[PATCH] Fix failure to invoke exception handler on Win64.

[Reid Kleckner]

================
Comment at: lib/Target/X86/X86FrameLowering.cpp:838
@@ +837,3 @@
+  MachineFunction* MF = MFI->getParent();
+Skip:
+  if (MBBI == MFI->begin()) {
----------------
This seems like a do { } while (...) loop.

================
Comment at: lib/Target/X86/X86FrameLowering.cpp:844
@@ +843,3 @@
+      --MFI;
+      MBBI = MFI->end();
+    }
----------------
Does this not return one-past-the-end?

That aside, I don't think this will work in general, because basic block placement runs *after* prologue-epilogue insertion.

For now I think we should always insert a nop if the epilogue is at the beginning of a basic block, just to be safe. The actual solution is probably to insert some kind of target-specific pseudo that we lower out after block placement to either a nop or nothing.

================
Comment at: test/CodeGen/X86/win64_call_epi.ll:1
@@ +1,2 @@
+; RUN: llc < %s -O0 -mcpu=corei7 -mtriple=x86_64-pc-mingw32 | FileCheck %s -check-prefix=WIN64
+
----------------
Does this fail without -O0?  Just curious.

================
Comment at: test/CodeGen/X86/win64_call_epi.ll:3
@@ +2,3 @@
+
+define void @foo() {
+
----------------
Can you come up with a test case that fails when block placement runs?  Something like this maybe:

  define void @bar(i1 zeroext %cond ) {
    br i1 %cond, label %a, label %b  ; <- Add metadata to indicate that %a is *highly* likely over %b.
  a:
    call void @foo()
    br label %done
  b:
    call void @baz()
    store i32 0, i32* @something
    br label %done
  done:
    ret void
  }

http://reviews.llvm.org/D4751