Keep the link register for uwtable.

The function with uwtable attribute might be visited by the
stack unwinder, thus the link register should be considered
as clobbered after the execution of the branch and link
instruction (i.e. the definition of the machine instruction
can't be ignored) even when the callee function are marked
with noreturn.

llvm-svn: 202165

llvm/lib/CodeGen/VirtRegMap.cpp

@@ -278,6 +278,11 @@ void VirtRegRewriter::rewrite() {
   PhysRegs.clear();
   PhysRegs.setUniverse(TRI->getNumRegs());
 
+  // The function with uwtable should guarantee that the stack unwinder
+  // can unwind the stack to the previous frame.  Thus, we can't apply the
+  // noreturn optimization if the caller function has uwtable attribute.
+  bool HasUWTable = MF->getFunction()->hasFnAttribute(Attribute::UWTable);
+
   for (MachineFunction::iterator MBBI = MF->begin(), MBBE = MF->end();
        MBBI != MBBE; ++MBBI) {
     DEBUG(MBBI->print(dbgs(), Indexes));
@@ -287,9 +292,12 @@ void VirtRegRewriter::rewrite() {
       MachineInstr *MI = MII;
       ++MII;
 
-      // Check if this instruction is a call to a noreturn function.
-      // If so, all the definitions set by this instruction can be ignored.
-      if (IsExitBB && MI->isCall())
+      // Check if this instruction is a call to a noreturn function.  If this
+      // is a call to noreturn function and we don't need the stack unwinding
+      // functionality (i.e. this function does not have uwtable attribute and
+      // the callee function has the nounwind attribute), then we can ignore
+      // the definitions set by this instruction.
+      if (!HasUWTable && IsExitBB && MI->isCall()) {
         for (MachineInstr::mop_iterator MOI = MI->operands_begin(),
                MOE = MI->operands_end(); MOI != MOE; ++MOI) {
           MachineOperand &MO = *MOI;
@@ -305,6 +313,7 @@ void VirtRegRewriter::rewrite() {
           NoReturnInsts.insert(MI);
           break;
         }
+      }
 
       for (MachineInstr::mop_iterator MOI = MI->operands_begin(),
            MOE = MI->operands_end(); MOI != MOE; ++MOI) {

llvm/test/CodeGen/ARM/noreturn.ll

@@ -43,6 +43,23 @@ entry:
   unreachable
 }
 
+; Test case for uwtable
+define i32 @test4() uwtable {
+; CHECK-LABEL: @test4
+; CHECK: push
+entry:
+  tail call void @overflow() #0
+  unreachable
+}
+
+define i32 @test5() uwtable {
+; CHECK-LABEL: @test5
+; CHECK: push
+entry:
+  tail call void @overflow_with_unwind() #1
+  unreachable
+}
+
 ; Function Attrs: noreturn
 declare void @overflow_with_unwind() #1