SequenceLength)
return false;

if ((flags[offset - SequenceLength] & OpcodeFlags.InstructionStart) == 0)
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Do we need to check that there are no jump targets in the sequence?

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ReadLdToken and ReadGetTypeFromHandle already do that check

return false;

ILReader reader = new ILReader(body, offset - SequenceLength);

TypeDesc type = ReadLdToken(ref reader, methodIL, flags);
if (type == null)
return false;

if (!ReadGetTypeFromHandle(ref reader, methodIL, flags))
return false;

// Dataflow runs on top of uninstantiated IL and we can't answer some questions there.
// Unfortunately this means dataflow will still see code that the rest of the system
// might have optimized away. It should not be a problem in practice.
if (type.IsSignatureVariable)
return false;

constant = type.IsValueType ? 1 : 0;

return true;
}

private static bool TryExpandTypeEquality(MethodIL methodIL, byte[] body, OpcodeFlags[] flags, int offset, string op, out int constant)
{
// We expect to see a sequence:
Expand Down Expand Up @@ -793,37 +834,37 @@ private static bool TryExpandTypeEquality(MethodIL methodIL, byte[] body, Opcode
constant ^= 1;

return true;
}

static TypeDesc ReadLdToken(ref ILReader reader, MethodIL methodIL, OpcodeFlags[] flags)
{
ILOpcode opcode = reader.ReadILOpcode();
if (opcode != ILOpcode.ldtoken)
return null;
private static TypeDesc ReadLdToken(ref ILReader reader, MethodIL methodIL, OpcodeFlags[] flags)
{
ILOpcode opcode = reader.ReadILOpcode();
if (opcode != ILOpcode.ldtoken)
return null;

TypeDesc t = (TypeDesc)methodIL.GetObject(reader.ReadILToken());
TypeDesc t = (TypeDesc)methodIL.GetObject(reader.ReadILToken());

if ((flags[reader.Offset] & OpcodeFlags.BasicBlockStart) != 0)
return null;
if ((flags[reader.Offset] & OpcodeFlags.BasicBlockStart) != 0)
return null;

return t;
}
return t;
}

static bool ReadGetTypeFromHandle(ref ILReader reader, MethodIL methodIL, OpcodeFlags[] flags)
{
ILOpcode opcode = reader.ReadILOpcode();
if (opcode != ILOpcode.call)
return false;
private static bool ReadGetTypeFromHandle(ref ILReader reader, MethodIL methodIL, OpcodeFlags[] flags)
{
ILOpcode opcode = reader.ReadILOpcode();
if (opcode != ILOpcode.call)
return false;

MethodDesc method = (MethodDesc)methodIL.GetObject(reader.ReadILToken());
MethodDesc method = (MethodDesc)methodIL.GetObject(reader.ReadILToken());

if (!method.IsIntrinsic || method.Name != "GetTypeFromHandle")
return false;
if (!method.IsIntrinsic || method.Name != "GetTypeFromHandle")
return false;

if ((flags[reader.Offset] & OpcodeFlags.BasicBlockStart) != 0)
return false;
if ((flags[reader.Offset] & OpcodeFlags.BasicBlockStart) != 0)
return false;

return true;
}
return true;
}

private sealed class SubstitutedMethodIL : MethodIL
Expand Down
26 changes: 26 additions & 0 deletions src/tests/nativeaot/SmokeTests/TrimmingBehaviors/DeadCodeElimination.cs
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Expand Up @@ -21,6 +21,7 @@ public static int Run()
TestArrayElementTypeOperations.Run();
TestStaticVirtualMethodOptimizations.Run();
TestTypeEquals.Run();
TestTypeIsValueType.Run();
TestBranchesInGenericCodeRemoval.Run();
TestUnmodifiableStaticFieldOptimization.Run();
TestUnmodifiableInstanceFieldOptimization.Run();
Expand Down Expand Up @@ -355,6 +356,31 @@ public static void Run()
}
}

class TestTypeIsValueType
{
class Never { }

class Ever { }

static void Generic<T>()
{
if (typeof(T).IsValueType)
{
Activator.CreateInstance(typeof(Never));
}

Activator.CreateInstance(typeof(Ever));
}

public static void Run()
{
Generic<object>();

ThrowIfPresent(typeof(TestTypeIsValueType), nameof(Never));
ThrowIfNotPresent(typeof(TestTypeIsValueType), nameof(Ever));
}
}

class TestBranchesInGenericCodeRemoval
{
class ClassWithUnusedVirtual
Expand Down
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