[ShrinkWrapping] Handle restores on no-return paths

Shrink-wrapping uses post-dominators to find a restore point that
post-dominates all the uses of CSR / stack.

The way dominator trees are modeled in LLVM today is that unreachable
blocks are not present in a generic dominator tree, so, an unreachable node is
dominated by anything: include/llvm/Support/GenericDomTree.h:467.

Since for post-dominators, a no-return block is considered
"unreachable", calling findNearestCommonDominator on an unreachable node
A and a non-unreachable node B, will return B, which can be false. If we
find such node, we bail out since there is no good restore point
available.

rdar://problem/30186931

llvm-svn: 303130

1 files changed, 8 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/ShrinkWrap.cpp b/llvm/lib/CodeGen/ShrinkWrap.cpp
index 4837495777d..e9df915bad1 100644
--- a/llvm/lib/CodeGen/ShrinkWrap.cpp
+++ b/llvm/lib/CodeGen/ShrinkWrap.cpp
@@ -282,8 +282,14 @@ void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB,
 
   if (!Restore)
     Restore = &MBB;
-  else
+  else if (MPDT->getNode(&MBB)) // If the block is not in the post dom tree, it
+                                // means the block never returns. If that's the
+                                // case, we don't want to call
+                                // `findNearestCommonDominator`, which will
+                                // return `Restore`.
     Restore = MPDT->findNearestCommonDominator(Restore, &MBB);
+  else
+    Restore = nullptr; // Abort, we can't find a restore point in this case.
 
   // Make sure we would be able to insert the restore code before the
   // terminator.
@@ -293,7 +299,7 @@ void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB,
         continue;
       // One of the terminator needs to happen before the restore point.
       if (MBB.succ_empty()) {
-        Restore = nullptr;
+        Restore = nullptr; // Abort, we can't find a restore point in this case.
         break;
       }
       // Look for a restore point that post-dominates all the successors.