Don't mark calls to allocation helpers as CSE candidates.

Marking them as CSE candidates usually blocks CSEs rather than enables them.
A typical case is:
 [1] GT_IND(x) = GT_CALL ALLOC_HELPER
...
[2] y = GT_IND(x)
...
[3] z = GT_IND(x)
If we mark CALL ALLOC_HELPER as a CSE candidate, we later discover
that it can't be a CSE def because GT_INDs in [2] and [3] can cause
more exceptions (NullRef) so we abandon this CSE.
If we don't mark CALL ALLOC_HELPER as a CSE candidate, we are able
to use GT_IND(x) in [2] as a CSE def and do a CSE on [2] and [3].

1 files changed, 23 insertions, 0 deletions

diff --git a/src/jit/optcse.cpp b/src/jit/optcse.cpp
index 52157c6479..f578b2aa7e 100644
--- a/src/jit/optcse.cpp
+++ b/src/jit/optcse.cpp
@@ -2564,6 +2564,29 @@ bool Compiler::optIsCSEcandidate(GenTree* tree)
     switch (oper)
     {
         case GT_CALL:
+
+            GenTreeCall* call;
+            call = tree->AsCall();
+
+            // Don't mark calls to allocation helpers as CSE candidates.
+            // Marking them as CSE candidates usually blocks CSEs rather than enables them.
+            // A typical case is:
+            // [1] GT_IND(x) = GT_CALL ALLOC_HELPER
+            // ...
+            // [2] y = GT_IND(x)
+            // ...
+            // [3] z = GT_IND(x)
+            // If we mark CALL ALLOC_HELPER as a CSE candidate, we later discover
+            // that it can't be a CSE def because GT_INDs in [2] and [3] can cause
+            // more exceptions (NullRef) so we abandon this CSE.
+            // If we don't mark CALL ALLOC_HELPER as a CSE candidate, we are able
+            // to use GT_IND(x) in [2] as a CSE def.
+            if ((call->gtCallType == CT_HELPER) &&
+                s_helperCallProperties.IsAllocator(eeGetHelperNum(call->gtCallMethHnd)))
+            {
+                return false;
+            }
+
             // If we have a simple helper call with no other persistent side-effects
             // then we allow this tree to be a CSE candidate
             //