Optimize flags reuse

Author: mikedn

src/jit/lower.cpp

@@ -2606,33 +2606,47 @@ GenTree* Lowering::LowerCompare(GenTree* cmp)
     {
         GenTree* op1 = cmp->gtGetOp1();
         GenTree* op2 = cmp->gtGetOp2();
-        LIR::Use cmpUse;
 
-        if (op1->OperIs(GT_AND, GT_OR, GT_XOR, GT_ADD, GT_SUB, GT_NEG) && op2->IsIntegralConst(0) &&
-            (op1->gtNext == op2) && (op2->gtNext == cmp) && BlockRange().TryGetUse(cmp, &cmpUse))
+        if (op2->IsIntegralConst(0) && op1->OperIs(GT_AND, GT_OR, GT_XOR, GT_ADD, GT_SUB, GT_NEG) &&
+            (op1->gtNext == op2) && (op2->gtNext == cmp))
         {
-            genTreeOps condition = cmp->OperGet();
-            GenTree*   next      = cmp->gtNext;
-            GenTreeCC* cc;
+            op1->gtFlags |= GTF_SET_FLAGS;
+            op1->SetUnusedValue();
 
-            if (cmpUse.User()->OperIs(GT_JTRUE))
+            BlockRange().Remove(op2);
+
+            GenTree*   next = cmp->gtNext;
+            GenTree*   cc;
+            genTreeOps ccOp;
+            LIR::Use   cmpUse;
+
+            // Fast check for the common case - relop used by a JTRUE that immediately follows it.
+            if ((next != nullptr) && next->OperIs(GT_JTRUE) && (next->gtGetOp1() == cmp))
             {
-                cmpUse.User()->ChangeOper(GT_JCC);
-                cc = cmpUse.User()->AsCC();
+                cc   = next;
+                ccOp = GT_JCC;
+                next = nullptr;
                 BlockRange().Remove(cmp);
             }
-            else
+            else if (BlockRange().TryGetUse(cmp, &cmpUse) && cmpUse.User()->OperIs(GT_JTRUE))
             {
-                cmp->ChangeOper(GT_SETCC);
-                cc = cmp->AsCC();
+                cc   = cmpUse.User();
+                ccOp = GT_JCC;
+                next = nullptr;
+                BlockRange().Remove(cmp);
+            }
+            else // The relop is not used by a JTRUE or it is not used at all.
+            {
+                // Transform the relop node it into a SETCC. If it's not used we could remove
+                // it completely but that means doing more work to handle a rare case.
+                cc   = cmp;
+                ccOp = GT_SETCC;
             }
 
-            cc->gtCondition = condition;
-            cc->gtFlags |= GTF_USE_FLAGS;
-            op1->gtFlags |= GTF_SET_FLAGS;
-            op1->SetUnusedValue();
-
-            BlockRange().Remove(op2);
+            genTreeOps condition = cmp->OperGet();
+            cc->ChangeOper(ccOp);
+            cc->AsCC()->gtCondition = condition;
+            cc->gtFlags |= GTF_USE_FLAGS | (cmp->gtFlags & GTF_UNSIGNED);
 
             return next;
         }