Replace `Unsafe.As` with `Unsafe.Read{Write}Unaligned` #113293

xtqqczze · 2025-03-08T14:45:17Z

Doesn't depend upon implementation specific behaviour, i.e. that data is properly aligned.

For System.Private.CoreLib

Doesn't depend upon implementation specific behaviour, i.e. that the array data is properly aligned.

teo-tsirpanis · 2025-03-08T16:35:23Z

src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/CastHelpers.cs

                else
                {
-                    Unsafe.As<byte, bool>(ref destPtr) = true;
+                    Unsafe.WriteUnaligned(ref destPtr, true);


Bools are one byte long so this is not needed; accessing them is always aligned.

Here and below, can you replace these byte <-> bool with ternary ops and run MihuBot?

EgorBo · 2025-03-08T17:36:51Z

src/coreclr/System.Private.CoreLib/src/System/RuntimeType.BoxCache.cs

                    // If the allocator is null, then we shouldn't allocate and make a copy,
                    // we should return the data as the object it currently is.
-                    return Unsafe.As<byte, object>(ref data);
+                    return Unsafe.ReadUnaligned<object>(ref data);


GC references are not expected to be not-aligned. We can probably do Unsafe.Read here with a debug assert checking the alignment.

Same in other places

I think we can introduce an internal helper in Unsafe: ReadAligned that checks alignment in Debug.Assert

EgorBo · 2025-03-08T17:40:44Z

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Enum.NativeAot.cs


                case EETypeElementType.Byte:
-                    result = (ulong)(long)Unsafe.As<byte, byte>(ref pValue);
+                    result = (ulong)(long)Unsafe.ReadUnaligned<byte>(ref pValue);


pValue is already byte, it doesn't need Unsafe. Same for SByte

EgorBo · 2025-03-08T17:42:01Z

src/libraries/System.Private.CoreLib/src/System/Enum.cs

            return InternalGetCorElementType() switch
            {
-                CorElementType.ELEMENT_TYPE_I1 => Unsafe.As<byte, sbyte>(ref data),
+                CorElementType.ELEMENT_TYPE_I1 => Unsafe.ReadUnaligned<sbyte>(ref data),


same here: sbyte is read from byte with unsafe

EgorBo · 2025-03-08T17:42:22Z

src/libraries/System.Private.CoreLib/src/System/Enum.cs

            return InternalGetCorElementType() switch
            {
-                CorElementType.ELEMENT_TYPE_I1 => Unsafe.As<byte, sbyte>(ref data).GetHashCode(),
+                CorElementType.ELEMENT_TYPE_I1 => Unsafe.ReadUnaligned<sbyte>(ref data).GetHashCode(),


EgorBo · 2025-03-08T17:42:38Z

src/libraries/System.Private.CoreLib/src/System/Enum.cs

            {
                case CorElementType.ELEMENT_TYPE_I1:
-                    return Unsafe.As<byte, sbyte>(ref pThisValue).CompareTo(Unsafe.As<byte, sbyte>(ref pTargetValue));
+                    return Unsafe.ReadUnaligned<sbyte>(ref pThisValue).CompareTo(Unsafe.ReadUnaligned<sbyte>(ref pTargetValue));


src/libraries/System.Private.CoreLib/src/System/Guid.cs

jkotas · 2025-03-09T00:42:47Z

src/coreclr/nativeaot/System.Private.CoreLib/src/System/Enum.NativeAot.cs

            {
                case EETypeElementType.Char:
-                    result = (ulong)(long)Unsafe.As<byte, char>(ref pValue);
+                    result = (ulong)(long)Unsafe.ReadUnaligned<char>(ref pValue);


The pValue is expected to be aligned properly. Changes in this method are unnecessary deoptimization.

jkotas · 2025-03-09T00:45:19Z

src/coreclr/nativeaot/System.Private.CoreLib/src/System/InvokeUtils.cs

                    {
                        case EETypeElementType.Single:
-                            Unsafe.As<byte, float>(ref dstValue) = srcValue;
+                            Unsafe.WriteUnaligned(ref dstValue, (float)srcValue);


Same here. All changes in this method are unnecessary deoptimizations.

jkotas

Many of the changes are unnecessary deoptimizations (I have not commented on all of them). You need to check what the surrounding code does and only fix this in places where unaligned access is actually possible.

EgorBo · 2025-03-09T22:20:05Z

@MihuBot

dotnet-policy-service · 2025-03-31T06:59:44Z

Tagging subscribers to this area: @dotnet/area-system-runtime
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dotnet-policy-service · 2025-05-09T14:34:30Z

Draft Pull Request was automatically closed for 30 days of inactivity. Please let us know if you'd like to reopen it.

Doesn't depend upon implementation specific behaviour, i.e. that data is properly aligned. Split from dotnet#113293.

Replace Unsafe.As with Unsafe.ReadUnaligned for loads

d03bb08

Doesn't depend upon implementation specific behaviour, i.e. that the array data is properly aligned.

xtqqczze requested review from MichalStrehovsky, steveisok and vitek-karas as code owners March 8, 2025 14:45

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teo-tsirpanis reviewed Mar 8, 2025

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xtqqczze changed the title ~~Replace Unsafe.As with Unsafe.ReadUnaligned for loads~~ Replace Unsafe.As with Unsafe.Read{Write}Unaligned Mar 8, 2025

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[JitDiff X64] [xtqqczze] Replace Unsafe.As with Unsafe.Read{Write}Unaligned MihuBot/runtime-utils#1044

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xtqqczze added a commit to xtqqczze/dotnet-runtime that referenced this pull request Jun 7, 2025

Replace Unsafe.As in System.Array

92b351f

Doesn't depend upon implementation specific behaviour, i.e. that data is properly aligned. Split from dotnet#113293.

xtqqczze mentioned this pull request Jun 7, 2025

Use Unsafe.BitCast in System.Array #116404

Merged

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Replace Unsafe.As with Unsafe.Read{Write}Unaligned #113293

Replace Unsafe.As with Unsafe.Read{Write}Unaligned #113293

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Replace `Unsafe.As` with `Unsafe.Read{Write}Unaligned` #113293

Replace `Unsafe.As` with `Unsafe.Read{Write}Unaligned` #113293

xtqqczze commented Mar 8, 2025 •

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EgorBo Mar 8, 2025 •

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jkotas Mar 9, 2025 •

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