- Stop simplifycfg from duplicating "ret" instructions into unconditional

  branches. PR8575, rdar://5134905, rdar://8911460.
- Allow codegen tail duplication to dup small return blocks after register
  allocation is done.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124462 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 13 insertions, 5 deletions

diff --git a/lib/CodeGen/TailDuplication.cpp b/lib/CodeGen/TailDuplication.cpp
index ce4b1be854..15aed3436c 100644
--- a/lib/CodeGen/TailDuplication.cpp
+++ b/lib/CodeGen/TailDuplication.cpp
@@ -465,9 +465,12 @@ TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
     MaxDuplicateCount = TailDuplicateSize;
 
   if (PreRegAlloc) {
-      // Pre-regalloc tail duplication hurts compile time and doesn't help
-      // much except for indirect branches.
-    if (TailBB->empty() || !TailBB->back().getDesc().isIndirectBranch())
+    if (TailBB->empty())
+      return false;
+    const TargetInstrDesc &TID = TailBB->back().getDesc();
+    // Pre-regalloc tail duplication hurts compile time and doesn't help
+    // much except for indirect branches and returns.
+    if (!TID.isIndirectBranch() && !TID.isReturn())
       return false;
     // If the target has hardware branch prediction that can handle indirect
     // branches, duplicating them can often make them predictable when there
@@ -502,7 +505,7 @@ TailDuplicatePass::TailDuplicate(MachineBasicBlock *TailBB, MachineFunction &MF,
   }
   // Heuristically, don't tail-duplicate calls if it would expand code size,
   // as it's less likely to be worth the extra cost.
-  if (InstrCount > 1 && HasCall)
+  if (InstrCount > 1 && (PreRegAlloc && HasCall))
     return false;
 
   DEBUG(dbgs() << "\n*** Tail-duplicating BB#" << TailBB->getNumber() << '\n');
diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index f6d7d76dbf..37e6d28d7b 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -28,6 +28,7 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/CFG.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ConstantRange.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
@@ -36,6 +37,10 @@
 #include <map>
 using namespace llvm;
 
+static cl::opt<bool>
+DupRet("simplifycfg-dup-ret", cl::Hidden, cl::init(false),
+       cl::desc("Duplicate return instructions into unconditional branches"));
+
 STATISTIC(NumSpeculations, "Number of speculative executed instructions");
 
 namespace {
@@ -2027,7 +2032,7 @@ bool SimplifyCFGOpt::SimplifyReturn(ReturnInst *RI) {
   }
   
   // If we found some, do the transformation!
-  if (!UncondBranchPreds.empty()) {
+  if (!UncondBranchPreds.empty() && DupRet) {
     while (!UncondBranchPreds.empty()) {
       BasicBlock *Pred = UncondBranchPreds.pop_back_val();
       DEBUG(dbgs() << "FOLDING: " << *BB