Index: cfe/trunk/lib/Format/BreakableToken.cpp
===================================================================
--- cfe/trunk/lib/Format/BreakableToken.cpp
+++ cfe/trunk/lib/Format/BreakableToken.cpp
@@ -545,15 +545,18 @@
 }
 
 BreakableToken::Split BreakableBlockComment::getSplitBefore(
-    unsigned LineIndex,
-    unsigned PreviousEndColumn,
-    unsigned ColumnLimit,
+    unsigned LineIndex, unsigned PreviousEndColumn, unsigned ColumnLimit,
     llvm::Regex &CommentPragmasRegex) const {
   if (!mayReflow(LineIndex, CommentPragmasRegex))
     return Split(StringRef::npos, 0);
   StringRef TrimmedContent = Content[LineIndex].ltrim(Blanks);
-  return getReflowSplit(TrimmedContent, ReflowPrefix, PreviousEndColumn,
-                        ColumnLimit);
+  Split Result = getReflowSplit(TrimmedContent, ReflowPrefix, PreviousEndColumn,
+                                ColumnLimit);
+  // Result is relative to TrimmedContent. Adapt it relative to
+  // Content[LineIndex].
+  if (Result.first != StringRef::npos)
+    Result.first += Content[LineIndex].size() - TrimmedContent.size();
+  return Result;
 }
 
 unsigned BreakableBlockComment::getReflownColumn(
@@ -633,17 +636,12 @@
         /*CurrentPrefix=*/ReflowPrefix, InPPDirective, /*Newlines=*/0,
         /*Spaces=*/0);
     // Check if we need to also insert a break at the whitespace range.
-    // For this we first adapt the reflow split relative to the beginning of the
-    // content.
     // Note that we don't need a penalty for this break, since it doesn't change
     // the total number of lines.
-    Split BreakSplit = SplitBefore;
-    BreakSplit.first += TrimmedContent.data() - Content[LineIndex].data();
     unsigned ReflownColumn =
         getReflownColumn(TrimmedContent, LineIndex, PreviousEndColumn);
-    if (ReflownColumn > ColumnLimit) {
-      insertBreak(LineIndex, 0, BreakSplit, Whitespaces);
-    }
+    if (ReflownColumn > ColumnLimit)
+      insertBreak(LineIndex, 0, SplitBefore, Whitespaces);
     return;
   }
 
Index: cfe/trunk/unittests/Format/FormatTestComments.cpp
===================================================================
--- cfe/trunk/unittests/Format/FormatTestComments.cpp
+++ cfe/trunk/unittests/Format/FormatTestComments.cpp
@@ -2803,6 +2803,23 @@
                    "    A = B;",
                    getLLVMStyleWithColumns(40)));
 }
+
+TEST_F(FormatTestComments, NoCrush_Bug34236) {
+  // This is a test case from a crasher reported in:
+  // https://bugs.llvm.org/show_bug.cgi?id=34236
+  EXPECT_EQ(
+      R"(
+/*                                                                */ /*
+                                                                      *       a
+                                                                      * b c
+                                                                      * d*/)",
+      format(
+          R"(
+/*                                                                */ /*
+ *       a b
+ *       c     d*/)",
+          getLLVMStyleWithColumns(80)));
+}
 } // end namespace
 } // end namespace format
 } // end namespace clang