Handle partial multireg COPY

A multireg COPY will only have a valid register for indices that require copying. Thus, the `GetRegCount` method must return the highest index that has a valid register.

Fix #20063

1 files changed, 9 insertions, 10 deletions

diff --git a/src/jit/gentree.h b/src/jit/gentree.h
index dbb5a75913..a0c86fbfbc 100644
--- a/src/jit/gentree.h
+++ b/src/jit/gentree.h
@@ -5535,22 +5535,21 @@ struct GenTreeCopyOrReload : public GenTreeUnOp
 
     unsigned GetRegCount()
     {
-        if (gtRegNum == REG_NA)
-        {
-            return 0;
-        }
 #if FEATURE_MULTIREG_RET
-        for (unsigned i = 0; i < MAX_RET_REG_COUNT - 1; ++i)
+        // We need to return the highest index for which we have a valid register.
+        // Note that the gtOtherRegs array is off by one (the 0th register is gtRegNum).
+        // If there's no valid register in gtOtherRegs, gtRegNum must be valid.
+        for (unsigned i = MAX_RET_REG_COUNT; i > 1; i--)
         {
-            if (gtOtherRegs[i] == REG_NA)
+            if (gtOtherRegs[i - 2] != REG_NA)
             {
-                return i + 1;
+                return i;
             }
         }
-        return MAX_RET_REG_COUNT;
-#else
-        return 1;
 #endif
+        // We should never have a COPY or RELOAD with no valid registers.
+        assert(gtRegNum != REG_NA);
+        return 1;
     }
 
     GenTreeCopyOrReload(genTreeOps oper, var_types type, GenTree* op1) : GenTreeUnOp(oper, type, op1)