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Use a simple temp instead of InlineArray1 #73086

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May 8, 2024
Merged

Use a simple temp instead of InlineArray1 #73086

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5ee9248
Use a simple temp instead of InlineArray1
RikkiGibson Apr 17, 2024
3e51b81
fix test failures
RikkiGibson Apr 18, 2024
60d90aa
fix tests
RikkiGibson Apr 18, 2024
9d69fa7
Pass isKnownToReferToTempIfReferenceType because the other place in t…
RikkiGibson Apr 19, 2024
754c7c4
Show that temps are not reused
RikkiGibson Apr 19, 2024
e9e7d2a
Undo unintentional change
RikkiGibson Apr 19, 2024
410ec68
Remove true argument for isKnownToReferToTempIfReferenceType
RikkiGibson Apr 26, 2024
35c2769
Address feedback
RikkiGibson Apr 26, 2024
8712cd2
Fix test
RikkiGibson Apr 26, 2024
1c459b3
fix test
RikkiGibson Apr 29, 2024
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13 changes: 6 additions & 7 deletions src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_CollectionExpression.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -195,8 +195,7 @@ private BoundExpression VisitArrayOrSpanCollectionExpression(BoundCollectionExpr
syntax,
elementType,
elements,
asReadOnlySpan: collectionTypeKind == CollectionExpressionTypeKind.ReadOnlySpan,
_additionalLocals);
asReadOnlySpan: collectionTypeKind == CollectionExpressionTypeKind.ReadOnlySpan);
}

Debug.Assert(IsAllocatingRefStructCollectionExpression(node, collectionTypeKind, elementType.Type, _compilation));
Expand Down Expand Up @@ -429,14 +428,14 @@ private static bool ShouldUseInlineArray(BoundCollectionExpressionBase node, CSh
SyntaxNode syntax,
TypeWithAnnotations elementType,
ImmutableArray<BoundNode> elements,
bool asReadOnlySpan,
ArrayBuilder<LocalSymbol> locals)
bool asReadOnlySpan)
{
Debug.Assert(elements.Length > 0);
Debug.Assert(elements.All(e => e is BoundExpression));
Debug.Assert(_factory.ModuleBuilderOpt is { });
Debug.Assert(_diagnostics.DiagnosticBag is { });
Debug.Assert(_compilation.Assembly.RuntimeSupportsInlineArrayTypes);
Debug.Assert(_additionalLocals is { });

int arrayLength = elements.Length;
if (arrayLength == 1
Expand All @@ -451,8 +450,8 @@ private static bool ShouldUseInlineArray(BoundCollectionExpressionBase node, CSh
var constructor = spanRefConstructor.AsMember(spanType);
var element = VisitExpression((BoundExpression)elements[0]);
var temp = _factory.StoreToTemp(element, out var assignment);
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What is the re-use policies on these temps? Basically is there any chance that the temp slot allocated here will be re-used or is it considered a temp that lives for the lifetime of the current method?

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I think the local slot can be reused outside the containing block, but that's fine, since the span value that references the temp can't escape outside the containing block.

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Hmm. Some of our temp are statement level. I agree block level temp is fine but we should be sure which this is. Had other bugs with statement temps being reused in span before

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I added a test. It doesn't look like the temp is reused.

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Not sure the test is sufficient here. Looking at the code it seems that it's a re-usable temp. The default kind of the temp is SynthesizedLocalKind.LoweringTemp and that is not a long lived temp. The doc mentions these cannot live across a statement boundary

    /// 1) Short-lived (temporary)
    ///    The lifespan of a temporary variable shall not cross a statement boundary (a PDB sequence point).
    ///    These variables are not tracked by EnC and don't have names. Only values less than 0 are considered
    ///    short-lived: new short-lived kinds should have a negative value.

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What is the re-use policies on these temps?

Slots can be reused for locals that go out of scope. The scope is defined by blocks and sequences that list locals they own. If I remember correctly, sometimes scope of locals is extended by codegen, when, for example, a sequence returns a ref to a local that it owns. One might say the bound tree violates scoping rules in such cases, but for whatever reason a decision was made to handle the case instead of enforcing correctness of the tree.

That said, symbols for temps are never reused by default. One might, of course, intentionally create a bound tree that shares the same temp for different purposes.

I hope that helps with concerns that prompted the original question.

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Regarding the short-lived/long-lived story. According to my understanding, these are mostly about PDB/ENC, and the statement boundary the comment is talking about is in terms of syntax (perhaps talking in terms of sequence point boundaries would be more accurate), not about bound statement nodes. Reuse of slots in IL is not based on that. It is based on what I said in the previous message on this thread. So as long as the local is added to the right BoundBlock/BoundSequence, it will not be reused while code for that node is emitted.

There is, however, something interesting going on with where we put inline array locals. The line locals.Add(inlineArrayLocal.LocalSymbol); below. According to comments on _additionalLocals field, the local is going to end up on the enclosing method outermost block. Hopefully that is not going to mess with EnC too much because effectively the local crosses a sequence point boundary. "Collection expressions" devs might want to take a close look at this.

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Great. Thansk for the explanation!

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There is, however, something interesting going on with where we put inline array locals. ...

Logged #73246 based on this comment and offline discussion.

locals.Add(temp.LocalSymbol);
var call = _factory.New(constructor, arguments: [temp], argumentRefKinds: [asReadOnlySpan ? RefKind.In : RefKind.Ref]);
_additionalLocals.Add(temp.LocalSymbol);
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var call = _factory.New(constructor, arguments: [temp], argumentRefKinds: [asReadOnlySpan ? RefKindExtensions.StrictIn : RefKind.Ref]);
return _factory.Sequence([assignment], call);
}

Expand All @@ -469,7 +468,7 @@ private static bool ShouldUseInlineArray(BoundCollectionExpressionBase node, CSh
BoundLocal inlineArrayLocal = _factory.StoreToTemp(new BoundDefaultExpression(syntax, inlineArrayType), out assignmentToTemp);
var sideEffects = ArrayBuilder<BoundExpression>.GetInstance();
sideEffects.Add(assignmentToTemp);
locals.Add(inlineArrayLocal.LocalSymbol);
_additionalLocals.Add(inlineArrayLocal.LocalSymbol);

// Populate the inline array.
// InlineArrayElementRef<<>y__InlineArrayN<ElementType>, ElementType>(ref tmp, 0) = element0;
Expand Down
190 changes: 160 additions & 30 deletions src/Compilers/CSharp/Test/Emit2/Semantics/CollectionExpressionTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -20919,7 +20919,7 @@ .maxstack 5
}

[Fact]
public void Span_SingleElement_TempReuse()
public void Span_SingleElement_TempsAreNotReused()
{
var source = """
using System;
Expand All @@ -20933,6 +20933,7 @@ static void M(int x)
{
Span<int> y = [x];
Console.Write(y[0]);
y[0]++;
}
{
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Do we have a test where we create single-element collection twice in the same block (not different blocks like in this test)?

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Indeed this is the type of test that I was most worried about. Re-using the temp between two different blocks is just fine. Re-using it within the same block would be a significant codegen error. Want to make sure that we're not doing that.

Span<int> y = [x];
Expand All @@ -20942,13 +20943,13 @@ static void M(int x)
}
""";

var verifier = CompileAndVerify(source, targetFramework: TargetFramework.Net80, verify: Verification.Skipped, expectedOutput: IncludeExpectedOutput("11"));
var verifier = CompileAndVerify(source, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe, verify: Verification.Skipped, expectedOutput: IncludeExpectedOutput("11"));
verifier.VerifyDiagnostics();

verifier.VerifyIL("Program.M", """
{
// Code size 49 (0x31)
.maxstack 2
// Code size 62 (0x3e)
.maxstack 3
.locals init (int V_0,
int V_1,
System.Span<int> V_2, //y
Expand All @@ -20963,26 +20964,34 @@ .maxstack 2
IL_000d: call "ref int System.Span<int>.this[int].get"
IL_0012: ldind.i4
IL_0013: call "void System.Console.Write(int)"
IL_0018: ldarg.0
IL_0019: stloc.1
IL_001a: ldloca.s V_1
IL_001c: newobj "System.Span<int>..ctor(ref int)"
IL_0021: stloc.3
IL_0022: ldloca.s V_3
IL_0024: ldc.i4.0
IL_0025: call "ref int System.Span<int>.this[int].get"
IL_002a: ldind.i4
IL_002b: call "void System.Console.Write(int)"
IL_0030: ret
IL_0018: ldloca.s V_2
IL_001a: ldc.i4.0
IL_001b: call "ref int System.Span<int>.this[int].get"
IL_0020: dup
IL_0021: ldind.i4
IL_0022: ldc.i4.1
IL_0023: add
IL_0024: stind.i4
IL_0025: ldarg.0
IL_0026: stloc.1
IL_0027: ldloca.s V_1
IL_0029: newobj "System.Span<int>..ctor(ref int)"
IL_002e: stloc.3
IL_002f: ldloca.s V_3
IL_0031: ldc.i4.0
IL_0032: call "ref int System.Span<int>.this[int].get"
IL_0037: ldind.i4
IL_0038: call "void System.Console.Write(int)"
IL_003d: ret
}
""");

verifier = CompileAndVerify(source, targetFramework: TargetFramework.Net70, verify: Verification.Skipped, expectedOutput: IncludeExpectedOutput("11"));
verifier = CompileAndVerify(source, targetFramework: TargetFramework.Net70, options: TestOptions.ReleaseExe, verify: Verification.Skipped, expectedOutput: IncludeExpectedOutput("11"));
verifier.VerifyDiagnostics();

verifier.VerifyIL("Program.M", """
{
// Code size 63 (0x3f)
// Code size 76 (0x4c)
.maxstack 5
.locals init (System.Span<int> V_0, //y
System.Span<int> V_1) //y
Expand All @@ -20999,20 +21008,141 @@ .maxstack 5
IL_0014: call "ref int System.Span<int>.this[int].get"
IL_0019: ldind.i4
IL_001a: call "void System.Console.Write(int)"
IL_001f: ldloca.s V_1
IL_0021: ldc.i4.1
IL_0022: newarr "int"
IL_001f: ldloca.s V_0
IL_0021: ldc.i4.0
IL_0022: call "ref int System.Span<int>.this[int].get"
IL_0027: dup
IL_0028: ldc.i4.0
IL_0029: ldarg.0
IL_002a: stelem.i4
IL_002b: call "System.Span<int>..ctor(int[])"
IL_0030: ldloca.s V_1
IL_0032: ldc.i4.0
IL_0033: call "ref int System.Span<int>.this[int].get"
IL_0038: ldind.i4
IL_0039: call "void System.Console.Write(int)"
IL_003e: ret
IL_0028: ldind.i4
IL_0029: ldc.i4.1
IL_002a: add
IL_002b: stind.i4
IL_002c: ldloca.s V_1
IL_002e: ldc.i4.1
IL_002f: newarr "int"
IL_0034: dup
IL_0035: ldc.i4.0
IL_0036: ldarg.0
IL_0037: stelem.i4
IL_0038: call "System.Span<int>..ctor(int[])"
IL_003d: ldloca.s V_1
IL_003f: ldc.i4.0
IL_0040: call "ref int System.Span<int>.this[int].get"
IL_0045: ldind.i4
IL_0046: call "void System.Console.Write(int)"
IL_004b: ret
}
""");
}

[Fact]
public void Span_SingleElement_TempsAreNotReused_SameBlock()
{
var source = """
using System;

class Program
{
static void Main() => M(1);

static void M(int x)
{
Span<int> y = [x];
Console.Write(y[0]);
y[0]++;

Span<int> z = [x];
Console.Write(z[0]);
}
}
""";

var verifier = CompileAndVerify(source, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe, verify: Verification.Skipped, expectedOutput: IncludeExpectedOutput("11"));
verifier.VerifyDiagnostics();

verifier.VerifyIL("Program.M", """
{
// Code size 62 (0x3e)
.maxstack 3
.locals init (System.Span<int> V_0, //y
System.Span<int> V_1, //z
int V_2,
int V_3)
IL_0000: ldarg.0
IL_0001: stloc.2
IL_0002: ldloca.s V_2
IL_0004: newobj "System.Span<int>..ctor(ref int)"
IL_0009: stloc.0
IL_000a: ldloca.s V_0
IL_000c: ldc.i4.0
IL_000d: call "ref int System.Span<int>.this[int].get"
IL_0012: ldind.i4
IL_0013: call "void System.Console.Write(int)"
IL_0018: ldloca.s V_0
IL_001a: ldc.i4.0
IL_001b: call "ref int System.Span<int>.this[int].get"
IL_0020: dup
IL_0021: ldind.i4
IL_0022: ldc.i4.1
IL_0023: add
IL_0024: stind.i4
IL_0025: ldarg.0
IL_0026: stloc.3
IL_0027: ldloca.s V_3
IL_0029: newobj "System.Span<int>..ctor(ref int)"
IL_002e: stloc.1
IL_002f: ldloca.s V_1
IL_0031: ldc.i4.0
IL_0032: call "ref int System.Span<int>.this[int].get"
IL_0037: ldind.i4
IL_0038: call "void System.Console.Write(int)"
IL_003d: ret
}
""");

verifier = CompileAndVerify(source, targetFramework: TargetFramework.Net70, options: TestOptions.ReleaseExe, verify: Verification.Skipped, expectedOutput: IncludeExpectedOutput("11"));
verifier.VerifyDiagnostics();

verifier.VerifyIL("Program.M", """
{
// Code size 76 (0x4c)
.maxstack 5
.locals init (System.Span<int> V_0, //y
System.Span<int> V_1) //z
IL_0000: ldloca.s V_0
IL_0002: ldc.i4.1
IL_0003: newarr "int"
IL_0008: dup
IL_0009: ldc.i4.0
IL_000a: ldarg.0
IL_000b: stelem.i4
IL_000c: call "System.Span<int>..ctor(int[])"
IL_0011: ldloca.s V_0
IL_0013: ldc.i4.0
IL_0014: call "ref int System.Span<int>.this[int].get"
IL_0019: ldind.i4
IL_001a: call "void System.Console.Write(int)"
IL_001f: ldloca.s V_0
IL_0021: ldc.i4.0
IL_0022: call "ref int System.Span<int>.this[int].get"
IL_0027: dup
IL_0028: ldind.i4
IL_0029: ldc.i4.1
IL_002a: add
IL_002b: stind.i4
IL_002c: ldloca.s V_1
IL_002e: ldc.i4.1
IL_002f: newarr "int"
IL_0034: dup
IL_0035: ldc.i4.0
IL_0036: ldarg.0
IL_0037: stelem.i4
IL_0038: call "System.Span<int>..ctor(int[])"
IL_003d: ldloca.s V_1
IL_003f: ldc.i4.0
IL_0040: call "ref int System.Span<int>.this[int].get"
IL_0045: ldind.i4
IL_0046: call "void System.Console.Write(int)"
IL_004b: ret
}
""");
}
Expand Down
71 changes: 71 additions & 0 deletions src/Compilers/CSharp/Test/Emit2/Semantics/ParamsCollectionTests.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -359,6 +359,77 @@ void assertAttributeData(string name)
}
}

[Fact]
public void Span_SingleElement_TempsAreNotReused()
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Did we file a bug to follow up on this? This is a place where future optimizations could very much benefit.

In this particular case the win is pretty minor. It's just an extra int or two on the stack. Consider though when we're passing instead 3 parameters and storing lots of redundant inline array temporaries. That can start to make the stack frame size increase be noticable and may deserve an optimization pass to do proper reuse.

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#73269

{
var source = """
using System;

class Program
{
static void Main()
{
M(1);
M(2);
}

static void M(params Span<int> span)
{
Console.Write(span[0]);
Console.Write(span.Length);
}
}
""";

var verifier = CompileAndVerify(source, targetFramework: TargetFramework.Net80, options: TestOptions.ReleaseExe, expectedOutput: "1121");
verifier.VerifyDiagnostics();
verifier.VerifyIL("Program.Main", """
{
// Code size 29 (0x1d)
.maxstack 1
.locals init (int V_0,
int V_1)
IL_0000: ldc.i4.1
IL_0001: stloc.0
IL_0002: ldloca.s V_0
IL_0004: newobj "System.Span<int>..ctor(ref int)"
IL_0009: call "void Program.M(params System.Span<int>)"
IL_000e: ldc.i4.2
IL_000f: stloc.1
IL_0010: ldloca.s V_1
IL_0012: newobj "System.Span<int>..ctor(ref int)"
IL_0017: call "void Program.M(params System.Span<int>)"
IL_001c: ret
}
""");

verifier = CompileAndVerify(source, targetFramework: TargetFramework.Net70, options: TestOptions.ReleaseExe, expectedOutput: "1121");
verifier.VerifyDiagnostics();
verifier.VerifyIL("Program.Main", """
{
// Code size 41 (0x29)
.maxstack 4
IL_0000: ldc.i4.1
IL_0001: newarr "int"
IL_0006: dup
IL_0007: ldc.i4.0
IL_0008: ldc.i4.1
IL_0009: stelem.i4
IL_000a: newobj "System.Span<int>..ctor(int[])"
IL_000f: call "void Program.M(params System.Span<int>)"
IL_0014: ldc.i4.1
IL_0015: newarr "int"
IL_001a: dup
IL_001b: ldc.i4.0
IL_001c: ldc.i4.2
IL_001d: stelem.i4
IL_001e: newobj "System.Span<int>..ctor(int[])"
IL_0023: call "void Program.M(params System.Span<int>)"
IL_0028: ret
}
""");
}

[Fact]
public void String()
{
Expand Down