[clr-interp] Add safepoints at method entry and backwards branch (#115567)

* [clr-interp] Add safepoints at method entry and backwards branch

Interpreter runs in GC unsafe mode, so without these safepoints for cooperative suspend, the GC could fail to suspend threads executing in the interpreter.

* [clr-interp] Add some starting contracts to interp exec method

Author: BrzVlad

src/coreclr/interpreter/compiler.cpp

@@ -1418,6 +1418,10 @@ void InterpCompiler::EmitBranch(InterpOpcode opcode, int32_t ilOffset)
     if (target < 0 || target >= m_ILCodeSize)
         assert(0);
 
+    // Backwards branch, emit safepoint
+    if (ilOffset < 0)
+        AddIns(INTOP_SAFEPOINT);
+
     InterpBasicBlock *pTargetBB = m_ppOffsetToBB[target];
     assert(pTargetBB != NULL);
 
@@ -2123,6 +2127,10 @@ int InterpCompiler::GenerateCode(CORINFO_METHOD_INFO* methodInfo)
 
     codeEnd = m_ip + m_ILCodeSize;
 
+    // Safepoint at each method entry. This could be done as part of a call, rather than
+    // adding an opcode.
+    AddIns(INTOP_SAFEPOINT);
+
     linkBBlocks = true;
     needsRetryEmit = false;
 retry_emit:

src/coreclr/interpreter/intops.def

@@ -35,6 +35,7 @@ OPDEF(INTOP_LDLOCA, "ldloca", 3, 1, 0, InterpOpInt)
 
 OPDEF(INTOP_SWITCH, "switch", 0, 0, 1, InterpOpSwitch)
 
+OPDEF(INTOP_SAFEPOINT, "safepoint", 1, 0, 0, InterpOpNoArgs)
 OPDEF(INTOP_BR, "br", 2, 0, 0, InterpOpBranch)
 
 OPDEF(INTOP_BRFALSE_I4, "brfalse.i4", 3, 0, 1, InterpOpBranch)

src/coreclr/vm/interpexec.cpp

@@ -36,6 +36,13 @@ static void InterpBreakpoint()
 
 void InterpExecMethod(InterpreterFrame *pInterpreterFrame, InterpMethodContextFrame *pFrame, InterpThreadContext *pThreadContext)
 {
+    CONTRACTL
+    {
+        GC_TRIGGERS;
+        MODE_COOPERATIVE;
+    }
+    CONTRACTL_END;
+
 #if defined(HOST_AMD64) && defined(HOST_WINDOWS)
     pInterpreterFrame->SetInterpExecMethodSSP((TADDR)_rdsspq());
 #endif // HOST_AMD64 && HOST_WINDOWS
@@ -293,6 +300,12 @@ void InterpExecMethod(InterpreterFrame *pInterpreterFrame, InterpMethodContextFr
                     break;
                 }
 
+                case INTOP_SAFEPOINT:
+                    if (g_TrapReturningThreads)
+                        JIT_PollGC();
+                    ip++;
+                    break;
+
                 case INTOP_BR:
                     ip += ip[1];
                     break;