Index: lib/Format/BreakableToken.h
===================================================================
--- lib/Format/BreakableToken.h
+++ lib/Format/BreakableToken.h
@@ -204,6 +204,10 @@
   // present) is also not considered part of the text.
   SmallVector<StringRef, 16> Lines;
 
+  // For each line in a block comment this stores how many characters overflowed
+  // into the next line if it was wrapped. Otherwise the value is 0.
+  SmallVector<unsigned, 16> WrappedLines;
+
   // LeadingWhitespace[i] is the number of characters regarded as whitespace in
   // front of Lines[i]. Note that this can include "* " sequences, which we
   // regard as whitespace when all lines have a "*" prefix.
Index: lib/Format/BreakableToken.cpp
===================================================================
--- lib/Format/BreakableToken.cpp
+++ lib/Format/BreakableToken.cpp
@@ -259,6 +259,7 @@
   TokenText.substr(2, TokenText.size() - 4).split(Lines, "\n");
 
   int IndentDelta = StartColumn - OriginalStartColumn;
+  WrappedLines.resize(Lines.size());
   LeadingWhitespace.resize(Lines.size());
   StartOfLineColumn.resize(Lines.size());
   StartOfLineColumn[0] = StartColumn + 2;
@@ -400,6 +401,8 @@
   Whitespaces.replaceWhitespaceInToken(
       Tok, BreakOffsetInToken, CharsToRemove, "", Prefix, InPPDirective, 1,
       IndentLevel, IndentAtLineBreak - Decoration.size());
+
+  WrappedLines[LineIndex] = Lines[LineIndex].size() - Split.first;
 }
 
 void BreakableBlockComment::replaceWhitespace(unsigned LineIndex,
@@ -438,13 +441,49 @@
     }
   }
 
-  unsigned WhitespaceOffsetInToken = Lines[LineIndex].data() -
-                                     Tok.TokenText.data() -
-                                     LeadingWhitespace[LineIndex];
+  const StringRef Line = Lines[LineIndex];
+  unsigned LeadingWhitespaceInToken = LeadingWhitespace[LineIndex];
+  unsigned WhitespaceOffsetInToken =
+      Line.data() - Tok.TokenText.data() - LeadingWhitespaceInToken;
+  unsigned NumNewLines = 1;
+  unsigned Spaces = StartOfLineColumn[LineIndex] - Prefix.size();
+
+  // At this point comment lines violating the column limit will are wrapped
+  // already. Reflow the comments by glueing the next line onto this one. We
+  // only do this if the line is non-empty (to avoid accidental removal of
+  // paragraphs) and starts with an alphanumeric character (to avoid breaking
+  // ASCII art). Otherwise keep the simple wrapping as-is. We also have to make
+  // sure that extending the current line won't push us over the column limit in
+  // the same word and cause a break to be inserted at the very same position.
+  const unsigned LastOverflow = WrappedLines[LineIndex - 1];
+  auto FirstWordLen = std::min(Line.find_first_of(Blanks), Line.size());
+  const bool Reflow =
+      LastOverflow > 0 && !Line.empty() && isAlphanumeric(Line[0]) &&
+      IndentLevel + Spaces + LastOverflow + FirstWordLen <= Style.ColumnLimit;
+  if (Reflow) {
+    Prefix = "";
+    unsigned NewBegin = Line.data() - Lines[LineIndex - 1].end();
+    WhitespaceOffsetInToken += LeadingWhitespaceInToken - NewBegin;
+    LeadingWhitespaceInToken = NewBegin;
+    NumNewLines = 0;
+    Spaces = 1;
+  }
+
   Whitespaces.replaceWhitespaceInToken(
-      Tok, WhitespaceOffsetInToken, LeadingWhitespace[LineIndex], "", Prefix,
-      InPPDirective, 1, IndentLevel,
-      StartOfLineColumn[LineIndex] - Prefix.size());
+      Tok, WhitespaceOffsetInToken, LeadingWhitespaceInToken, "", Prefix,
+      InPPDirective, NumNewLines, IndentLevel, Spaces);
+
+  // Reflowing can bring this line over the column limit too. Break if
+  // necessary.
+  auto LineLength = encoding::columnWidthWithTabs(Line, IndentLevel,
+                                                  Style.TabWidth, Encoding);
+  unsigned NewLengthOfLine = IndentLevel + Spaces + LineLength + LastOverflow;
+  if (Reflow && NewLengthOfLine > Style.ColumnLimit &&
+      Line.size() <= Style.ColumnLimit) {
+    auto Split = getCommentSplit(Line, NewLengthOfLine - Line.size(),
+                                 Style.ColumnLimit, Style.TabWidth, Encoding);
+    insertBreak(LineIndex, 0, Split, Whitespaces);
+  }
 }
 
 unsigned
Index: unittests/Format/FormatTest.cpp
===================================================================
--- unittests/Format/FormatTest.cpp
+++ unittests/Format/FormatTest.cpp
@@ -1568,8 +1568,8 @@
             "     *   a comment\n"
             "* that we break\n"
             " * another comment\n"
-            "* we have to break\n"
-            "* a left comment\n"
+            "* we have to break a\n"
+            "* left comment\n"
             " */",
             format("  /* some comment\n"
                    "       *   a comment that we break\n"
@@ -1647,6 +1647,57 @@
                    getLLVMStyleWithColumns(20)));
 }
 
+TEST_F(FormatTest, ReflowsBlockComments) {
+  EXPECT_EQ("/*\n"
+            " * aaaa aaaa\n"
+            " * aaaa\n"
+            " */",
+            format("/*\n"
+                   " * aaaa aaaa\n"
+                   " * aaaa\n"
+                   " */",
+                   getLLVMStyleWithColumns(20)));
+
+  EXPECT_EQ("/*\n"
+            " * aaaaaaaaaaaaaaaaa\n"
+            " * aaaa\n"
+            " *\n"
+            " * aaaa\n"
+            " */",
+            format("/*\n"
+                   " * aaaaaaaaaaaaaaaaa aaaa\n"
+                   " *\n"
+                   " * aaaa\n"
+                   " */",
+                   getLLVMStyleWithColumns(20)));
+
+  EXPECT_EQ("/*\n"
+            " * aaaaaaaaaaaaaaaaa\n"
+            " * aaaa\n"
+            " * |--|\n"
+            " * aaaa\n"
+            " */",
+            format("/*\n"
+                   " * aaaaaaaaaaaaaaaaa aaaa\n"
+                   " * |--|\n"
+                   " * aaaa\n"
+                   " */",
+                   getLLVMStyleWithColumns(20)));
+
+  EXPECT_EQ("/*\n"
+            " * aaaaaaaaaaaaaaaaa\n"
+            " * aaaa aaaa\n"
+            " * aaaaaaaaaaaaaaaaa\n"
+            " * aaaa\n"
+            " */",
+            format("/*\n"
+                   " * aaaaaaaaaaaaaaaaa aaaa\n"
+                   " * aaaa aaaaaaaaaaaaaaaaa\n"
+                   " * aaaa\n"
+                   " */",
+                   getLLVMStyleWithColumns(20)));
+}
+
 TEST_F(FormatTest, CommentsInStaticInitializers) {
   EXPECT_EQ(
       "static SomeType type = {aaaaaaaaaaaaaaaaaaaa, /* comment */\n"