Fix lowering's containment analysis.

This fixes a silent bad code generation issue that arose during internal
testing. The original repro is a test failure under COMPlus_JitStress=2.
Due to explicit null check insertion, we (eventually) end up with the
following LIR:

    t6096 =    lclVar    ref    V86 cse10         <l:$4ad, c:$1b5>

            /--*  t6096  ref
            *  st.lclVar ref    V41 tmp29        d:26

    t2733 =    lclVar    ref    V41 tmp29        u:26 <l:$4ad, c:$1b5>

            /--*  t2733  ref
            *  nullcheck byte   <l:$4b8, c:$58a>

    t2736 =    lclVar    ref    V41 tmp29        u:26 (last use) <l:$4ad, c:$1b5>

    t2737 =    const     long   20 field offset Fseq[y] $107

            /--*  t2736  ref
            +--*  t2737  long
    t2735 = *  +         byref  <l:$2ad, c:$2ac>

    t6081 =    lclVar    ref    V83 cse7          <l:$4bd, c:$1b7>

            /--*  t6081  ref
            *  st.lclVar ref    V41 tmp29        d:27

    t2762 =    lclVar    ref    V41 tmp29        u:27 <l:$4bd, c:$1b7>

            /--*  t2762  ref
            *  nullcheck byte   <l:$583, c:$58f>

    t2765 =    lclVar    ref    V41 tmp29        u:27 (last use) <l:$4bd, c:$1b7>

    t2766 =    const     long   20 field offset Fseq[y] $107

            /--*  t2765  ref
            +--*  t2766  long
    t2764 = *  +         byref  <l:$2af, c:$2ae>

            /--*  t2764  byref
    t2763 = *  indir     int    <l:$54e, c:$1ed>

    t2767 =    lclVar    int   (AX) V07 loc4          $1ee

            /--*  t2763  int
            +--*  t2767  int
    t2738 = *  +         int    <l:$554, c:$553>

            /--*  t2735  byref
            +--*  t2738  int
            *  storeIndir int

During lowering, we attempt to form an RMW add rooted at the final
storeIndir. The pattern matching that attempts to form RMW operations,
however, does not consider whether or not it is safe to perform the
code motion involved in making the destination and source addresses
for the operator contained. In this case, lowering moves the evaluation
of the address (i.e. the dataflow tree rooted at the add that produces
t2735) into the storeIndir. This moves a use of tmp29 across a def of
the same and causes the program to store a value to an incorrect
address.

There are many variations on this pattern. For example, given the
following C#:

	static int T(C[] a, C c)
	{
	    return a.Length != c.M() ? 100 : 0;
	}

The evaluation of a.Length (including the necessary null check) should
occur before the call to c.M(). The lack of correct checks for safe
code motion that caused the original repro, however, cause the JIT to
generate bad code in this case as well: the null check for a is folded
into the load of a.Length, which is then made contained by the compare.
This results in the call to c.M() executing before the null check, which
causes the program to behave incorrectly in the case that a is null.

In order to fix the code motion analysis, this change introduces a new
type, `SideEffectSet`, that can be used to summarize the side effects
of a set of nodes and check whether or not they interfere with another
set of side effects. This change then uses the new type to ensure that
it is safe to perform the code motion necessary to make an operand
contained before doing so.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/jit/gentree.h b/src/jit/gentree.h
index a4c023aaee..f46b0a976e 100644
--- a/src/jit/gentree.h
+++ b/src/jit/gentree.h
@@ -1372,6 +1372,7 @@ public:
         {
             case GT_LOCKADD:
             case GT_XADD:
+            case GT_XCHG:
             case GT_CMPXCHG:
             case GT_BLK:
             case GT_OBJ:
@@ -1410,7 +1411,7 @@ public:
         return (gtOper == GT_XADD || gtOper == GT_XCHG || gtOper == GT_LOCKADD || gtOper == GT_CMPXCHG);
     }
 
-    bool OperIsAtomicOp()
+    bool OperIsAtomicOp() const
     {
         return OperIsAtomicOp(gtOper);
     }
@@ -3418,6 +3419,8 @@ struct GenTreeCall final : public GenTree
         return (gtCallMoreFlags & GTF_CALL_M_DOES_NOT_RETURN) != 0;
     }
 
+    bool IsPure(Compiler* compiler) const;
+
     unsigned short gtCallMoreFlags; // in addition to gtFlags
 
     unsigned char gtCallType : 3;   // value from the gtCallTypes enumeration