Matrix swizzle writes (#2713)

* Add a bunch of builder emit wrappers for constant indices

To avoid cluttering any calling code with int instruction construction

* Matrix swizzle stores

Closes https://github.com/shader-slang/slang/issues/2512

* Matrix swizzle store tests

* Squash vs warnings

* Select scalar for singular swizzles

* Test singular swizzle materialization

* Use IRIntegerValue over UInt for IR wrappers

* Correct size of swizzle vector type

* Remove variable shadowing

11 files changed, 457 insertions, 42 deletions

diff --git a/source/slang/slang-ir-insts.h b/source/slang/slang-ir-insts.h
index bf0a5d4cd..fe658566c 100644
--- a/source/slang/slang-ir-insts.h
+++ b/source/slang/slang-ir-insts.h
@@ -2810,6 +2810,10 @@ public:
         IRType* elementType,
         IRInst* elementCount);
 
+    IRVectorType* getVectorType(
+        IRType* elementType,
+        IRIntegerValue elementCount);
+
     IRMatrixType* getMatrixType(
         IRType* elementType,
         IRInst* rowCount,
@@ -3324,6 +3328,10 @@ public:
 
     IRInst* emitElementExtract(
         IRInst* base,
+        IRIntegerValue index);
+
+    IRInst* emitElementExtract(
+        IRInst* base,
         const ArrayView<IRInst*>& accessChain);
 
     IRInst* emitElementAddress(
@@ -3337,9 +3345,14 @@ public:
 
     IRInst* emitElementAddress(
         IRInst* basePtr,
+        IRIntegerValue index);
+
+    IRInst* emitElementAddress(
+        IRInst* basePtr,
         const ArrayView<IRInst*>& accessChain);
 
     IRInst* emitUpdateElement(IRInst* base, IRInst* index, IRInst* newElement);
+    IRInst* emitUpdateElement(IRInst* base, IRIntegerValue index, IRInst* newElement);
     IRInst* emitUpdateElement(IRInst* base, const List<IRInst*>& accessChain, IRInst* newElement);
 
     IRInst* emitGetAddress(
diff --git a/source/slang/slang-ir.cpp b/source/slang/slang-ir.cpp
index bd2271953..e624ef1fd 100644
--- a/source/slang/slang-ir.cpp
+++ b/source/slang/slang-ir.cpp
@@ -2809,6 +2809,13 @@ namespace Slang
             operands);
     }
 
+    IRVectorType* IRBuilder::getVectorType(
+        IRType* elementType,
+        IRIntegerValue elementCount)
+    {
+        return getVectorType(elementType, getIntValue(getIntType(), elementCount));
+    }
+
     IRMatrixType* IRBuilder::getMatrixType(
         IRType* elementType,
         IRInst* rowCount,
@@ -4607,6 +4614,13 @@ namespace Slang
 
     IRInst* IRBuilder::emitElementExtract(
         IRInst* base,
+        IRIntegerValue index)
+    {
+        return emitElementExtract(base, getIntValue(getIntType(), index));
+    }
+
+    IRInst* IRBuilder::emitElementExtract(
+        IRInst* base,
         const ArrayView<IRInst*>& accessChain)
     {
         for (auto access : accessChain)
@@ -4653,6 +4667,13 @@ namespace Slang
 
     IRInst* IRBuilder::emitElementAddress(
         IRInst* basePtr,
+        IRIntegerValue index)
+    {
+        return emitElementAddress(basePtr, getIntValue(getIntType(), index));
+    }
+
+    IRInst* IRBuilder::emitElementAddress(
+        IRInst* basePtr,
         IRInst* index)
     {
         IRType* type = nullptr;
@@ -4726,6 +4747,11 @@ namespace Slang
         return inst;
     }
 
+    IRInst* IRBuilder::emitUpdateElement(IRInst* base, IRIntegerValue index, IRInst* newElement)
+    {
+        return emitUpdateElement(base, getIntValue(getIntType(), index), newElement);
+    }
+
     IRInst* IRBuilder::emitUpdateElement(IRInst* base, const List<IRInst*>& accessChain, IRInst* newElement)
     {
         List<IRInst*> args;
diff --git a/source/slang/slang-lower-to-ir.cpp b/source/slang/slang-lower-to-ir.cpp
index ada2e043e..1fba0e2f8 100644
--- a/source/slang/slang-lower-to-ir.cpp
+++ b/source/slang/slang-lower-to-ir.cpp
@@ -90,6 +90,7 @@ struct SubscriptInfo : ExtendedValueInfo
 struct BoundStorageInfo;
 struct BoundMemberInfo;
 struct SwizzledLValueInfo;
+struct SwizzledMatrixLValueInfo;
 struct CopiedValInfo;
 struct ExtractedExistentialValInfo;
 
@@ -125,6 +126,9 @@ struct LoweredValInfo
         // The result of applying swizzling to an l-value
         SwizzledLValue,
 
+        // The result of applying swizzling to an l-value matrix
+        SwizzledMatrixLValue,
+
         // The value extracted from an opened existential
         ExtractedExistential,
     };
@@ -194,12 +198,21 @@ struct LoweredValInfo
     static LoweredValInfo swizzledLValue(
         SwizzledLValueInfo* extInfo);
 
+    static LoweredValInfo swizzledMatrixLValue(
+        SwizzledMatrixLValueInfo* extInfo);
+
     SwizzledLValueInfo* getSwizzledLValueInfo()
     {
         SLANG_ASSERT(flavor == Flavor::SwizzledLValue);
         return (SwizzledLValueInfo*)ext;
     }
 
+    SwizzledMatrixLValueInfo* getSwizzledMatrixLValueInfo()
+    {
+        SLANG_ASSERT(flavor == Flavor::SwizzledMatrixLValue);
+        return (SwizzledMatrixLValueInfo*)ext;
+    }
+
     static LoweredValInfo extractedExistential(
         ExtractedExistentialValInfo* extInfo);
 
@@ -295,6 +308,23 @@ struct SwizzledLValueInfo : ExtendedValueInfo
     UInt            elementIndices[4];
 };
 
+// Represents the result of a matrix swizzle operation in an l-value context.
+// The same non-contiguous and no-duplicate rules as above apply.
+struct SwizzledMatrixLValueInfo : ExtendedValueInfo
+{
+    // The type of the expression.
+    IRType*         type;
+
+    // The base expression (this should be an l-value)
+    LoweredValInfo  base;
+
+    // The number of elements in the swizzle
+    UInt            elementCount;
+
+    // The coords for the elements being swizzled, zero indexed
+    MatrixCoord     elementCoords[4];
+};
+
 // Represents the results of extractng a value of
 // some (statically unknown) concrete type from
 // an existential, in an l-value context.
@@ -352,6 +382,15 @@ LoweredValInfo LoweredValInfo::swizzledLValue(
     return info;
 }
 
+LoweredValInfo LoweredValInfo::swizzledMatrixLValue(
+        SwizzledMatrixLValueInfo* extInfo)
+{
+    LoweredValInfo info;
+    info.flavor = Flavor::SwizzledMatrixLValue;
+    info.ext = extInfo;
+    return info;
+}
+
 LoweredValInfo LoweredValInfo::extractedExistential(
     ExtractedExistentialValInfo* extInfo)
 {
@@ -1036,6 +1075,32 @@ top:
                 swizzleInfo->elementIndices));
         }
 
+    case LoweredValInfo::Flavor::SwizzledMatrixLValue:
+        {
+            auto swizzleInfo = lowered.getSwizzledMatrixLValueInfo();
+            auto base = getSimpleVal(context, swizzleInfo->base);
+            if(const auto type = as<IRMatrixType>(base->getDataType()))
+            {
+                IRInst* components[4];
+                for(UInt i = 0; i < swizzleInfo->elementCount; ++i)
+                {
+                    components[i] = builder->emitElementExtract(
+                        builder->emitElementExtract(base,swizzleInfo->elementCoords[i].row),
+                        swizzleInfo->elementCoords[i].col);
+                }
+                return swizzleInfo->elementCount == 1
+                    ? LoweredValInfo::simple(components[0])
+                    : LoweredValInfo::simple(builder->emitMakeVector(
+                        builder->getVectorType(type->getElementType(), swizzleInfo->elementCount),
+                        swizzleInfo->elementCount,
+                        components));
+            }
+            else
+            {
+                SLANG_UNEXPECTED("Expected a matrix type in matrix swizzle");
+            }
+        }
+
     case LoweredValInfo::Flavor::ExtractedExistential:
         {
             auto info = lowered.getExtractedExistentialValInfo();
@@ -2468,6 +2533,7 @@ void addInArg(
     case LoweredValInfo::Flavor::Simple:
     case LoweredValInfo::Flavor::Ptr:
     case LoweredValInfo::Flavor::SwizzledLValue:
+    case LoweredValInfo::Flavor::SwizzledMatrixLValue:
     case LoweredValInfo::Flavor::BoundStorage:
     case LoweredValInfo::Flavor::BoundMember:
     case LoweredValInfo::Flavor::ExtractedExistential:
@@ -4571,31 +4637,55 @@ struct LValueExprLoweringVisitor : ExprLoweringVisitorBase<LValueExprLoweringVis
 
     // When visiting a swizzle expression in an l-value context,
     // we need to construct a "swizzled l-value."
-    LoweredValInfo visitMatrixSwizzleExpr(MatrixSwizzleExpr*)
-    {
-        SLANG_UNIMPLEMENTED_X("matrix swizzle lvalue case");
-    }
-
-    // When visiting a swizzle expression in an l-value context,
-    // we need to construct a "sizzled l-value."
-    LoweredValInfo visitSwizzleExpr(SwizzleExpr* expr)
+    LoweredValInfo visitMatrixSwizzleExpr(MatrixSwizzleExpr* expr)
     {
         auto irType = lowerType(context, expr->type);
         auto loweredBase = lowerRValueExpr(context, expr->base);
 
-        RefPtr<SwizzledLValueInfo> swizzledLValue = new SwizzledLValueInfo();
+        RefPtr<SwizzledMatrixLValueInfo> swizzledLValue = new SwizzledMatrixLValueInfo();
         swizzledLValue->type = irType;
 
         UInt elementCount = (UInt)expr->elementCount;
         swizzledLValue->elementCount = elementCount;
 
-        // As a small optimization, we will detect if the base expression
-        // has also lowered into a swizzle and only return a single
-        // swizzle instead of nested swizzles.
+        // In the default case, we can just copy the indices being
+        // used for the swizzle over directly from the expression,
+        // and use the base as-is.
+        //
+        swizzledLValue->base = loweredBase;
+        for (UInt ii = 0; ii < elementCount; ++ii)
+        {
+            swizzledLValue->elementCoords[ii] = expr->elementCoords[ii];
+        }
+
+        context->shared->extValues.add(swizzledLValue);
+        return LoweredValInfo::swizzledMatrixLValue(swizzledLValue);
+    }
+
+    // When visiting a swizzle expression in an l-value context,
+    // we need to construct a "swizzled l-value."
+    LoweredValInfo visitSwizzleExpr(SwizzleExpr* expr)
+    {
+        auto irType = lowerType(context, expr->type);
+        auto loweredBase = lowerLValueExpr(context, expr->base);
+        UInt elementCount = (UInt)expr->elementCount;
+
+        // Assign to 'bs' the elements from 'as' according to the first 'n' indices in 'is'
+        auto backpermute = [](UInt n, const auto* as, const int* is, auto* bs)
+        {
+            for(UInt i = 0; i < n; ++i)
+            {
+                bs[i] = as[is[i]];
+            }
+        };
+
+        // As required by the implementation of 'assign' and as a small
+        // optimization, we will detect if the base expression has also lowered
+        // into a swizzle and only return a single swizzle instead of nested
+        // swizzles.
         //
         // E.g., if we have input like `foo[i].zw.y` we should optimize it
         // down to just `foo[i].w`.
-        //
         if(loweredBase.flavor == LoweredValInfo::Flavor::SwizzledLValue)
         {
             auto baseSwizzleInfo = loweredBase.getSwizzledLValueInfo();
@@ -4604,43 +4694,66 @@ struct LValueExprLoweringVisitor : ExprLoweringVisitorBase<LValueExprLoweringVis
             // `foo[i]` in our example above), but will need to remap
             // the swizzle indices it uses.
             //
+
+            RefPtr<SwizzledLValueInfo> swizzledLValue = new SwizzledLValueInfo;
+            swizzledLValue->type = irType;
             swizzledLValue->base = baseSwizzleInfo->base;
-            for (UInt ii = 0; ii < elementCount; ++ii)
-            {
-                // First we get the swizzle element of the "outer" swizzle,
-                // as it was written by the user. In our running example of
-                // `foo[i].zw.y` this is the `y` element reference.
-                //
-                UInt originalElementIndex = UInt(expr->elementIndices[ii]);
+            swizzledLValue->elementCount = elementCount;
 
-                // Next we will use that original element index to figure
-                // out which of the elements of the original swizzle this
-                // should map to.
-                //
-                // In our example, `y` means index 1, and so we fetch
-                // element 1 from the inner swizzle sequence `zw`, to get `w`.
-                //
-                SLANG_ASSERT(originalElementIndex < baseSwizzleInfo->elementCount);
-                UInt remappedElementIndex = baseSwizzleInfo->elementIndices[originalElementIndex];
+            // Take the swizzle element of the "outer" swizzle, as it was
+            // written by the user. In our running example of `foo[i].zw.y`
+            // this is the `y` element reference.
+            //
+            // Use that original element index to figure out which of the
+            // elements of the original swizzle this should map to.
+            backpermute(
+                swizzledLValue->elementCount,
+                baseSwizzleInfo->elementIndices,
+                expr->elementIndices,
+                swizzledLValue->elementIndices);
 
-                swizzledLValue->elementIndices[ii] = remappedElementIndex;
-            }
+            context->shared->extValues.add(swizzledLValue);
+            return LoweredValInfo::swizzledLValue(swizzledLValue);
+        }
+        else if(loweredBase.flavor == LoweredValInfo::Flavor::SwizzledMatrixLValue)
+        {
+            auto baseSwizzleInfo = loweredBase.getSwizzledMatrixLValueInfo();
+
+            RefPtr<SwizzledMatrixLValueInfo> swizzledLValue = new SwizzledMatrixLValueInfo();
+            swizzledLValue->type = irType;
+            swizzledLValue->base = baseSwizzleInfo->base;
+            swizzledLValue->elementCount = elementCount;
+
+            // Use the index of our swizzle to permute the index of the base
+            // swizzle as above
+            backpermute(
+                swizzledLValue->elementCount,
+                baseSwizzleInfo->elementCoords,
+                expr->elementIndices,
+                swizzledLValue->elementCoords);
+
+            context->shared->extValues.add(swizzledLValue);
+            return LoweredValInfo::swizzledMatrixLValue(swizzledLValue);
         }
         else
         {
+            RefPtr<SwizzledLValueInfo> swizzledLValue = new SwizzledLValueInfo;
+            swizzledLValue->type = irType;
+            swizzledLValue->base = loweredBase;
+            swizzledLValue->elementCount = elementCount;
+
             // In the default case, we can just copy the indices being
             // used for the swizzle over directly from the expression,
             // and use the base as-is.
             //
-            swizzledLValue->base = loweredBase;
             for (UInt ii = 0; ii < elementCount; ++ii)
             {
                 swizzledLValue->elementIndices[ii] = (UInt) expr->elementIndices[ii];
             }
-        }
 
-        context->shared->extValues.add(swizzledLValue);
-        return LoweredValInfo::swizzledLValue(swizzledLValue);
+            context->shared->extValues.add(swizzledLValue);
+            return LoweredValInfo::swizzledLValue(swizzledLValue);
+        }
     }
 };
 
@@ -5845,6 +5958,30 @@ LoweredValInfo tryGetAddress(
         }
         break;
 
+    // TODO(Ellie): There's an uncomfortable level of duplication here...
+    case LoweredValInfo::Flavor::SwizzledMatrixLValue:
+        {
+            auto originalSwizzleInfo = val.getSwizzledMatrixLValueInfo();
+            auto originalBase = originalSwizzleInfo->base;
+
+            UInt elementCount = originalSwizzleInfo->elementCount;
+
+            auto newBase = tryGetAddress(context, originalBase, TryGetAddressMode::Aggressive);
+            RefPtr<SwizzledMatrixLValueInfo> newSwizzleInfo = new SwizzledMatrixLValueInfo();
+            context->shared->extValues.add(newSwizzleInfo);
+
+            newSwizzleInfo->base = newBase;
+            newSwizzleInfo->type = originalSwizzleInfo->type;
+            newSwizzleInfo->elementCount = elementCount;
+            for(UInt ee = 0; ee < elementCount; ++ee)
+            {
+                newSwizzleInfo->elementCoords[ee] = originalSwizzleInfo->elementCoords[ee];
+            }
+
+            return LoweredValInfo::swizzledMatrixLValue(newSwizzleInfo);
+        }
+        break;
+
     // TODO: are there other cases we need to handled here?
 
     default:
@@ -5889,6 +6026,26 @@ void assign(
 
     auto builder = context->irBuilder;
 
+    // If there's a single element, just emit a regular store, otherwise
+    // proceed with a swizzle store
+    auto swizzledStore = [builder](
+        IRInst*         dest,
+        IRInst*         source,
+        UInt            elementCount,
+        UInt const*     elementIndices){
+        if(elementCount == 1)
+        {
+            return builder->emitStore(
+                builder->emitElementAddress(dest, elementIndices[0]),
+                source);
+        }
+        return builder->emitSwizzledStore(
+            dest,
+            source,
+            elementCount,
+            elementIndices);
+    };
+
 top:
     switch (left.flavor)
     {
@@ -5967,14 +6124,8 @@ top:
                     // it around, in comparison to a simpler model where
                     // we simply form a pointer to each of the vector
                     // elements and write to them individually.
-                    //
-                    // TODO: we might also consider just special-casing
-                    // single-element swizzles so that the common case
-                    // can turn into a simple `store` instead of a
-                    // `swizzledStore`.
-                    //
                     IRInst* irRightVal = getSimpleVal(context, right);
-                    builder->emitSwizzledStore(
+                    swizzledStore(
                         loweredBase.val,
                         irRightVal,
                         swizzleInfo->elementCount,
@@ -5985,6 +6136,108 @@ top:
         }
         break;
 
+    case LoweredValInfo::Flavor::SwizzledMatrixLValue:
+        {
+            // The `left` value is of the form `<base>.<swizzleElements>`.
+            // How we will handle this depends on what `base` looks like:
+            auto swizzleInfo = left.getSwizzledMatrixLValueInfo();
+            auto loweredBase = swizzleInfo->base;
+
+            IRInst* irRightVal = getSimpleVal(context, right);
+
+            const UInt maxRowIndex = 4;
+            const UInt maxCols = 4; // swizzleInfo->elementCount;
+
+            // Sort the swizzle elements according to the row to which they
+            // write.
+            // Using row-major terminology
+
+            // The number of element writes in each row
+            UInt rowSizes[maxRowIndex] = {};
+            // The columns being written to in each row
+            UInt rowWrites[maxRowIndex][maxCols];
+            // The RHS element indices being written in each row
+            UInt rowIndices[maxRowIndex][maxCols];
+            for(UInt i = 0; i < swizzleInfo->elementCount; ++i)
+            {
+                const auto& c = swizzleInfo->elementCoords[i];
+                auto& rowSize = rowSizes[c.row];
+                rowWrites[c.row][rowSize] = c.col;
+                rowIndices[c.row][rowSize] = i;
+                ++rowSize;
+            }
+
+            const auto rElemType =
+                composeGetters<IRType>(irRightVal, &IRInst::getDataType, &IRVectorType::getElementType);
+
+            switch( loweredBase.flavor )
+            {
+            case LoweredValInfo::Flavor::Ptr:
+                {
+                    // Matrix swizzle writes are implemented as several vector swizzle writes
+                    for(UInt r = 0; r < maxRowIndex; ++r)
+                    {
+                        // Skip if we have nothing in this row
+                        if(rowSizes[r] == 0)
+                        {
+                            continue;
+                        }
+                        const auto rowAddr = builder->emitElementAddress(loweredBase.val, r);
+                        // Only select the RHS elements if it's a vector
+                        const auto rSwizzled = rElemType
+                            ? builder->emitSwizzle(
+                                builder->getVectorType(rElemType, rowSizes[r]),
+                                irRightVal,
+                                rowSizes[r],
+                                rowIndices[r])
+                            : irRightVal;
+                        swizzledStore(
+                            rowAddr,
+                            rSwizzled,
+                            rowSizes[r],
+                            rowWrites[r]);
+                    }
+                }
+                break;
+            default:
+                {
+                    // As above, our fallback position is to lower via a
+                    // temporary, e.g.:
+                    //
+                    //      float4x3 tmp = <base>;
+                    //      tmp[0].xzy = float3(...);
+                    //      tmp[1].yxz = float3(...);
+                    //      tmp[4].yzx = float3(...);
+                    //      <base> = tmp;
+                    //
+                    // Create a variable, and use the ptr writing matrix
+                    // swizzle assignment above to fill it, then write that back
+                    // to the l value. This approach generates the neatest IR
+                    const auto beforeLValue = getSimpleVal(context, loweredBase);
+                    const auto type = beforeLValue->getDataType();
+
+                    // Store our initial lvalue in tmp
+                    const auto tmpVar = builder->emitVar(type);
+                    builder->emitStore(tmpVar, beforeLValue);
+
+                    // Make a new swizzle write to write into this pointer
+                    auto nextSwizzleInfo = left.getSwizzledMatrixLValueInfo();
+                    SwizzledMatrixLValueInfo nextInfo = *nextSwizzleInfo;
+                    nextInfo.base = LoweredValInfo::ptr(tmpVar);
+
+                    // Perform that swizzling assignment
+                    assign(context, LoweredValInfo::swizzledMatrixLValue(&nextInfo), right);
+
+                    // Write (non-swizzled) into the l value
+                    left = loweredBase;
+                    right = LoweredValInfo::ptr(tmpVar);
+                    goto top;
+                }
+                break;
+            }
+        }
+        break;
+
     case LoweredValInfo::Flavor::BoundStorage:
         {
             // The `left` value refers to a subscript operation on
diff --git a/tests/language-feature/swizzles/matrix-swizzle-write-array.slang b/tests/language-feature/swizzles/matrix-swizzle-write-array.slang
new file mode 100644
index 000000000..7d266b8cd
--- /dev/null
+++ b/tests/language-feature/swizzles/matrix-swizzle-write-array.slang
@@ -0,0 +1,36 @@
+//TEST(compute):COMPARE_COMPUTE: -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE: -vk -compute -shaderobj -output-using-type
+
+// Test that matrix swizzle writes work correctly
+// Matrix swizzles can either be one or zero indexed
+// Reference: https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-per-component-math
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float2x2> outputBuffer;
+
+struct MySubscriptable
+{
+    float2x2 m;
+    __subscript() -> float2x2
+    {
+        get { return m; }
+        set { m = newValue; }
+    }
+};
+
+[numthreads(1, 1, 1)]
+void computeMain(uint tid : SV_GroupIndex)
+{
+    MySubscriptable s;
+    s.m = float2x2(0,0,0,0);
+
+    // _ 1
+    // 4 5
+    s[]._12_21_22 = float3(1, 4, 5);
+
+    // 2 1
+    // 4 3
+    s[]._m00_m11 = float2(2, 3);
+
+    outputBuffer[tid] = s.m;
+}
diff --git a/tests/language-feature/swizzles/matrix-swizzle-write-array.slang.expected.txt b/tests/language-feature/swizzles/matrix-swizzle-write-array.slang.expected.txt
new file mode 100644
index 000000000..089d975fe
--- /dev/null
+++ b/tests/language-feature/swizzles/matrix-swizzle-write-array.slang.expected.txt
@@ -0,0 +1,5 @@
+type: float
+2.000000
+1.000000
+4.000000
+3.000000
diff --git a/tests/language-feature/swizzles/matrix-swizzle-write-single.slang b/tests/language-feature/swizzles/matrix-swizzle-write-single.slang
new file mode 100644
index 000000000..e622d216c
--- /dev/null
+++ b/tests/language-feature/swizzles/matrix-swizzle-write-single.slang
@@ -0,0 +1,21 @@
+//TEST(compute):COMPARE_COMPUTE: -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE: -vk -compute -shaderobj -output-using-type
+
+// Test that writes to single matrix elements with swizzles work
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[numthreads(4, 1, 1)]
+void computeMain(uint tid : SV_GroupIndex)
+{
+    // 0 0
+    // 0 0
+    float2x2 m = float2x2(0);
+
+    // 0 2
+    // 0 0
+    const float x = m._m01 = 2;
+
+    outputBuffer[tid] = x + m[tid/2][tid%2];
+}
diff --git a/tests/language-feature/swizzles/matrix-swizzle-write-single.slang.expected.txt b/tests/language-feature/swizzles/matrix-swizzle-write-single.slang.expected.txt
new file mode 100644
index 000000000..16af82e12
--- /dev/null
+++ b/tests/language-feature/swizzles/matrix-swizzle-write-single.slang.expected.txt
@@ -0,0 +1,5 @@
+type: float
+2.000000
+4.000000
+2.000000
+2.000000
diff --git a/tests/language-feature/swizzles/matrix-swizzle-write-swizzle.slang b/tests/language-feature/swizzles/matrix-swizzle-write-swizzle.slang
new file mode 100644
index 000000000..a0d267c7a
--- /dev/null
+++ b/tests/language-feature/swizzles/matrix-swizzle-write-swizzle.slang
@@ -0,0 +1,19 @@
+//TEST(compute):COMPARE_COMPUTE: -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE: -vk -compute -shaderobj -output-using-type
+
+// Test that writing to swizzles of matrix swizzles works correctly
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[numthreads(4, 1, 1)]
+void computeMain(uint tid : SV_GroupIndex)
+{
+    float2x2 m = float2x2(0);
+
+    // 1 4
+    // 3 2
+    m._m00_m01_m10_m11.xwzy.wzyx.wzyx = float4(1,2,3,4);
+
+    outputBuffer[tid] = m[tid/2][tid%2];
+}
diff --git a/tests/language-feature/swizzles/matrix-swizzle-write-swizzle.slang.expected.txt b/tests/language-feature/swizzles/matrix-swizzle-write-swizzle.slang.expected.txt
new file mode 100644
index 000000000..e8f3f9609
--- /dev/null
+++ b/tests/language-feature/swizzles/matrix-swizzle-write-swizzle.slang.expected.txt
@@ -0,0 +1,5 @@
+type: float
+1.000000
+4.000000
+3.000000
+2.000000
diff --git a/tests/language-feature/swizzles/matrix-swizzle-write.slang b/tests/language-feature/swizzles/matrix-swizzle-write.slang
new file mode 100644
index 000000000..5baa4407c
--- /dev/null
+++ b/tests/language-feature/swizzles/matrix-swizzle-write.slang
@@ -0,0 +1,27 @@
+//TEST(compute):COMPARE_COMPUTE: -compute -shaderobj -output-using-type
+//TEST(compute, vulkan):COMPARE_COMPUTE: -vk -compute -shaderobj -output-using-type
+
+// Test that matrix swizzle writes work correctly
+// Matrix swizzles can either be one or zero indexed
+// Reference: https://docs.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-per-component-math
+
+//TEST_INPUT:ubuffer(data=[0 0 0 0], stride=4):out,name=outputBuffer
+RWStructuredBuffer<float> outputBuffer;
+
+[numthreads(4, 1, 1)]
+void computeMain(uint tid : SV_GroupIndex)
+{
+    // 0 0
+    // 0 0
+    float2x2 m = float2x2(0);
+
+    // 0 1
+    // 4 0
+    m._12_21 = float2(1, 4);
+
+    // 2 1
+    // 4 3
+    m._m00_m11 = float2(2, 3);
+
+    outputBuffer[tid] = m[tid/2][tid%2];
+}
diff --git a/tests/language-feature/swizzles/matrix-swizzle-write.slang.expected.txt b/tests/language-feature/swizzles/matrix-swizzle-write.slang.expected.txt
new file mode 100644
index 000000000..089d975fe
--- /dev/null
+++ b/tests/language-feature/swizzles/matrix-swizzle-write.slang.expected.txt
@@ -0,0 +1,5 @@
+type: float
+2.000000
+1.000000
+4.000000
+3.000000