[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303130 91177308-0d34-0410-b5e6-96231b3b80d8

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.

test/CodeGen/Thumb2/v8_IT_5.ll

@@ -9,7 +9,7 @@
 ; CHECK-NEXT: b
 ; CHECK: [[JUMPTARGET]]:{{.*}}%if.else173
 ; CHECK-NEXT: mov.w
-; CHECK-NEXT: bx lr
+; CHECK-NEXT: pop
 ; CHECK-NEXT: %if.else145
 ; CHECK-NEXT: mov.w
 

test/CodeGen/X86/x86-shrink-wrapping.ll

@@ -270,8 +270,6 @@ if.end:                                           ; preds = %if.else, %for.end
   ret i32 %sum.1
 }
 
-declare void @somethingElse(...)
-
 ; Check with a more complex case that we do not have restore within the loop and
 ; save outside.
 ; CHECK-LABEL: loopInfoRestoreOutsideLoop:
@@ -982,3 +980,54 @@ for.inc:
 }
 
 attributes #4 = { "no-frame-pointer-elim"="true" }
+
+@x = external global i32, align 4
+@y = external global i32, align 4
+
+; The post-dominator tree does not include the branch containing the infinite
+; loop, which can occur into a misplacement of the restore block, if we're
+; looking for the nearest common post-dominator of an "unreachable" block.
+
+; CHECK-LABEL: infiniteLoopNoSuccessor:
+; CHECK: ## BB#0:
+; Make sure the prologue happens in the entry block.
+; CHECK-NEXT: pushq %rbp
+; ...
+; Make sure we don't shrink-wrap.
+; CHECK: ## BB#1
+; CHECK-NOT: pushq %rbp
+; ...
+; Make sure the epilogue happens in the exit block.
+; CHECK: ## BB#5
+; CHECK: popq %rbp
+; CHECK-NEXT: retq
+define void @infiniteLoopNoSuccessor() #5 {
+  %1 = load i32, i32* @x, align 4
+  %2 = icmp ne i32 %1, 0
+  br i1 %2, label %3, label %4
+
+; <label>:3:
+  store i32 0, i32* @x, align 4
+  br label %4
+
+; <label>:4:
+  call void (...) @somethingElse()
+  %5 = load i32, i32* @y, align 4
+  %6 = icmp ne i32 %5, 0
+  br i1 %6, label %10, label %7
+
+; <label>:7:
+  %8 = call i32 (...) @something()
+  br label %9
+
+; <label>:9:
+  call void (...) @somethingElse()
+  br label %9
+
+; <label>:10:
+  ret void
+}
+
+declare void @somethingElse(...)
+
+attributes #5 = { nounwind  "no-frame-pointer-elim-non-leaf" }