From 86f8e7887eb73b94525f2a51462ef97d9c435fbf Mon Sep 17 00:00:00 2001
From: Andy Ayers <andya@microsoft.com>
Date: Wed, 30 Aug 2017 17:36:07 -0700
Subject: [PATCH] JIT: don't reuse box temps when optimizing (#13703)

The importer reuses temps for different box operations that do not overlap.
This keeps the number of temps to a minimum and reduces prolog zeroing if the
temps end up untracked. But reuse prevents the importer from accurately typing
the temp.

So now, when optimizing, allocate a new temp for each box operation, and type
the temp with the type of the box.

This, along with a small update in `gtGetClassHandle` to obtain the box type,
enables some devirtualization of interface calls on boxes and will facilitate
future changes to optimize away boxes entirely (eg #5626).
---
 src/jit/gentree.cpp  | 15 +++++++++++++++
 src/jit/importer.cpp | 18 ++++++++++++++++--
 2 files changed, 31 insertions(+), 2 deletions(-)

diff --git a/src/jit/gentree.cpp b/src/jit/gentree.cpp
index df700a8884..753bb56455 100644
--- a/src/jit/gentree.cpp
+++ b/src/jit/gentree.cpp
@@ -15978,6 +15978,21 @@ CORINFO_CLASS_HANDLE Compiler::gtGetClassHandle(GenTreePtr tree, bool* isExact,
             break;
         }
 
+        case GT_BOX:
+        {
+            // Box should just wrap a local var reference which has
+            // the type we're looking for. Also box only represents a
+            // non-nullable value type so result cannot be null.
+            GenTreeBox* box     = obj->AsBox();
+            GenTree*    boxTemp = box->BoxOp();
+            assert(boxTemp->IsLocal());
+            const unsigned boxTempLcl = boxTemp->AsLclVar()->GetLclNum();
+            objClass                  = lvaTable[boxTempLcl].lvClassHnd;
+            *isExact                  = lvaTable[boxTempLcl].lvClassIsExact;
+            *isNonNull                = true;
+            break;
+        }
+
         default:
         {
             break;
diff --git a/src/jit/importer.cpp b/src/jit/importer.cpp
index e325d05c5e..0e456a8eae 100644
--- a/src/jit/importer.cpp
+++ b/src/jit/importer.cpp
@@ -5255,9 +5255,23 @@ void Compiler::impImportAndPushBox(CORINFO_RESOLVED_TOKEN* pResolvedToken)
         // and the other you get
         //    *(temp+4) = expr
 
-        if (impBoxTempInUse || impBoxTemp == BAD_VAR_NUM)
+        if (opts.MinOpts() || opts.compDbgCode)
         {
-            impBoxTemp = lvaGrabTemp(true DEBUGARG("Box Helper"));
+            // For minopts/debug code, try and minimize the total number
+            // of box temps by reusing an existing temp when possible.
+            if (impBoxTempInUse || impBoxTemp == BAD_VAR_NUM)
+            {
+                impBoxTemp = lvaGrabTemp(true DEBUGARG("Reusable Box Helper"));
+            }
+        }
+        else
+        {
+            // When optimizing, use a new temp for each box operation
+            // since we then know the exact class of the box temp.
+            impBoxTemp                  = lvaGrabTemp(true DEBUGARG("Single-def Box Helper"));
+            lvaTable[impBoxTemp].lvType = TYP_REF;
+            const bool isExact          = true;
+            lvaSetClass(impBoxTemp, pResolvedToken->hClass, isExact);
         }
 
         // needs to stay in use until this box expression is appended
-- 
2.34.1