[ARM] Prevent use of a value pointed by end() iterator when placing a jump table

Function ARMConstantIslands::doInitialJumpTablePlacement() iterates over all
basic blocks in a machine function. It calls `MI = MBB.getLastNonDebugInstr()`
to get the last instruction in each block and then uses MI->getOpcode() to
decide what to do. If getLastNonDebugInstr() returns MBB.end() (for example,
when the block does not contain any instructions) then calling getOpcode() on
this value is incorrect. Avoid this problem by checking the result of
getLastNonDebugInstr().

Differential Revision: http://reviews.llvm.org/D14694

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253222 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/ARM/ARMConstantIslandPass.cpp b/lib/Target/ARM/ARMConstantIslandPass.cpp
index 416c5fd..0bf2d37 100644 (file)
--- a/lib/Target/ARM/ARMConstantIslandPass.cpp
+++ b/lib/Target/ARM/ARMConstantIslandPass.cpp
@@ -589,6 +589,8 @@ void ARMConstantIslands::doInitialJumpTablePlacement(
   MachineBasicBlock *LastCorrectlyNumberedBB = nullptr;
   for (MachineBasicBlock &MBB : *MF) {
     auto MI = MBB.getLastNonDebugInstr();
+    if (MI == MBB.end())
+      continue;
 
     unsigned JTOpcode;
     switch (MI->getOpcode()) {