Remove GT_ARGPLACE nodes (#68140)

These do not serve much purpose today -- instead just use null and add a
helper function to iterate non-null early args, which is somewhat
common.

In addition to saving some TP and memory, teaching the backend about
null early nodes will also be beneficial because I am planning to change
rationalization to null out non-values in the early arg list so that all
nodes have only values as their operands in LIR.

Throughput diff:
```
Collection                                  Base # instructions Diff # instructions PDIFF
aspnet.run.windows.x64.checked.mch           69,717,468,395      69,206,312,087     -0.73%
benchmarks.run.windows.x64.checked.mch       54,695,846,729      54,294,078,768     -0.73%
coreclr_tests.pmi.windows.x64.checked.mch   340,169,515,528     337,478,749,067     -0.79%
libraries.crossgen2.windows.x64.checked.mch 128,653,906,043     126,926,566,191     -1.34%
libraries.pmi.windows.x64.checked.mch       228,653,702,806     226,554,618,843     -0.92%
libraries_tests.pmi.windows.x64.checked.mch 531,053,530,645     525,233,144,101     -1.10%
```

Memory stats (libraries.pmi)
Before: 25961399533 bytes
After: 25770612141 bytes (-0.7%)

docs/design/coreclr/jit/jit-call-morphing.md

@@ -144,11 +144,9 @@ For arguments that are marked as not needing a temp:
 -----------------
 
 1. If this is an argument that is passed in a register, then the existing
-node is moved to the late argument list and a new `GT_ARGPLACE` (placeholder)
-node replaces it in the early argument list.
-2. Additionally, if `m_needPlace` is true (only for `FEATURE_FIXED_OUT_ARGS`)
-then the existing node is moved to the late argument list and a new
-`GT_ARGPLACE` (placeholder) node replaces it in the `early argument list.
+node is moved to the late argument list.
+2. Similarly, if `m_needPlace` is true (only for `FEATURE_FIXED_OUT_ARGS`)
+then the existing node is moved to the late argument list.
 3. Otherwise the argument is left in the early argument and it will be
 evaluated directly into the outgoing arg area or pushed on the stack.
 

src/coreclr/jit/codegenarmarch.cpp

@@ -5478,13 +5478,6 @@ unsigned CodeGenInterface::InferStructOpSizeAlign(GenTree* op, unsigned* alignme
         opSize    = TARGET_POINTER_SIZE * 2;
         alignment = TARGET_POINTER_SIZE;
     }
-    else if (op->IsArgPlaceHolderNode())
-    {
-        CORINFO_CLASS_HANDLE clsHnd = op->AsArgPlace()->gtArgPlaceClsHnd;
-        assert(clsHnd != 0);
-        opSize    = roundUp(compiler->info.compCompHnd->getClassSize(clsHnd), TARGET_POINTER_SIZE);
-        alignment = roundUp(compiler->info.compCompHnd->getClassAlignmentRequirement(clsHnd), TARGET_POINTER_SIZE);
-    }
     else
     {
         assert(!"Unhandled gtOper");

src/coreclr/jit/codegenlinear.cpp

@@ -1328,13 +1328,6 @@ void CodeGen::genConsumeRegAndCopy(GenTree* node, regNumber needReg)
 void CodeGen::genNumberOperandUse(GenTree* const operand, int& useNum) const
 {
     assert(operand != nullptr);
-
-    // Ignore argument placeholders.
-    if (operand->OperGet() == GT_ARGPLACE)
-    {
-        return;
-    }
-
     assert(operand->gtUseNum == -1);
 
     if (!operand->isContained() && !operand->IsCopyOrReload())

src/coreclr/jit/codegenloongarch64.cpp

@@ -9343,13 +9343,6 @@ unsigned CodeGenInterface::InferStructOpSizeAlign(GenTree* op, unsigned* alignme
         opSize    = TARGET_POINTER_SIZE * 2;
         alignment = TARGET_POINTER_SIZE;
     }
-    else if (op->IsArgPlaceHolderNode())
-    {
-        CORINFO_CLASS_HANDLE clsHnd = op->AsArgPlace()->gtArgPlaceClsHnd;
-        assert(clsHnd != 0);
-        opSize    = roundUp(compiler->info.compCompHnd->getClassSize(clsHnd), TARGET_POINTER_SIZE);
-        alignment = roundUp(compiler->info.compCompHnd->getClassAlignmentRequirement(clsHnd), TARGET_POINTER_SIZE);
-    }
     else
     {
         assert(!"Unhandled gtOper");

src/coreclr/jit/codegenxarch.cpp

@@ -5377,7 +5377,7 @@ void CodeGen::genCall(GenTreeCall* call)
     // The call will pop its arguments.
     // for each putarg_stk:
     target_ssize_t stackArgBytes = 0;
-    for (CallArg& arg : call->gtArgs.Args())
+    for (CallArg& arg : call->gtArgs.EarlyArgs())
     {
         GenTree* argNode = arg.GetEarlyNode();
         if (argNode->OperIs(GT_PUTARG_STK) && ((argNode->gtFlags & GTF_LATE_ARG) == 0))

src/coreclr/jit/compiler.h

@@ -2605,8 +2605,6 @@ class Compiler
 
     GenTree* gtNewNothingNode();
 
-    GenTree* gtNewArgPlaceHolderNode(var_types type, CORINFO_CLASS_HANDLE clsHnd);
-
     GenTree* gtUnusedValNode(GenTree* expr);
 
     GenTree* gtNewKeepAliveNode(GenTree* op);
@@ -10651,7 +10649,6 @@ class GenTreeVisitor
             case GT_JMPTABLE:
             case GT_CLS_VAR:
             case GT_CLS_VAR_ADDR:
-            case GT_ARGPLACE:
             case GT_PHYSREG:
             case GT_EMITNOP:
             case GT_PINVOKE_PROLOG:
@@ -10838,7 +10835,7 @@ class GenTreeVisitor
             {
                 GenTreeCall* const call = node->AsCall();
 
-                for (CallArg& arg : call->gtArgs.Args())
+                for (CallArg& arg : call->gtArgs.EarlyArgs())
                 {
                     result = WalkTree(&arg.EarlyNodeRef(), call);
                     if (result == fgWalkResult::WALK_ABORT)

src/coreclr/jit/compiler.hpp

@@ -1256,15 +1256,6 @@ inline void GenTree::gtBashToNOP()
     gtFlags &= ~(GTF_ALL_EFFECT | GTF_REVERSE_OPS);
 }
 
-// return new arg placeholder node.  Does not do anything but has a type associated
-// with it so we can keep track of register arguments in lists associated w/ call nodes
-
-inline GenTree* Compiler::gtNewArgPlaceHolderNode(var_types type, CORINFO_CLASS_HANDLE clsHnd)
-{
-    GenTree* node = new (this, GT_ARGPLACE) GenTreeArgPlace(type, clsHnd);
-    return node;
-}
-
 /*****************************************************************************/
 
 inline GenTree* Compiler::gtUnusedValNode(GenTree* expr)
@@ -4208,7 +4199,6 @@ void GenTree::VisitOperands(TVisitor visitor)
         case GT_JMPTABLE:
         case GT_CLS_VAR:
         case GT_CLS_VAR_ADDR:
-        case GT_ARGPLACE:
         case GT_PHYSREG:
         case GT_EMITNOP:
         case GT_PINVOKE_PROLOG:
@@ -4370,7 +4360,7 @@ void GenTree::VisitOperands(TVisitor visitor)
         {
             GenTreeCall* const call = this->AsCall();
 
-            for (CallArg& arg : call->gtArgs.Args())
+            for (CallArg& arg : call->gtArgs.EarlyArgs())
             {
                 if (visitor(arg.GetEarlyNode()) == VisitResult::Abort)
                 {

src/coreclr/jit/fgdiagnostic.cpp

@@ -3047,7 +3047,11 @@ void Compiler::fgDebugCheckFlags(GenTree* tree)
             {
                 // TODO-Cleanup: this is a patch for a violation in our GT_ASG propagation.
                 // see https://github.com/dotnet/runtime/issues/13758
-                actualFlags |= arg.GetEarlyNode()->gtFlags & GTF_ASG;
+                if (arg.GetEarlyNode() != nullptr)
+                {
+                    actualFlags |= arg.GetEarlyNode()->gtFlags & GTF_ASG;
+                }
+
                 if (arg.GetLateNode() != nullptr)
                 {
                     actualFlags |= arg.GetLateNode()->gtFlags & GTF_ASG;

src/coreclr/jit/fgprofile.cpp

@@ -1560,6 +1560,7 @@ class ClassProbeInserter
         uint8_t* classProfile = m_schema[*m_currentSchemaIndex].Offset + m_profileMemory;
         *m_currentSchemaIndex += 2; // There are 2 schema entries per class probe
 
+        assert(!call->gtArgs.AreArgsComplete());
         CallArg* objUse = nullptr;
         if (compiler->impIsCastHelperEligibleForClassProbe(call))
         {

src/coreclr/jit/flowgraph.cpp

@@ -1057,7 +1057,8 @@ GenTree* Compiler::fgOptimizeDelegateConstructor(GenTreeCall*            call,
 
     assert(call->gtArgs.HasThisPointer());
     assert(call->gtArgs.CountArgs() == 3);
-    GenTree* targetMethod = call->gtArgs.GetArgByIndex(2)->GetEarlyNode();
+    assert(!call->gtArgs.AreArgsComplete());
+    GenTree* targetMethod = call->gtArgs.GetArgByIndex(2)->GetNode();
     noway_assert(targetMethod->TypeGet() == TYP_I_IMPL);
     genTreeOps            oper            = targetMethod->OperGet();
     CORINFO_METHOD_HANDLE targetMethodHnd = nullptr;
@@ -1071,7 +1072,7 @@ GenTree* Compiler::fgOptimizeDelegateConstructor(GenTreeCall*            call,
     else if (oper == GT_CALL && targetMethod->AsCall()->gtCallMethHnd == eeFindHelper(CORINFO_HELP_VIRTUAL_FUNC_PTR))
     {
         assert(targetMethod->AsCall()->gtArgs.CountArgs() == 3);
-        GenTree* handleNode = targetMethod->AsCall()->gtArgs.GetArgByIndex(2)->GetEarlyNode();
+        GenTree* handleNode = targetMethod->AsCall()->gtArgs.GetArgByIndex(2)->GetNode();
 
         if (handleNode->OperGet() == GT_CNS_INT)
         {
@@ -1110,7 +1111,7 @@ GenTree* Compiler::fgOptimizeDelegateConstructor(GenTreeCall*            call,
         GenTreeCall* runtimeLookupCall = qmarkNode->AsOp()->gtOp2->AsOp()->gtOp1->AsCall();
 
         // This could be any of CORINFO_HELP_RUNTIMEHANDLE_(METHOD|CLASS)(_LOG?)
-        GenTree* tokenNode = runtimeLookupCall->gtArgs.GetArgByIndex(1)->GetEarlyNode();
+        GenTree* tokenNode = runtimeLookupCall->gtArgs.GetArgByIndex(1)->GetNode();
         noway_assert(tokenNode->OperGet() == GT_CNS_INT);
         targetMethodHnd = CORINFO_METHOD_HANDLE(tokenNode->AsIntCon()->gtCompileTimeHandle);
     }
@@ -1142,8 +1143,8 @@ GenTree* Compiler::fgOptimizeDelegateConstructor(GenTreeCall*            call,
             {
                 JITDUMP("optimized\n");
 
-                GenTree*       thisPointer       = call->gtArgs.GetThisArg()->GetEarlyNode();
-                GenTree*       targetObjPointers = call->gtArgs.GetArgByIndex(1)->GetEarlyNode();
+                GenTree*       thisPointer       = call->gtArgs.GetThisArg()->GetNode();
+                GenTree*       targetObjPointers = call->gtArgs.GetArgByIndex(1)->GetNode();
                 CORINFO_LOOKUP pLookup;
                 info.compCompHnd->getReadyToRunDelegateCtorHelper(&ldftnToken->m_token, ldftnToken->m_tokenConstraint,
                                                                   clsHnd, &pLookup);
@@ -1175,8 +1176,8 @@ GenTree* Compiler::fgOptimizeDelegateConstructor(GenTreeCall*            call,
         {
             JITDUMP("optimized\n");
 
-            GenTree* thisPointer       = call->gtArgs.GetArgByIndex(0)->GetEarlyNode();
-            GenTree* targetObjPointers = call->gtArgs.GetArgByIndex(1)->GetEarlyNode();
+            GenTree* thisPointer       = call->gtArgs.GetArgByIndex(0)->GetNode();
+            GenTree* targetObjPointers = call->gtArgs.GetArgByIndex(1)->GetNode();
             call = gtNewHelperCallNode(CORINFO_HELP_READYTORUN_DELEGATE_CTOR, TYP_VOID, thisPointer, targetObjPointers);
 
             CORINFO_LOOKUP entryPoint;
@@ -3852,10 +3853,8 @@ BasicBlock* Compiler::fgRngChkTarget(BasicBlock* block, SpecialCodeKind kind)
 //
 // Arguments:
 //    tree  - the tree to sequence
-//    isLIR - whether the sequencing is being done for LIR. If so,
-//            ARGPLACE nodes will not be threaded into the linear
-//            order, and the GTF_REVERSE_OPS flag will be cleared
-//            on all the nodes.
+//    isLIR - whether the sequencing is being done for LIR. If so, the
+//            GTF_REVERSE_OPS flag will be cleared on all nodes.
 //
 // Return Value:
 //    The first node to execute in the sequenced tree.
@@ -3887,12 +3886,6 @@ GenTree* Compiler::fgSetTreeSeq(GenTree* tree, bool isLIR)
             if (m_isLIR)
             {
                 node->ClearReverseOp();
-
-                // ARGPLACE nodes are not threaded into the LIR sequence.
-                if (node->OperIs(GT_ARGPLACE))
-                {
-                    return fgWalkResult::WALK_CONTINUE;
-                }
             }
 
             node->gtPrev       = m_prevNode;

src/coreclr/jit/forwardsub.cpp

@@ -288,7 +288,7 @@ class ForwardSubVisitor final : public GenTreeVisitor<ForwardSubVisitor>
 
             for (CallArg& arg : m_callAncestor->gtArgs.Args())
             {
-                m_useFlags |= (arg.GetEarlyNode()->gtFlags & GTF_GLOB_EFFECT);
+                m_useFlags |= (arg.GetNode()->gtFlags & GTF_GLOB_EFFECT);
             }
 
             if (oldUseFlags != m_useFlags)