RyuJIT/x86: implement tailcall via helper

Fixes #4185

Also: (1) added a class to simplify non-standard arg handling in fgMorphArgs(),
(2) fixed minor tree output alignment bug.

Author: BruceForstall

src/jit/assertionprop.cpp

@@ -1911,15 +1911,15 @@ void Compiler::optAssertionGen(GenTreePtr tree)
         {
             //  Retrieve the 'this' arg
             GenTreePtr thisArg = gtGetThisArg(tree);
-#if defined(_TARGET_AMD64_) || defined(_TARGET_ARM_)
+#if defined(_TARGET_X86_) || defined(_TARGET_AMD64_) || defined(_TARGET_ARM_)
             if (thisArg == nullptr)
             {
                 // For tail calls we lose the this pointer in the argument list but that's OK because a null check
                 // was made explicit, so we get the assertion when we walk the GT_IND in the argument list.
                 noway_assert(tree->gtCall.IsTailCall());
                 break;
             }
-#endif // _TARGET_AMD64_ || _TARGET_ARM_
+#endif // _TARGET_X86_ || _TARGET_AMD64_ || _TARGET_ARM_
             noway_assert(thisArg != nullptr);
             assertionIndex = optCreateAssertion(thisArg, nullptr, OAK_NOT_EQUAL);
         }

src/jit/codegencommon.cpp

@@ -1391,10 +1391,6 @@ void                CodeGenInterface::reloadFloatReg(var_types type, TempDsc* tm
 regNumber           CodeGenInterface::genGetThisArgReg(GenTreePtr  call)
 {
     noway_assert(call->IsCall());
-#if RETBUFARG_PRECEDES_THIS
-    if (call->AsCall()->HasRetBufArg())
-        return REG_ARG_1;
-#endif // RETBUFARG_PRECEEDS_THIS
     return REG_ARG_0;
 }
 
@@ -7813,6 +7809,18 @@ void                CodeGen::genFinalizeFrame()
 
     // Set various registers as "modified" for special code generation scenarios: Edit & Continue, P/Invoke calls, etc.
 
+#if defined(_TARGET_X86_)
+    if (compiler->compTailCallUsed)
+    {
+        // If we are generating a helper-based tailcall, we've set the tailcall helper "flags"
+        // argument to "1", indicating to the tailcall helper that we've saved the callee-saved
+        // registers (ebx, esi, edi). So, we need to make sure all the callee-saved registers
+        // actually get saved.
+
+        regSet.rsSetRegsModified(RBM_INT_CALLEE_SAVED);
+    }
+#endif // _TARGET_X86_
+
 #if defined(_TARGET_ARMARCH_)
     // We need to determine if we will change SP larger than a specific amount to determine if we want to use a loop
     // to touch stack pages, that will require multiple registers. See genAllocLclFrame() for details.

src/jit/compiler.h

@@ -1140,7 +1140,7 @@ struct fgArgTabEntry
     bool           processed    :1; // True when we have decided the evaluation order for this argument in the gtCallLateArgs 
     bool           isHfaRegArg  :1; // True when the argument is passed as a HFA in FP registers.
     bool           isBackFilled :1; // True when the argument fills a register slot skipped due to alignment requirements of previous arguments.
-    bool           isNonStandard:1; // True if it is an arg that is passed in a reg other than a standard arg reg
+    bool           isNonStandard:1; // True if it is an arg that is passed in a reg other than a standard arg reg, or is forced to be on the stack despite its arg list position.
 
 #if defined(FEATURE_UNIX_AMD64_STRUCT_PASSING)
     bool           isStruct     :1; // True if this is a struct arg
@@ -1279,7 +1279,6 @@ class  fgArgInfo
     unsigned            GetNextSlotNum() { return nextSlotNum; }
     bool                HasRegArgs()     { return hasRegArgs; } 
     bool                HasStackArgs()   { return hasStackArgs; }
-
 };
 
 

src/jit/gentree.cpp

@@ -8087,18 +8087,23 @@ void                Compiler::gtDispNode(GenTreePtr     tree,
 
         // If we have an indent stack, don't add additional characters,
         // as it will mess up the alignment.
-        if (tree->gtOper != GT_STMT && hasSeqNum && (indentStack == nullptr))
+        bool displayDotNum = tree->gtOper != GT_STMT && hasSeqNum && (indentStack == nullptr);
+        if (displayDotNum)
+        {
             printf("N%03u.%02u ", prev->gtSeqNum, dotNum);
+        }
         else
+        {
             printf("     ");
+        }
 
         if (tree->gtCostsInitialized)
         {
             printf("(%3u,%3u) ", tree->gtCostEx, tree->gtCostSz);
         }
         else
         {
-            if (tree->gtOper != GT_STMT && hasSeqNum)
+            if (displayDotNum)
             {
                 // Do better alignment in this case
                 printf("       ");