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

[efriedma]

Summary

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.)

[mgrang]

LGTM. I have verified that this patch resolves the problem in our custom code base.

llvm/trunk/lib/CodeGen/EarlyIfConversion.cpp

bool SSAIfConv::canConvertIf(MachineBasicBlock *MBB) {
   }
 
   // This is weird, probably some sort of degenerate CFG.
   if (!TBB) {
     LLVM_DEBUG(dbgs() << "AnalyzeBranch didn't find conditional branch.\n");
     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;
 
   // Any phis in the tail block must be convertible to selects.
   PHIs.clear();
   MachineBasicBlock *TPred = getTPred();
   MachineBasicBlock *FPred = getFPred();

llvm/trunk/test/CodeGen/AArch64/landingpad-ifcvt.ll

+; RUN: llc < %s | FileCheck %s
+
+; Make sure this doesn't crash (and the output is sane).
+; CHECK: ; %__except.ret
+; CHECK-NEXT: mov     x0, xzr
+
+target datalayout = "e-m:w-p:64:64-i32:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64-pc-windows-msvc19.11.0"
+
+define i64 @f(i32* %hwnd, i32 %message, i64 %wparam, i64 %lparam) personality i8* bitcast (i32 (...)* @__C_specific_handler to i8*) {
+entry:
+  %call = invoke i64 @callee(i32* %hwnd, i32 %message, i64 %wparam, i64 %lparam)
+          to label %__try.cont unwind label %catch.dispatch
+
+catch.dispatch:                                   ; preds = %entry
+  %0 = catchswitch within none [label %__except.ret] unwind to caller
+
+__except.ret:                                     ; preds = %catch.dispatch
+  %1 = catchpad within %0 [i8* bitcast (i32 (i8*, i8*)* @filt to i8*)]
+  catchret from %1 to label %__try.cont
+
+__try.cont:                                       ; preds = %__except.ret, %entry
+  %rv.0 = phi i64 [ 0, %__except.ret ], [ %call, %entry ]
+  ret i64 %rv.0
+}
+
+declare dso_local i64 @callee(i32*, i32, i64, i64)
+declare i32 @filt(i8*, i8* nocapture readnone)
+declare dso_local i32 @__C_specific_handler(...)