From 973dc3b1d8f81389ed87664772595afd75567dd3 Mon Sep 17 00:00:00 2001
From: Bill Wendling <isanbard@gmail.com>
Date: Thu, 15 Jul 2010 23:32:40 +0000
Subject: Handle code gen for the unreachable instruction if it's the only
 instruction in the function. We'll just turn it into a "trap" instruction
 instead.

The problem with not handling this is that it might generate a prologue without
the equivalent epilogue to go with it:

$ cat t.ll
define void @foo() {
entry:
  unreachable
}
$ llc -o - t.ll -relocation-model=pic -disable-fp-elim -unwind-tables
        .section        __TEXT,__text,regular,pure_instructions
        .globl  _foo
        .align  4, 0x90
_foo:                                   ## @foo
Leh_func_begin0:
## BB#0:                                ## %entry
        pushq   %rbp
Ltmp0:
        movq    %rsp, %rbp
Ltmp1:
Leh_func_end0:
...

The unwind tables then have bad data in them causing all sorts of problems.

Fixes <rdar://problem/8096481>.

llvm-svn: 108473
---
 llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

(limited to 'llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp')

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 458e865a6b3..1fa887c69e0 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -796,6 +796,7 @@ void SelectionDAGBuilder::visit(unsigned Opcode, const User &I) {
 #define HANDLE_INST(NUM, OPCODE, CLASS) \
     case Instruction::OPCODE: visit##OPCODE((CLASS&)I); break;
 #include "llvm/Instruction.def"
+#undef HANDLE_INST
   }
 
   // Assign the ordering to the freshly created DAG nodes.
@@ -2194,6 +2195,19 @@ void SelectionDAGBuilder::visitIndirectBr(const IndirectBrInst &I) {
                           getValue(I.getAddress())));
 }
 
+void SelectionDAGBuilder::visitUnreachable(const UnreachableInst &I) {
+  // If the function consists of a single "unreachable" instruction, emit a
+  // "trap". This prevents the back-ends from generating empty functions or
+  // functions which have a prologue, but no epilogue.
+  const BasicBlock *BB = I.getParent();
+  const Function *F = BB->getParent();
+
+  if (F->size() == 1 && BB->size() == 1 &&
+      isa<UnreachableInst>(BB->getTerminator()))
+      DAG.setRoot(DAG.getNode(ISD::TRAP, getCurDebugLoc(),
+                              MVT::Other, getRoot()));
+}
+
 void SelectionDAGBuilder::visitFSub(const User &I) {
   // -0.0 - X --> fneg
   const Type *Ty = I.getType();
-- 
cgit v1.2.3