JIT: Improve isinst expansion

[EgorBo]

Small change - big (-400kb) diffs.

What it does, for op1 isinst ExactCls it changes the resulting shape like this:

      tmp = op1;
      if (tmp != null) // qmarkNull
      {
          if (tmp->pMT == ExactCls) // qmarkMT
              result = tmp;
          else
              result = null;
      }
      else
-        result = tmp; // tmp is always null here anyway
+        result = null;

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Issue Details

---

Small change - big (-400kb) diffs.

What it does, for x isinst Cls it changes the resulting shape like this:

    //  tmp = op1;
    //  if (tmp != null) // qmarkNull
    //  {
    //      if (tmp->pMT == op2) // qmarkMT
    //          result = tmp;
    //      else
    //          result = null;
    //  }
    //  else
-    //     result = tmp;
+   //      result = null;

Author:	EgorBo
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

@dotnet/jit-contrib PTAL, trivial change with nice diffs

[AndyAyersMS]

Nice find.

Where does this unblock things? I would have thought RBO would have already handled something like this. Maybe assignments get in the way?

[EgorBo]

Where does this unblock things? I would have thought RBO would have already handled something like this. Maybe assignments get in the way?

Assertion prop definitly handles this (lcl prop), but looks like it's better to do it early so it can help RBO. My guess that JitOptRepeat would solve this too 🙂