[EarlyIfConversion] Don't if-convert unconditional branches.

A block ending in an unconditional branch can have two successors if one
is a landing pad.  In practice, I think this only has an effect on
Windows because landing pads are never empty for Itanium unwinding.

(Alternatively, I could add a check to
AArch64InstrInfo::canInsertSelect, but this seems more obvious.)

Differential Revision: https://reviews.llvm.org/D56468

llvm-svn: 351142

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/EarlyIfConversion.cpp b/llvm/lib/CodeGen/EarlyIfConversion.cpp
index 098afd885f2..364e1f03094 100644
--- a/llvm/lib/CodeGen/EarlyIfConversion.cpp
+++ b/llvm/lib/CodeGen/EarlyIfConversion.cpp
@@ -398,6 +398,13 @@ bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB) {
     return false;
   }
 
+  // Make sure the analyzed branch is conditional; one of the successors
+  // could be a landing pad. (Empty landing pads can be generated on Windows.)
+  if (Cond.empty()) {
+    LLVM_DEBUG(dbgs() << "AnalyzeBranch found an unconditional branch.\n");
+    return false;
+  }
+
   // AnalyzeBranch doesn't set FBB on a fall-through branch.
   // Make sure it is always set.
   FBB = TBB == Succ0 ? Succ1 : Succ0;