From b7700e2074652d3e60278a2b60fecac10ef0fdad Mon Sep 17 00:00:00 2001
From: Rajat Dua <radua@microsoft.com>
Date: Thu, 26 Jul 2018 11:25:08 -0700
Subject: [PATCH] Don't add compensation code for array values in prepass.
 OS#17527968

---
 lib/Backend/GlobOptBlockData.cpp             | 91 +++++++++-----------
 test/Optimizer/Miscellaneous_MaxInterpret.js | 20 +++++
 2 files changed, 63 insertions(+), 48 deletions(-)

diff --git a/lib/Backend/GlobOptBlockData.cpp b/lib/Backend/GlobOptBlockData.cpp
index 19bcc514d51..31ad9566b1a 100644
--- a/lib/Backend/GlobOptBlockData.cpp
+++ b/lib/Backend/GlobOptBlockData.cpp
@@ -1086,7 +1086,7 @@ ValueInfo *GlobOptBlockData::MergeArrayValueInfo(
     // but in different syms, create a new sym and record that the array sym requires compensation. Compensation will be
     // inserted later to initialize this new sym from all predecessors of the merged block.
 
-    StackSym *newHeadSegmentSym;
+    StackSym *newHeadSegmentSym = nullptr;
     if(toDataValueInfo->HeadSegmentSym() && fromDataValueInfo->HeadSegmentSym())
     {
         if(toDataValueInfo->HeadSegmentSym() == fromDataValueInfo->HeadSegmentSym())
@@ -1095,27 +1095,26 @@ ValueInfo *GlobOptBlockData::MergeArrayValueInfo(
         }
         else
         {
-            Assert(!this->globOpt->IsLoopPrePass());
-            Assert(symsRequiringCompensation);
-            symsRequiringCompensation->Set(arraySym->m_id);
-            Assert(symsCreatedForMerge);
-            if(symsCreatedForMerge->Test(toDataValueInfo->HeadSegmentSym()->m_id))
+            if (!this->globOpt->IsLoopPrePass())
             {
-                newHeadSegmentSym = toDataValueInfo->HeadSegmentSym();
-            }
-            else
-            {
-                newHeadSegmentSym = StackSym::New(TyMachPtr, this->globOpt->func);
-                symsCreatedForMerge->Set(newHeadSegmentSym->m_id);
+                // Adding compensation code in the prepass won't help, as the symstores would again be different in the main pass.
+                Assert(symsRequiringCompensation);
+                symsRequiringCompensation->Set(arraySym->m_id);
+                Assert(symsCreatedForMerge);
+                if (symsCreatedForMerge->Test(toDataValueInfo->HeadSegmentSym()->m_id))
+                {
+                    newHeadSegmentSym = toDataValueInfo->HeadSegmentSym();
+                }
+                else
+                {
+                    newHeadSegmentSym = StackSym::New(TyMachPtr, this->globOpt->func);
+                    symsCreatedForMerge->Set(newHeadSegmentSym->m_id);
+                }
             }
         }
     }
-    else
-    {
-        newHeadSegmentSym = nullptr;
-    }
 
-    StackSym *newHeadSegmentLengthSym;
+    StackSym *newHeadSegmentLengthSym = nullptr;
     if(toDataValueInfo->HeadSegmentLengthSym() && fromDataValueInfo->HeadSegmentLengthSym())
     {
         if(toDataValueInfo->HeadSegmentLengthSym() == fromDataValueInfo->HeadSegmentLengthSym())
@@ -1124,27 +1123,25 @@ ValueInfo *GlobOptBlockData::MergeArrayValueInfo(
         }
         else
         {
-            Assert(!this->globOpt->IsLoopPrePass());
-            Assert(symsRequiringCompensation);
-            symsRequiringCompensation->Set(arraySym->m_id);
-            Assert(symsCreatedForMerge);
-            if(symsCreatedForMerge->Test(toDataValueInfo->HeadSegmentLengthSym()->m_id))
+            if (!this->globOpt->IsLoopPrePass())
             {
-                newHeadSegmentLengthSym = toDataValueInfo->HeadSegmentLengthSym();
-            }
-            else
-            {
-                newHeadSegmentLengthSym = StackSym::New(TyUint32, this->globOpt->func);
-                symsCreatedForMerge->Set(newHeadSegmentLengthSym->m_id);
+                Assert(symsRequiringCompensation);
+                symsRequiringCompensation->Set(arraySym->m_id);
+                Assert(symsCreatedForMerge);
+                if (symsCreatedForMerge->Test(toDataValueInfo->HeadSegmentLengthSym()->m_id))
+                {
+                    newHeadSegmentLengthSym = toDataValueInfo->HeadSegmentLengthSym();
+                }
+                else
+                {
+                    newHeadSegmentLengthSym = StackSym::New(TyUint32, this->globOpt->func);
+                    symsCreatedForMerge->Set(newHeadSegmentLengthSym->m_id);
+                }
             }
         }
     }
-    else
-    {
-        newHeadSegmentLengthSym = nullptr;
-    }
 
-    StackSym *newLengthSym;
+    StackSym *newLengthSym = nullptr;
     if(toDataValueInfo->LengthSym() && fromDataValueInfo->LengthSym())
     {
         if(toDataValueInfo->LengthSym() == fromDataValueInfo->LengthSym())
@@ -1153,25 +1150,23 @@ ValueInfo *GlobOptBlockData::MergeArrayValueInfo(
         }
         else
         {
-            Assert(!this->globOpt->IsLoopPrePass());
-            Assert(symsRequiringCompensation);
-            symsRequiringCompensation->Set(arraySym->m_id);
-            Assert(symsCreatedForMerge);
-            if(symsCreatedForMerge->Test(toDataValueInfo->LengthSym()->m_id))
-            {
-                newLengthSym = toDataValueInfo->LengthSym();
-            }
-            else
+            if (!this->globOpt->IsLoopPrePass())
             {
-                newLengthSym = StackSym::New(TyUint32, this->globOpt->func);
-                symsCreatedForMerge->Set(newLengthSym->m_id);
+                Assert(symsRequiringCompensation);
+                symsRequiringCompensation->Set(arraySym->m_id);
+                Assert(symsCreatedForMerge);
+                if (symsCreatedForMerge->Test(toDataValueInfo->LengthSym()->m_id))
+                {
+                    newLengthSym = toDataValueInfo->LengthSym();
+                }
+                else
+                {
+                    newLengthSym = StackSym::New(TyUint32, this->globOpt->func);
+                    symsCreatedForMerge->Set(newLengthSym->m_id);
+                }
             }
         }
     }
-    else
-    {
-        newLengthSym = nullptr;
-    }
 
     if(newHeadSegmentSym || newHeadSegmentLengthSym || newLengthSym)
     {
diff --git a/test/Optimizer/Miscellaneous_MaxInterpret.js b/test/Optimizer/Miscellaneous_MaxInterpret.js
index 0936cc99df0..e60e7e3bf0b 100644
--- a/test/Optimizer/Miscellaneous_MaxInterpret.js
+++ b/test/Optimizer/Miscellaneous_MaxInterpret.js
@@ -1071,3 +1071,23 @@ function test66() {
 }
 WScript.Echo("test66: " + test66());
 WScript.Echo("test66: " + test66());
+
+(function test67Runner(){
+    function test67(a,b,x){
+        var i = a[0];
+        var j = b[0];
+        var t = a;
+        for (var p = 0; p < x; p++) {
+            if (p > 2) {
+                t = b;
+                b = a;
+            }
+        }
+    }
+
+    var arr1 = [1,2];
+    var arr2 = [3,4];
+    var x = 1;
+    test67(arr1, arr2, x);
+    test67(arr1, arr2, x)
+})();
\ No newline at end of file