JIT: Optimize unused array allocations (#67205)

Co-authored-by: Jan Kotas <jkotas@microsoft.com>
Co-authored-by: Jakob Botsch Nielsen <Jakob.botsch.nielsen@gmail.com>

src/coreclr/inc/corinfo.h

@@ -1919,6 +1919,8 @@ struct CORINFO_VarArgInfo
 
 #define SIZEOF__CORINFO_Object                            TARGET_POINTER_SIZE /* methTable */
 
+#define CORINFO_Array_MaxLength                           0x7FFFFFC7
+
 #define OFFSETOF__CORINFO_Array__length                   SIZEOF__CORINFO_Object
 #ifdef TARGET_64BIT
 #define OFFSETOF__CORINFO_Array__data                     (OFFSETOF__CORINFO_Array__length + sizeof(uint32_t) /* length */ + sizeof(uint32_t) /* alignpad */)

src/coreclr/jit/earlyprop.cpp

@@ -29,58 +29,6 @@ bool Compiler::optDoEarlyPropForBlock(BasicBlock* block)
     return bbHasArrayRef || bbHasNullCheck;
 }
 
-//------------------------------------------------------------------------------
-// getArrayLengthFromAllocation: Return the array length for an array allocation
-//                               helper call.
-//
-// Arguments:
-//    tree           - The array allocation helper call.
-//    block          - tree's basic block.
-//
-// Return Value:
-//    Return the array length node.
-
-GenTree* Compiler::getArrayLengthFromAllocation(GenTree* tree DEBUGARG(BasicBlock* block))
-{
-    assert(tree != nullptr);
-
-    GenTree* arrayLength = nullptr;
-
-    if (tree->OperGet() == GT_CALL)
-    {
-        GenTreeCall* call = tree->AsCall();
-
-        if (call->gtCallType == CT_HELPER)
-        {
-            if (call->gtCallMethHnd == eeFindHelper(CORINFO_HELP_NEWARR_1_DIRECT) ||
-                call->gtCallMethHnd == eeFindHelper(CORINFO_HELP_NEWARR_1_OBJ) ||
-                call->gtCallMethHnd == eeFindHelper(CORINFO_HELP_NEWARR_1_VC) ||
-                call->gtCallMethHnd == eeFindHelper(CORINFO_HELP_NEWARR_1_ALIGN8))
-            {
-                // This is an array allocation site. Grab the array length node.
-                arrayLength = gtArgEntryByArgNum(call, 1)->GetNode();
-            }
-            else if (call->gtCallMethHnd == eeFindHelper(CORINFO_HELP_READYTORUN_NEWARR_1))
-            {
-                // On arm when compiling on certain platforms for ready to run, a handle will be
-                // inserted before the length. To handle this case, we will grab the last argument
-                // as that's always the length. See fgInitArgInfo for where the handle is inserted.
-                int arrLenArgNum = call->fgArgInfo->ArgCount() - 1;
-                arrayLength      = gtArgEntryByArgNum(call, arrLenArgNum)->GetNode();
-            }
-#ifdef DEBUG
-            if (arrayLength != nullptr)
-            {
-                optCheckFlagsAreSet(OMF_HAS_NEWARRAY, "OMF_HAS_NEWARRAY", BBF_HAS_NEWARRAY, "BBF_HAS_NEWARRAY", tree,
-                                    block);
-            }
-#endif
-        }
-    }
-
-    return arrayLength;
-}
-
 #ifdef DEBUG
 //-----------------------------------------------------------------------------
 // optCheckFlagsAreSet: Check that the method flag and the basic block flag are set.

src/coreclr/jit/gentree.cpp

@@ -1015,6 +1015,78 @@ bool GenTreeCall::IsPure(Compiler* compiler) const
            compiler->s_helperCallProperties.IsPure(compiler->eeGetHelperNum(gtCallMethHnd));
 }
 
+//------------------------------------------------------------------------------
+// getArrayLengthFromAllocation: Return the array length for an array allocation
+//                               helper call.
+//
+// Arguments:
+//    tree           - The array allocation helper call.
+//    block          - tree's basic block.
+//
+// Return Value:
+//    Return the array length node.
+
+GenTree* Compiler::getArrayLengthFromAllocation(GenTree* tree DEBUGARG(BasicBlock* block))
+{
+    assert(tree != nullptr);
+
+    GenTree* arrayLength = nullptr;
+
+    if (tree->OperGet() == GT_CALL)
+    {
+        GenTreeCall* call = tree->AsCall();
+        if (call->fgArgInfo == nullptr)
+        {
+            // Currently this function is only profitable during the late stages
+            // so we avoid complicating below code to access the early args.
+            return nullptr;
+        }
+
+        if (call->gtCallType == CT_HELPER)
+        {
+            switch (eeGetHelperNum(call->gtCallMethHnd))
+            {
+                case CORINFO_HELP_NEWARR_1_DIRECT:
+                case CORINFO_HELP_NEWARR_1_OBJ:
+                case CORINFO_HELP_NEWARR_1_VC:
+                case CORINFO_HELP_NEWARR_1_ALIGN8:
+                {
+                    // This is an array allocation site. Grab the array length node.
+                    arrayLength = gtArgEntryByArgNum(call, 1)->GetNode();
+                    break;
+                }
+
+                case CORINFO_HELP_READYTORUN_NEWARR_1:
+                {
+                    // On arm when compiling on certain platforms for ready to run, a handle will be
+                    // inserted before the length. To handle this case, we will grab the last argument
+                    // as that's always the length. See fgInitArgInfo for where the handle is inserted.
+                    int arrLenArgNum = call->fgArgInfo->ArgCount() - 1;
+                    arrayLength      = gtArgEntryByArgNum(call, arrLenArgNum)->GetNode();
+                    break;
+                }
+
+                default:
+                    break;
+            }
+#ifdef DEBUG
+            if ((arrayLength != nullptr) && (block != nullptr))
+            {
+                optCheckFlagsAreSet(OMF_HAS_NEWARRAY, "OMF_HAS_NEWARRAY", BBF_HAS_NEWARRAY, "BBF_HAS_NEWARRAY", tree,
+                                    block);
+            }
+#endif
+        }
+    }
+
+    if (arrayLength != nullptr)
+    {
+        arrayLength = arrayLength->OperIsPutArg() ? arrayLength->gtGetOp1() : arrayLength;
+    }
+
+    return arrayLength;
+}
+
 //-------------------------------------------------------------------------
 // HasSideEffects:
 //    Returns true if this call has any side effects. All non-helpers are considered to have side-effects. Only helpers
@@ -1052,6 +1124,21 @@ bool GenTreeCall::HasSideEffects(Compiler* compiler, bool ignoreExceptions, bool
         return true;
     }
 
+    // Consider array allocators side-effect free for constant length (if it's not negative and fits into i32)
+    if (helperProperties.IsAllocator(helper))
+    {
+        GenTree* arrLen = compiler->getArrayLengthFromAllocation((GenTree*)this DEBUGARG(nullptr));
+        // if arrLen is nullptr it means it wasn't an array allocator
+        if ((arrLen != nullptr) && arrLen->IsIntCnsFitsInI32())
+        {
+            ssize_t cns = arrLen->AsIntConCommon()->IconValue();
+            if ((cns >= 0) && (cns <= CORINFO_Array_MaxLength))
+            {
+                return false;
+            }
+        }
+    }
+
     // If we also care about exceptions then check if the helper can throw
     if (!ignoreExceptions && !helperProperties.NoThrow(helper))
     {

src/tests/GC/API/GC/GetGCMemoryInfo.cs

@@ -71,7 +71,7 @@ static void MakeTemporarySOHAllocations()
         int byteArraySize = 1000;
         for (int i = 0; i < (totalTempAllocBytes / byteArraySize); i++)
         {
-            byte[] byteArray = new byte[byteArraySize];
+            GC.KeepAlive(new byte[byteArraySize]);
         }
     }
 

src/tests/profiler/gc/gcallocate.cs

@@ -14,6 +14,11 @@ public static int RunTest(String[] args)
         {
             int[] large = new int[100000];
             int[] pinned = GC.AllocateArray<int>(32, true);
+
+            // don't let the jit to optimize these allocations
+            GC.KeepAlive(large);
+            GC.KeepAlive(pinned);
+
             Console.WriteLine("Test Passed");
             return 100;
         }