Add cmdline option to apply more aggressive cfg merging

Author: HighW4y2H3ll

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -198,6 +198,10 @@ static cl::opt<unsigned> MaxSwitchCasesPerResult(
     "max-switch-cases-per-result", cl::Hidden, cl::init(16),
     cl::desc("Limit cases to analyze when converting a switch to select"));
 
+static cl::opt<bool> MergeBlockIntoPredecessorWithTwoSuccessor(
+    "simplifycfg-merge-into-pred-with-two-succ", cl::Hidden, cl::init(false),
+    cl::desc("Allow SimplifyCFG to merge blocks into predecessor even when the predecessor has two successors"));
+
 STATISTIC(NumBitMaps, "Number of switch instructions turned into bitmaps");
 STATISTIC(NumLinearMaps,
           "Number of switch instructions turned into linear mapping");
@@ -8339,7 +8343,7 @@ bool SimplifyCFGOpt::simplifyOnce(BasicBlock *BB) {
   // Merge basic blocks into their predecessor if there is only one distinct
   // pred, and if there is only one distinct successor of the predecessor, and
   // if there are no PHI nodes.
-  if (MergeBlockIntoPredecessor(BB, DTU))
+  if (MergeBlockIntoPredecessor(BB, DTU, nullptr, nullptr, nullptr, MergeBlockIntoPredecessorWithTwoSuccessor))
     return true;
 
   if (SinkCommon && Options.SinkCommonInsts)

llvm/test/Transforms/SimplifyCFG/two-succ-merge-block.ll

@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --tool ./opt --version 5
-; RUN: opt < %s -passes=simplifycfg -S | FileCheck %s
+; RUN: opt < %s -passes=simplifycfg -simplifycfg-merge-into-pred-with-two-succ -S | FileCheck %s
 
 define i1 @_Z7compareRK1SS1_(ptr %a, ptr %b) {
 ; CHECK-LABEL: @_Z7compareRK1SS1_(