[Arm64] Additional Insert comments

diff --git a/src/jit/codegenarm64.cpp b/src/jit/codegenarm64.cpp
index d151d3b..5463e2a 100644 (file)
--- a/src/jit/codegenarm64.cpp
+++ b/src/jit/codegenarm64.cpp
@@ -5354,9 +5354,16 @@ void CodeGen::genHWIntrinsicSimdInsertOp(GenTreeHWIntrinsic* node)
     if (op3->isContained())
     {
         // Handle vector element to vector element case
+        //
+        // If op3 is contained this is because lowering found an opportunity to contain a Simd.Extract in a Simd.Insert
+        //
         regNumber op3Reg = op3->gtGetOp1()->gtRegNum;
 
         assert(genIsValidFloatReg(op3Reg));
+
+        // op3 containment currently only occurs when
+        //   + op3 is a Simd.Extract() (gtHWIntrinsicId == NI_ARM64_SIMD_GetItem)
+        //   + element & srcLane are immediate constants
         assert(op2->isContainedIntOrIImmed());
         assert(op3->OperIs(GT_HWIntrinsic));
         assert(op3->AsHWIntrinsic()->gtHWIntrinsicId == NI_ARM64_SIMD_GetItem);
@@ -5365,12 +5372,13 @@ void CodeGen::genHWIntrinsicSimdInsertOp(GenTreeHWIntrinsic* node)
         int element = (int)op2->AsIntConCommon()->IconValue();
         int srcLane = (int)op3->gtGetOp2()->AsIntConCommon()->IconValue();
 
+        // Emit mov targetReg[element], op3Reg[srcLane]
         getEmitter()->emitIns_R_R_I_I(INS_mov, baseTypeSize, targetReg, op3Reg, element, srcLane);
     }
     else
     {
         // Handle scalar to vector element case
-        // TODO-ARM64-CQ handle containing scalar const where possible
+        // TODO-ARM64-CQ handle containing op3 scalar const where possible
         regNumber op3Reg = op3->gtRegNum;
 
         auto emitSwCase = [&](int element) {

diff --git a/src/jit/lowerarmarch.cpp b/src/jit/lowerarmarch.cpp
index 6c154ce..a518964 100644 (file)
--- a/src/jit/lowerarmarch.cpp
+++ b/src/jit/lowerarmarch.cpp
@@ -916,7 +916,12 @@ void Lowering::ContainCheckHWIntrinsic(GenTreeHWIntrinsic* node)
 
                 GenTree* op3 = argList->Rest()->Rest()->Current();
 
-                // TODO-ARM64-CQ Support containing NI_ARM64_SIMD_GetItem (vector element to element move)
+                // In the HW intrinsics C# API there is no direct way to specify a vector element to element mov
+                //   VX[a] = VY[b]
+                // In C# this would naturally be expressed by
+                //   Insert(VX, a, Extract(VY, b))
+                // If both a & b are immediate constants contain the extract/getItem so that we can emit
+                //   the single instruction mov Vx[a], Vy[b]
                 if (op3->OperIs(GT_HWIntrinsic) && (op3->AsHWIntrinsic()->gtHWIntrinsicId == NI_ARM64_SIMD_GetItem))
                 {
                     ContainCheckHWIntrinsic(op3->AsHWIntrinsic());