Jitstress fixes for tailcall tests (dotnet#1771)

Make STRESS_GENERIC_VARN more compatible with methods that make explicit tail
calls. Don't add gc checks for explicit tail calls, and remove the code in
morph that blocks tail calls if gc checks are active.

Update the tailcall test to to disable STRESS_UNSAFE_BUFFER_CHECKS. This can
be reverted when dotnet#341 is merged.

Fixes dotnet#1752.

src/coreclr/src/jit/compiler.h

@@ -4021,7 +4021,7 @@ class Compiler
     }
     void impLoadArg(unsigned ilArgNum, IL_OFFSET offset);
     void impLoadLoc(unsigned ilLclNum, IL_OFFSET offset);
-    bool impReturnInstruction(BasicBlock* block, int prefixFlags, OPCODE& opcode);
+    bool impReturnInstruction(int prefixFlags, OPCODE& opcode);
 
 #ifdef _TARGET_ARM_
     void impMarkLclDstNotPromotable(unsigned tmpNum, GenTree* op, CORINFO_CLASS_HANDLE hClass);

src/coreclr/src/jit/importer.cpp

@@ -11508,7 +11508,7 @@ void Compiler::impImportBlockCode(BasicBlock* block)
             case CEE_RET:
                 prefixFlags &= ~PREFIX_TAILCALL; // ret without call before it
             RET:
-                if (!impReturnInstruction(block, prefixFlags, opcode))
+                if (!impReturnInstruction(prefixFlags, opcode))
                 {
                     return; // abort
                 }
@@ -16254,7 +16254,7 @@ GenTree* Compiler::impAssignSmallStructTypeToVar(GenTree* op, CORINFO_CLASS_HAND
 //    registers return values to suitable temps.
 //
 // Arguments:
-//     op -- call returning a struct in a registers
+//     op -- call returning a struct in registers
 //     hClass -- class handle for struct
 //
 // Returns:
@@ -16278,11 +16278,20 @@ GenTree* Compiler::impAssignMultiRegTypeToVar(GenTree* op, CORINFO_CLASS_HANDLE
 }
 #endif // FEATURE_MULTIREG_RET
 
-// do import for a return
-// returns false if inlining was aborted
-// opcode can be ret or call in the case of a tail.call
-bool Compiler::impReturnInstruction(BasicBlock* block, int prefixFlags, OPCODE& opcode)
+//------------------------------------------------------------------------
+// impReturnInstruction: import a return or an explicit tail call
+//
+// Arguments:
+//     prefixFlags -- active IL prefixes
+//     opcode -- [in, out] IL opcode
+//
+// Returns:
+//     True if import was successful (may fail for some inlinees)
+//
+bool Compiler::impReturnInstruction(int prefixFlags, OPCODE& opcode)
 {
+    const bool isTailCall = (prefixFlags & PREFIX_TAILCALL) != 0;
+
     if (tiVerificationNeeded)
     {
         verVerifyThisPtrInitialised();
@@ -16334,7 +16343,7 @@ bool Compiler::impReturnInstruction(BasicBlock* block, int prefixFlags, OPCODE&
                       (varTypeIsStruct(op2) && varTypeIsStruct(info.compRetType)));
 
 #ifdef DEBUG
-            if (opts.compGcChecks && info.compRetType == TYP_REF)
+            if (!isTailCall && opts.compGcChecks && (info.compRetType == TYP_REF))
             {
                 // DDB 3483  : JIT Stress: early termination of GC ref's life time in exception code path
                 // VSW 440513: Incorrect gcinfo on the return value under COMPlus_JitGCChecks=1 for methods with
@@ -16741,7 +16750,7 @@ bool Compiler::impReturnInstruction(BasicBlock* block, int prefixFlags, OPCODE&
     }
 
     // We must have imported a tailcall and jumped to RET
-    if (prefixFlags & PREFIX_TAILCALL)
+    if (isTailCall)
     {
 #if defined(FEATURE_CORECLR) || !defined(_TARGET_AMD64_)
         // Jit64 compat:

src/coreclr/src/jit/morph.cpp

@@ -6975,15 +6975,6 @@ GenTree* Compiler::fgMorphPotentialTailCall(GenTreeCall* call)
     }
 #endif
 
-#ifdef DEBUG
-    // DDB 99324: Just disable tailcall under compGcChecks stress mode.
-    if (opts.compGcChecks)
-    {
-        failTailCall("GcChecks");
-        return nullptr;
-    }
-#endif
-
     // We have to ensure to pass the incoming retValBuf as the
     // outgoing one. Using a temp will not do as this function will
     // not regain control to do the copy. This can happen when inlining

src/coreclr/tests/src/JIT/Directed/tailcall/tailcall.ilproj

@@ -9,4 +9,14 @@
   <ItemGroup>
     <Compile Include="$(MSBuildProjectName).il" />
   </ItemGroup>
+  <PropertyGroup>
+    <CLRTestBatchPreCommands><![CDATA[
+$(CLRTestBatchPreCommands)
+set COMPlus_JitStressModeNamesNot=STRESS_UNSAFE_BUFFER_CHECKS
+]]></CLRTestBatchPreCommands>
+    <BashCLRTestPreCommands><![CDATA[
+$(BashCLRTestPreCommands)
+export COMPlus_JitStressModeNamesNot=STRESS_UNSAFE_BUFFER_CHECKS
+]]></BashCLRTestPreCommands>
+  </PropertyGroup>
 </Project>