ARM: avoid duplicating branches during constant islands.

We were using a naive heuristic to determine whether a basic block already had
an unconditional branch at the end. This mostly corresponded to reality
(assuming branches got optimised) because there's not much point in a branch to
the next block, but could go wrong.

llvm-svn: 221904

1 files changed, 10 insertions, 6 deletions

diff --git a/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp b/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
index fa9bfa2b327..1d3ca1a9ce3 100644
--- a/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ b/llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -275,6 +275,7 @@ namespace {
 
   private:
     void doInitialPlacement(std::vector<MachineInstr*> &CPEMIs);
+    bool BBHasFallthrough(MachineBasicBlock *MBB);
     CPEntry *findConstPoolEntry(unsigned CPI, const MachineInstr *CPEMI);
     unsigned getCPELogAlign(const MachineInstr *CPEMI);
     void scanFunctionJumpTables();
@@ -566,7 +567,7 @@ ARMConstantIslands::doInitialPlacement(std::vector<MachineInstr*> &CPEMIs) {
 
 /// BBHasFallthrough - Return true if the specified basic block can fallthrough
 /// into the block immediately after it.
-static bool BBHasFallthrough(MachineBasicBlock *MBB) {
+bool ARMConstantIslands::BBHasFallthrough(MachineBasicBlock *MBB) {
   // Get the next machine basic block in the function.
   MachineFunction::iterator MBBI = MBB;
   // Can't fall off end of function.
@@ -574,12 +575,15 @@ static bool BBHasFallthrough(MachineBasicBlock *MBB) {
     return false;
 
   MachineBasicBlock *NextBB = std::next(MBBI);
-  for (MachineBasicBlock::succ_iterator I = MBB->succ_begin(),
-       E = MBB->succ_end(); I != E; ++I)
-    if (*I == NextBB)
-      return true;
+  if (std::find(MBB->succ_begin(), MBB->succ_end(), NextBB) == MBB->succ_end())
+    return false;
 
-  return false;
+  // Try to analyze the end of the block. A potential fallthrough may already
+  // have an unconditional branch for whatever reason.
+  MachineBasicBlock *TBB, *FBB;
+  SmallVector<MachineOperand, 4> Cond;
+  bool TooDifficult = TII->AnalyzeBranch(*MBB, TBB, FBB, Cond);
+  return TooDifficult || FBB == nullptr;
 }
 
 /// findConstPoolEntry - Given the constpool index and CONSTPOOL_ENTRY MI,