Optimize VectorX<T>.ConditionalSelect for constant masks

src/coreclr/jit/gentree.cpp

@@ -29442,6 +29442,57 @@ bool GenTree::IsInvariant() const
     return OperIsConst() || OperIs(GT_LCL_ADDR) || OperIs(GT_FTN_ADDR);
 }
 
+//-------------------------------------------------------------------
+// IsVectorPerElementMask: returns true if this node is a vector constant per-element mask
+//                         (every element has either all bits set or none of them).
+//
+// Arguments:
+//    simdBaseType - the base type of the constant being checked.
+//    simdSize     - the size of the SIMD type of the intrinsic.
+//
+// Returns:
+//     True if this node is a vector constant per-element mask.
+//
+bool GenTree::IsVectorPerElementMask(var_types simdBaseType, unsigned simdSize) const
+{
+#ifdef FEATURE_SIMD
+    if (IsCnsVec())
+    {
+        const GenTreeVecCon* vecCon = AsVecCon();
+
+        int elementCount = vecCon->ElementCount(simdSize, simdBaseType);
+
+        switch (simdBaseType)
+        {
+            case TYP_BYTE:
+            case TYP_UBYTE:
+                return ElementsAreAllBitsSetOrZero(&vecCon->gtSimdVal.u8[0], elementCount);
+            case TYP_SHORT:
+            case TYP_USHORT:
+                return ElementsAreAllBitsSetOrZero(&vecCon->gtSimdVal.u16[0], elementCount);
+            case TYP_INT:
+            case TYP_UINT:
+            case TYP_FLOAT:
+                return ElementsAreAllBitsSetOrZero(&vecCon->gtSimdVal.u32[0], elementCount);
+            case TYP_LONG:
+            case TYP_ULONG:
+            case TYP_DOUBLE:
+                return ElementsAreAllBitsSetOrZero(&vecCon->gtSimdVal.u64[0], elementCount);
+            default:
+                unreached();
+        }
+    }
+    else if (OperIsHWIntrinsic())
+    {
+        // TODO-XARCH-AVX512 Use VPBLENDM* and take input directly from K registers if cond is from
+        // MoveMaskToVectorSpecial.
+        return HWIntrinsicInfo::ReturnsPerElementMask(AsHWIntrinsic()->GetHWIntrinsicId());
+    }
+#endif // FEATURE_SIMD
+
+    return false;
+}
+
 //------------------------------------------------------------------------
 // IsNeverNegative: returns true if the given tree is known to be never
 //                  negative, i. e. the upper bit will always be zero.

src/coreclr/jit/gentree.h

@@ -2317,6 +2317,7 @@ struct GenTree
     bool Precedes(GenTree* other);
 
     bool IsInvariant() const;
+    bool IsVectorPerElementMask(var_types simdBaseType, unsigned simdSize) const;
 
     bool IsNeverNegative(Compiler* comp) const;
     bool IsNeverNegativeOne(Compiler* comp) const;

src/coreclr/jit/lowerxarch.cpp

@@ -2994,13 +2994,12 @@ GenTree* Lowering::LowerHWIntrinsicCndSel(GenTreeHWIntrinsic* node)
     GenTree* op3 = node->Op(3);
 
     // If the condition vector comes from a hardware intrinsic that
-    // returns a per-element mask (marked with HW_Flag_ReturnsPerElementMask),
-    // we can optimize the entire conditional select to
-    // a single BlendVariable instruction (if supported by the architecture)
+    // returns a per-element mask, we can optimize the entire
+    // conditional select to a single BlendVariable instruction
+    // (if supported by the architecture)
 
-    // TODO-XARCH-AVX512 Use VPBLENDM* and take input directly from K registers if cond is from MoveMaskToVectorSpecial.
     // First, determine if the condition is a per-element mask
-    if (op1->OperIsHWIntrinsic() && HWIntrinsicInfo::ReturnsPerElementMask(op1->AsHWIntrinsic()->GetHWIntrinsicId()))
+    if (op1->IsVectorPerElementMask(simdBaseType, simdSize))
     {
         // Next, determine if the target architecture supports BlendVariable
         NamedIntrinsic blendVariableId = NI_Illegal;

src/coreclr/jit/simd.h

@@ -15,6 +15,17 @@ static bool ElementsAreSame(T* array, size_t size)
     return true;
 }
 
+template <typename T>
+static bool ElementsAreAllBitsSetOrZero(T* array, size_t size)
+{
+    for (size_t i = 0; i < size; i++)
+    {
+        if (array[i] != static_cast<T>(0) && array[i] != static_cast<T>(~0))
+            return false;
+    }
+    return true;
+}
+
 struct simd8_t
 {
     union