[clr-interp] Add support for DOTNET_InterpHalt

This flag adds a `INTOP_BREAKPOINT` instruction at the beginning of the method. This opcode calls the `InterpBreakpoint` method. In order to make use of this debug option, you need to add a breakpoint to this method.

An alternative would be to use special instructions to trigger a trap in the debugger, but there is no clean cross platform way to do this currently. Using `__builtin_debugtrap()` on arm64 doesn't allow for step/continue, you would need to set the PC register manually to the next instruction in order to exit the trap, making it a pain to use.

Author: BrzVlad

src/coreclr/interpreter/compiler.cpp

@@ -1803,9 +1803,15 @@ int InterpCompiler::GenerateCode(CORINFO_METHOD_INFO* methodInfo)
         goto exit_bad_code;
     }
 
+    m_currentILOffset = -1;
+
+#if DEBUG
+    if (m_methodName && InterpConfig.InterpHalt() && !strcmp(m_methodName, InterpConfig.InterpHalt()))
+        AddIns(INTOP_BREAKPOINT);
+#endif
+
     if ((methodInfo->options & CORINFO_OPT_INIT_LOCALS) && m_ILLocalsSize > 0)
     {
-        m_currentILOffset = 0;
         AddIns(INTOP_INITLOCALS);
         m_pLastNewIns->data[0] = m_ILLocalsOffset;
         m_pLastNewIns->data[1] = m_ILLocalsSize;

src/coreclr/interpreter/intops.def

@@ -252,6 +252,7 @@ OPDEF(INTOP_NEWOBJ_VT, "newobj.vt", 5, 1, 1, InterpOpMethodToken)
 OPDEF(INTOP_CALL_HELPER_PP, "call.helper.pp", 5, 1, 0, InterpOpThreeInts)
 
 OPDEF(INTOP_ZEROBLK_IMM, "zeroblk.imm", 3, 0, 1, InterpOpInt)
+OPDEF(INTOP_BREAKPOINT, "breakpoint", 1, 0, 0, InterpOpNoArgs)
 OPDEF(INTOP_FAILFAST, "failfast", 1, 0, 0, InterpOpNoArgs)
 
 // All instructions after this point are IROPS, instructions that are not emitted/executed

src/coreclr/vm/interpexec.cpp

@@ -31,6 +31,13 @@ InterpThreadContext* InterpGetThreadContext()
     }
 }
 
+#ifdef DEBUG
+static void InterpBreakpoint()
+{
+
+}
+#endif
+
 #define LOCAL_VAR_ADDR(offset,type) ((type*)(stack + (offset)))
 #define LOCAL_VAR(offset,type) (*LOCAL_VAR_ADDR(offset, type))
 // TODO once we have basic EH support
@@ -60,6 +67,12 @@ void InterpExecMethod(InterpreterFrame *pInterpreterFrame, InterpMethodContextFr
 
         switch (*ip)
         {
+#ifdef DEBUG
+            case INTOP_BREAKPOINT:
+                InterpBreakpoint();
+                ip++;
+                break;
+#endif
             case INTOP_INITLOCALS:
                 memset(stack + ip[1], 0, ip[2]);
                 ip += 3;