AOT: Enable branch merging for checked smi comparisons

runtime/vm/flow_graph.cc

Index: runtime/vm/flow_graph.cc
diff --git a/runtime/vm/flow_graph.cc b/runtime/vm/flow_graph.cc
index f513b304c012f92220fa118ad8641bb91b59e133..46ecb5d92b4ea886d1d24522fefebbf07c71fa6b 100644
--- a/runtime/vm/flow_graph.cc
+++ b/runtime/vm/flow_graph.cc
@@ -2014,7 +2014,7 @@ void FlowGraph::EliminateEnvironments() {
     }
     for (ForwardInstructionIterator it(block); !it.Done(); it.Advance()) {
       Instruction* current = it.Current();
-      if (!current->CanDeoptimize()) {
+      if (!current->CanDeoptimize() && !current->MayThrow()) {

[Vyacheslav Egorov (Google), 2016/11/01 13:19:06]

BranchInstr::CanDeoptimize(...) has some special code for the case when branches
can throw. Maybe we should remove it from there. 

Also I find it confusing that throwing needs deopt info (I always forget why one
needs deopt environments for throwing). So maybe comment should be added here
explaining that.

[Florian Schneider, 2016/11/01 15:26:25]

I'll add a check if the instruction is inside a try-block: It's try-catch that
needs the environment to emit the call-prologue inside try-blocks in optimized
code. Stacktraces don't need them, and deopt-infos are not written in the
snapshot. We omit inlined frames from stacktraces in AOT compiled code for
snapshot size reasons.

         // TODO(srdjan): --source-lines needs deopt environments to get at
         // the code for this instruction, however, leaving the environment
         // changes code.