runtime: Bad interaction between constant prop and CSE

Exploring the invariant hoisting code I found that it doesn’t work with the assignments.

In optimizer.cpp there’s a code:

   if (treeIsInvariant)
    {
        // Tree must be a suitable CSE candidate for us to be able to hoist it.
        treeIsHoistable = optIsCSEcandidate(tree);

optIsCSEcandidate is false for assignments.

Though it won’t work with the assignment the analysis will detect that the the expression subtree of the assignment is invariant and will actually clone the expression to the prehead block of the loop.

We’ll get a tree looking e.g. like this:

------------ BB04 [029..???), preds={BB01} succs={BB02}

***** BB04, stmt 8 (top level)
     (  5,  5) [000098] -------------             *  stmtExpr  void  (top level) (IL   ???...  ???)
N006 (  0,  0) [000096] -------------             |  /--*  nop       void  
N007 (  5,  5) [000097] -------------             \--*  comma     void  
N004 (  1,  1) [000095] -------H-----                |  /--*  lclVar    int    V04 loc3         u:3 <l:$46, c:$1c1>
N005 (  5,  5) [000091] -------H-----                \--*  +         int    <l:$4a, c:$1c6>
N002 (  1,  1) [000094] -------H-----                   |  /--*  lclVar    int    V03 loc2         u:3 <l:$44, c:$1c0>
N003 (  3,  3) [000092] -------H-----                   \--*  +         int    <l:$49, c:$1c5>
N001 (  1,  1) [000093] -------H-----                      \--*  lclVar    int    V02 loc1         u:3 <l:$42, c:$101>

------------ BB02 [029..043) -> BB02 (cond), preds={BB02,BB04} succs={BB03,BB02}

The result of such a tree is not used and on the code generator no code is produced for such a tree.

/cc @BruceForstall @brucehoult @briansull @Dmitri-Botcharnikov