[PATCH] Fix for crash during SjLj preparation step

[Valerii Hiora]

Added a test case and used 'select' as Bill proposed. 

Still need advice if it is possible to unify both aggregate type branches into one, as I'm not sure if there is a combination of codegen/optimization flags which might not eliminate loading from null.

http://reviews.llvm.org/D4256

Files:
  lib/CodeGen/SjLjEHPrepare.cpp
  test/CodeGen/Thumb/sjljehprepare-lower-empty-struct.ll

Index: lib/CodeGen/SjLjEHPrepare.cpp
===================================================================
--- lib/CodeGen/SjLjEHPrepare.cpp
+++ lib/CodeGen/SjLjEHPrepare.cpp
@@ -248,19 +248,38 @@
   for (Function::arg_iterator AI = F.arg_begin(), AE = F.arg_end(); AI != AE;
        ++AI) {
     Type *Ty = AI->getType();
-
-    // Aggregate types can't be cast, but are legal argument types, so we have
-    // to handle them differently. We use an extract/insert pair as a
-    // lightweight method to achieve the same goal.
-    if (isa<StructType>(Ty) || isa<ArrayType>(Ty)) {
-      Instruction *EI = ExtractValueInst::Create(AI, 0, "", AfterAllocaInsPt);
-      Instruction *NI = InsertValueInst::Create(AI, EI, 0);
-      NI->insertAfter(EI);
-      AI->replaceAllUsesWith(NI);
-
-      // Set the operand of the instructions back to the AllocaInst.
-      EI->setOperand(0, AI);
-      NI->setOperand(0, AI);
+    StructType *ST = dyn_cast<StructType>(Ty);
+    ArrayType *AT = dyn_cast<ArrayType>(Ty);
+
+    if (ST || AT) {
+      // Aggregate types can't be cast, but are legal argument types, so we have
+      // to handle them differently. We use an always true selection to achieve
+      // the same goal
+      if ((ST && ST->getNumElements() != 0)
+          || (AT && AT->getNumElements() != 0)) {
+        Instruction *EI = ExtractValueInst::Create(AI, 0, "", AfterAllocaInsPt);
+        Instruction *NI = InsertValueInst::Create(AI, EI, 0);
+        NI->insertAfter(EI);
+        AI->replaceAllUsesWith(NI);
+
+        // Set the operand of the instructions back to the AllocaInst.
+        EI->setOperand(0, AI);
+        NI->setOperand(0, AI);
+      } else {
+        // If it is empty structure or 0-length array it is impossible
+        // to use ExtractValue/InsertValue pair, so instead
+        // always-true select is used against arg and reconstructed
+        // from null value
+        Value *NullValue = ConstantPointerNull::get(Ty->getPointerTo());
+
+        Instruction *LI = new LoadInst(NullValue, AI->getName() + ".null", AfterAllocaInsPt);
+        Value *TrueValue = ConstantInt::getTrue(F.getContext());
+        Instruction *SI = SelectInst::Create(TrueValue, AI, LI);
+        SI->insertAfter(LI);
+        AI->replaceAllUsesWith(SI);
+
+        SI->setOperand(1, AI);
+      }
     } else {
       // This is always a no-op cast because we're casting AI to AI->getType()
       // so src and destination types are identical. BitCast is the only
Index: test/CodeGen/Thumb/sjljehprepare-lower-empty-struct.ll
===================================================================
--- /dev/null
+++ test/CodeGen/Thumb/sjljehprepare-lower-empty-struct.ll
@@ -0,0 +1,22 @@
+; RUN: llc -mtriple=thumbv7-apple-ios < %s
+; SjLjEHPrepare shouldn't crash when lowering empty structs.
+
+target datalayout = "e-p:32:32:32-i1:8:32-i8:8:32-i16:16:32-i32:32:32-i64:32:64-f32:32:32-f64:32:64-v64:32:64-v128:32:128-a0:0:32-n32-S32"
+target triple = "thumbv7-apple-ios"
+
+define i8* @foo(i32 %d, {} %c) {
+entry:
+  invoke void @bar ()
+    to label %unreachable unwind label %handler
+
+unreachable:
+  unreachable
+
+handler:
+  %tmp = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @baz to i8*)
+  cleanup
+  resume { i8*, i32 } undef
+}
+
+declare void @bar()
+declare i32 @baz(...)
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