Address comments ykjit#1

ykllvmwrap/src/jitmodbuilder.cc

@@ -37,6 +37,10 @@ uint64_t getNewTraceIdx() {
 
 #define YK_OUTLINE_FNATTR "yk_outline"
 
+// The first two arguments of a stackmap call are it's id and shadow bytes and
+// need to be skipped when scanning the operands for live values.
+#define YK_STACKMAP_SKIP_ARGS 2
+
 // Return value telling the caller of the compiled trace that no guard failure
 // occurred and it's safe to continue running the fake trace stitching loop.
 #define TRACE_RETURN_SUCCESS 0
@@ -612,13 +616,16 @@ class JITModBuilder {
     for (size_t I = 0; I < ActiveFrames.size(); I++) {
       FrameInfo FI = ActiveFrames[I];
 
-      // Read live AOT variables from stackmap calls and match them to
-      // variables in the JIT so we they can be optimised.
+      // Read live AOTMod values from stackmap calls and find their
+      // corresponding values in JITMod. These are exactly the values that are
+      // live at each guard failure and need to be deoptimised.
       CallBase *SMC = FI.SMCall;
-      for (size_t Idx = 2; Idx < SMC->arg_size(); Idx++) {
+      for (size_t Idx = YK_STACKMAP_SKIP_ARGS; Idx < SMC->arg_size(); Idx++) {
         Value *Arg = SMC->getArgOperand(Idx);
         if (VMap.find(Arg) != VMap.end()) {
           if (Arg == NewControlPointCall) {
+            // There's no corresponding JIT value for the return value of the
+            // control point call, so just skip it.
             continue;
           }
           Value *JITArg = VMap[Arg];