Generalize CmmUnwind and pass unwind information through NCG

As discussed in D1532, Trac Trac #11337, and Trac Trac #11338, the stack
unwinding information produced by GHC is currently quite approximate.
Essentially we assume that register values do not change at all within a
basic block. While this is somewhat true in normal Haskell code, blocks
containing foreign calls often break this assumption. This results in
unreliable call stacks, especially in the code containing foreign calls.
This is worse than it sounds as unreliable unwinding information can at
times result in segmentation faults.

This patch set attempts to improve this situation by tracking unwinding
information with finer granularity. By dispensing with the assumption of
one unwinding table per block, we allow the compiler to accurately
represent the areas surrounding foreign calls.

Towards this end we generalize the representation of unwind information
in the backend in three ways,

 * Multiple CmmUnwind nodes can occur per block

 * CmmUnwind nodes can now carry unwind information for multiple
   registers (while not strictly necessary; this makes emitting
   unwinding information a bit more convenient in the compiler)

 * The NCG backend is given an opportunity to modify the unwinding
   records since it may need to make adjustments due to, for instance,
   native calling convention requirements for foreign calls (see
   #11353).

This sets the stage for resolving #11337 and #11338.

Test Plan: Validate

Reviewers: scpmw, simonmar, austin, erikd

Subscribers: qnikst, thomie

Differential Revision: https://phabricator.haskell.org/D2741

6 files changed, 220 insertions, 62 deletions

diff --git a/compiler/cmm/CmmLayoutStack.hs b/compiler/cmm/CmmLayoutStack.hs
index 0f8495f3e6..60f89704f0 100644
--- a/compiler/cmm/CmmLayoutStack.hs
+++ b/compiler/cmm/CmmLayoutStack.hs
@@ -275,10 +275,11 @@ layout dflags procpoints liveness entry entry_args final_stackmaps final_sp_high
        --
        let middle_pre = blockToList $ foldl blockSnoc middle1 middle2
 
-           final_blocks = manifestSp dflags final_stackmaps stack0 sp0 final_sp_high entry0
-                              middle_pre sp_off last1 fixup_blocks
+       let final_blocks =
+               manifestSp dflags final_stackmaps stack0 sp0 final_sp_high
+                          entry0 middle_pre sp_off last1 fixup_blocks
 
-           acc_stackmaps' = mapUnion acc_stackmaps out
+       let acc_stackmaps' = mapUnion acc_stackmaps out
 
            -- If this block jumps to the GC, then we do not take its
            -- stack usage into account for the high-water mark.
@@ -793,19 +794,20 @@ manifestSp dflags stackmaps stack0 sp0 sp_high
     adj_pre_sp  = mapExpDeep (areaToSp dflags sp0            sp_high area_off)
     adj_post_sp = mapExpDeep (areaToSp dflags (sp0 - sp_off) sp_high area_off)
 
-    -- Add unwind pseudo-instructions to document Sp level for debugging
+    -- Add unwind pseudo-instructions at the beginning of each block to
+    -- document Sp level for debugging
     add_unwind_info block
-      | debugLevel dflags > 0 = CmmUnwind Sp sp_unwind : block
+      | debugLevel dflags > 0 =
+          CmmUnwind [(Sp, sp_unwind)] : block
       | otherwise             = block
     sp_unwind = CmmRegOff (CmmGlobal Sp) (sp0 - wORD_SIZE dflags)
 
-    final_middle = maybeAddSpAdj dflags sp_off $
-                   blockFromList $
-                   add_unwind_info $
-                   map adj_pre_sp $
-                   elimStackStores stack0 stackmaps area_off $
-                   middle_pre
-
+    final_middle = maybeAddSpAdj dflags sp_off
+                 . blockFromList
+                 . add_unwind_info
+                 . map adj_pre_sp
+                 . elimStackStores stack0 stackmaps area_off
+                 $ middle_pre
     final_last    = optStackCheck (adj_post_sp last)
 
     final_block   = blockJoin first final_middle final_last
@@ -823,9 +825,9 @@ getAreaOff stackmaps (Young l) =
 
 maybeAddSpAdj :: DynFlags -> ByteOff -> Block CmmNode O O -> Block CmmNode O O
 maybeAddSpAdj _      0      block = block
-maybeAddSpAdj dflags sp_off block
-   = block `blockSnoc` CmmAssign spReg (cmmOffset dflags (CmmReg spReg) sp_off)
-
+maybeAddSpAdj dflags sp_off block = block `blockSnoc` adj
+  where
+    adj = CmmAssign spReg (cmmOffset dflags (CmmReg spReg) sp_off)
 
 {-
 Sp(L) is the Sp offset on entry to block L relative to the base of the
diff --git a/compiler/cmm/CmmNode.hs b/compiler/cmm/CmmNode.hs
index 1103fdb6ec..7acf4c6d8b 100644
--- a/compiler/cmm/CmmNode.hs
+++ b/compiler/cmm/CmmNode.hs
@@ -61,7 +61,9 @@ data CmmNode e x where
     -- the "old" value of a register if we want to navigate the stack
     -- up one frame. Having unwind information for @Sp@ will allow the
     -- debugger to "walk" the stack.
-  CmmUnwind :: !GlobalReg -> !CmmExpr -> CmmNode O O
+    --
+    -- See Note [What is this unwinding business?] in Debug
+  CmmUnwind :: [(GlobalReg, CmmExpr)] -> CmmNode O O
 
   CmmAssign :: !CmmReg -> !CmmExpr -> CmmNode O O
     -- Assign to register
@@ -459,7 +461,7 @@ mapExp :: (CmmExpr -> CmmExpr) -> CmmNode e x -> CmmNode e x
 mapExp _ f@(CmmEntry{})                          = f
 mapExp _ m@(CmmComment _)                        = m
 mapExp _ m@(CmmTick _)                           = m
-mapExp f   (CmmUnwind r e)                       = CmmUnwind r (f e)
+mapExp f   (CmmUnwind regs)                      = CmmUnwind (map (fmap f) regs)
 mapExp f   (CmmAssign r e)                       = CmmAssign r (f e)
 mapExp f   (CmmStore addr e)                     = CmmStore (f addr) (f e)
 mapExp f   (CmmUnsafeForeignCall tgt fs as)      = CmmUnsafeForeignCall (mapForeignTarget f tgt) fs (map f as)
@@ -490,7 +492,7 @@ mapExpM :: (CmmExpr -> Maybe CmmExpr) -> CmmNode e x -> Maybe (CmmNode e x)
 mapExpM _ (CmmEntry{})              = Nothing
 mapExpM _ (CmmComment _)            = Nothing
 mapExpM _ (CmmTick _)               = Nothing
-mapExpM f (CmmUnwind r e)           = CmmUnwind r `fmap` f e
+mapExpM f (CmmUnwind regs)          = CmmUnwind `fmap` mapM (\(r,e) -> f e >>= \e' -> pure (r,e')) regs
 mapExpM f (CmmAssign r e)           = CmmAssign r `fmap` f e
 mapExpM f (CmmStore addr e)         = (\[addr', e'] -> CmmStore addr' e') `fmap` mapListM f [addr, e]
 mapExpM _ (CmmBranch _)             = Nothing
@@ -543,7 +545,7 @@ foldExp :: (CmmExpr -> z -> z) -> CmmNode e x -> z -> z
 foldExp _ (CmmEntry {}) z                         = z
 foldExp _ (CmmComment {}) z                       = z
 foldExp _ (CmmTick {}) z                          = z
-foldExp f (CmmUnwind _ e) z                       = f e z
+foldExp f (CmmUnwind xs) z                        = foldr f z (map snd xs)
 foldExp f (CmmAssign _ e) z                       = f e z
 foldExp f (CmmStore addr e) z                     = f addr $ f e z
 foldExp f (CmmUnsafeForeignCall t _ as) z         = foldr f (foldExpForeignTarget f t z) as
diff --git a/compiler/cmm/CmmParse.y b/compiler/cmm/CmmParse.y
index e7425930a6..cfadc61e84 100644
--- a/compiler/cmm/CmmParse.y
+++ b/compiler/cmm/CmmParse.y
@@ -635,8 +635,15 @@ stmt    :: { CmmParse () }
                 { pushStackFrame $3 $5 }
         | 'reserve' expr '=' lreg maybe_body
                 { reserveStackFrame $2 $4 $5 }
-        | 'unwind' GLOBALREG '=' expr
-                { $4 >>= code . emitUnwind $2 }
+        | 'unwind' unwind_regs ';'
+                { $2 >>= code . emitUnwind }
+
+unwind_regs
+        :: { CmmParse [(GlobalReg, CmmExpr)] }
+        : GLOBALREG '=' expr ',' unwind_regs
+                { do e <- $3; rest <- $5; return (($1, e) : rest) }
+        | GLOBALREG '=' expr
+                { do e <- $3; return [($1, e)] }
 
 foreignLabel     :: { CmmParse CmmExpr }
         : NAME                          { return (CmmLit (CmmLabel (mkForeignLabel $1 Nothing ForeignLabelInThisPackage IsFunction))) }
diff --git a/compiler/cmm/Debug.hs b/compiler/cmm/Debug.hs
index 7b9383569b..02daa3686c 100644
--- a/compiler/cmm/Debug.hs
+++ b/compiler/cmm/Debug.hs
@@ -12,15 +12,17 @@
 module Debug (
 
   DebugBlock(..), dblIsEntry,
-  UnwindTable, UnwindExpr(..),
   cmmDebugGen,
   cmmDebugLabels,
   cmmDebugLink,
-  debugToMap
+  debugToMap,
 
+  -- * Unwinding information
+  UnwindTable, UnwindPoint(..),
+  UnwindExpr(..), toUnwindExpr
   ) where
 
-import BlockId         ( blockLbl )
+import BlockId
 import CLabel
 import Cmm
 import CmmUtils
@@ -56,7 +58,7 @@ data DebugBlock =
   , dblPosition   :: !(Maybe Int)  -- ^ Output position relative to
                                    -- other blocks. @Nothing@ means
                                    -- the block was optimized out
-  , dblUnwind     :: !UnwindTable  -- ^ Unwind information
+  , dblUnwind     :: [UnwindPoint]
   , dblBlocks     :: ![DebugBlock] -- ^ Nested blocks
   }
 
@@ -74,14 +76,12 @@ instance Outputable DebugBlock where
             (maybe empty ppr (dblSourceTick blk)) <+>
             (maybe (text "removed") ((text "pos " <>) . ppr)
                    (dblPosition blk)) <+>
-            pprUwMap (dblUnwind blk) $$
+            (ppr (dblUnwind blk)) <+>
             (if null (dblBlocks blk) then empty else ppr (dblBlocks blk))
-    where pprUw (g, expr) = ppr g <> char '=' <> ppr expr
-          pprUwMap = braces . hsep . punctuate comma . map pprUw . Map.toList
 
 -- | Intermediate data structure holding debug-relevant context information
 -- about a block.
-type BlockContext = (CmmBlock, RawCmmDecl, UnwindTable)
+type BlockContext = (CmmBlock, RawCmmDecl)
 
 -- | Extract debug data from a group of procedures. We will prefer
 -- source notes that come from the given module (presumably the module
@@ -127,7 +127,7 @@ cmmDebugGen modLoc decls = map (blocksForScope Nothing) topScopes
                    | otherwise                = panic "ticksToCopy impossible"
                 where ticks = bCtxsTicks $ fromMaybe [] $ Map.lookup s blockCtxs
       ticksToCopy _ = []
-      bCtxsTicks = concatMap (blockTicks . fstOf3)
+      bCtxsTicks = concatMap (blockTicks . fst)
 
       -- Finding the "best" source tick is somewhat arbitrary -- we
       -- select the first source span, while preferring source ticks
@@ -151,7 +151,9 @@ cmmDebugGen modLoc decls = map (blocksForScope Nothing) topScopes
               nested = fromMaybe [] $ Map.lookup scope scopeMap
               childs = map (mkBlock False) (tail bctxs) ++
                        map (blocksForScope stick) nested
-              mkBlock top (block, prc, unwind)
+
+              mkBlock :: Bool -> BlockContext -> DebugBlock
+              mkBlock top (block, prc)
                 = DebugBlock { dblProcedure    = g_entry graph
                              , dblLabel        = label
                              , dblCLabel       = case info of
@@ -163,9 +165,9 @@ cmmDebugGen modLoc decls = map (blocksForScope Nothing) topScopes
                              , dblParent       = Nothing
                              , dblTicks        = ticks
                              , dblPosition     = Nothing -- see cmmDebugLink
-                             , dblUnwind       = unwind
                              , dblSourceTick   = stick
                              , dblBlocks       = blocks
+                             , dblUnwind       = []
                              }
                 where (CmmProc infos entryLbl _ graph) = prc
                       label = entryLabel block
@@ -189,29 +191,33 @@ cmmDebugGen modLoc decls = map (blocksForScope Nothing) topScopes
 --
 -- This involves a pre-order traversal, as we want blocks in rough
 -- control flow order (so ticks have a chance to be sorted in the
--- right order). We also use this opportunity to have blocks inherit
--- unwind information from their predecessor blocks where it is
--- lacking.
+-- right order).
 blockContexts :: RawCmmGroup -> Map.Map CmmTickScope [BlockContext]
 blockContexts decls = Map.map reverse $ foldr walkProc Map.empty decls
-  where walkProc CmmData{}                 m = m
+  where walkProc :: RawCmmDecl
+                 -> Map.Map CmmTickScope [BlockContext]
+                 -> Map.Map CmmTickScope [BlockContext]
+        walkProc CmmData{}                 m = m
         walkProc prc@(CmmProc _ _ _ graph) m
           | mapNull blocks = m
-          | otherwise      = snd $ walkBlock prc entry Map.empty (emptyLbls, m)
+          | otherwise      = snd $ walkBlock prc entry (emptyLbls, m)
           where blocks = toBlockMap graph
                 entry  = [mapFind (g_entry graph) blocks]
                 emptyLbls = setEmpty :: LabelSet
-        walkBlock _   []             _      c            = c
-        walkBlock prc (block:blocks) unwind (visited, m)
+
+        walkBlock :: RawCmmDecl -> [Block CmmNode C C]
+                  -> (LabelSet, Map.Map CmmTickScope [BlockContext])
+                  -> (LabelSet, Map.Map CmmTickScope [BlockContext])
+        walkBlock _   []             c            = c
+        walkBlock prc (block:blocks) (visited, m)
           | lbl `setMember` visited
-          = walkBlock prc blocks unwind (visited, m)
+          = walkBlock prc blocks (visited, m)
           | otherwise
-          = walkBlock prc blocks unwind $
-            walkBlock prc succs unwind'
+          = walkBlock prc blocks $
+            walkBlock prc succs
               (lbl `setInsert` visited,
-               insertMulti scope (block, prc, unwind') m)
+               insertMulti scope (block, prc) m)
           where CmmEntry lbl scope = firstNode block
-                unwind' = extractUnwind block `Map.union` unwind
                 (CmmProc _ _ _ graph) = prc
                 succs = map (flip mapFind (toBlockMap graph))
                             (successors (lastNode block))
@@ -234,14 +240,17 @@ cmmDebugLabels isMeta nats = seqList lbls lbls
         getBlocks _other                         = []
         allMeta (BasicBlock _ instrs) = all isMeta instrs
 
--- | Sets position fields in the debug block tree according to native
--- generated code.
-cmmDebugLink :: [Label] -> [DebugBlock] -> [DebugBlock]
-cmmDebugLink labels blocks = map link blocks
+-- | Sets position and unwind table fields in the debug block tree according to
+-- native generated code.
+cmmDebugLink :: [Label] -> LabelMap [UnwindPoint]
+             -> [DebugBlock] -> [DebugBlock]
+cmmDebugLink labels unwindPts blocks = map link blocks
   where blockPos :: LabelMap Int
         blockPos = mapFromList $ flip zip [0..] labels
         link block = block { dblPosition = mapLookup (dblLabel block) blockPos
                            , dblBlocks   = map link (dblBlocks block)
+                           , dblUnwind   = fromMaybe mempty
+                                         $ mapLookup (dblLabel block) unwindPts
                            }
 
 -- | Converts debug blocks into a label map for easier lookups
@@ -249,14 +258,158 @@ debugToMap :: [DebugBlock] -> LabelMap DebugBlock
 debugToMap = mapUnions . map go
    where go b = mapInsert (dblLabel b) b $ mapUnions $ map go (dblBlocks b)
 
+{-
+Note [What is this unwinding business?]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Unwinding tables are a variety of debugging information used by debugging tools
+to reconstruct the execution history of a program at runtime. These tables
+consist of sets of "instructions", one set for every instruction in the program,
+which describe how to reconstruct the state of the machine at the point where
+the current procedure was called. For instance, consider the following annotated
+pseudo-code,
+
+  a_fun:
+    add rsp, 8            -- unwind: rsp = rsp - 8
+    mov rax, 1            -- unwind: rax = unknown
+    call another_block
+    sub rsp, 8            -- unwind: rsp = rsp
+
+We see that attached to each instruction there is an "unwind" annotation, which
+provides a relationship between each updated register and its value at the
+time of entry to a_fun. This is the sort of information that allows gdb to give
+you a stack backtrace given the execution state of your program. This
+unwinding information is captured in various ways by various debug information
+formats; in the case of DWARF (the only format supported by GHC) it is known as
+Call Frame Information (CFI) and can be found in the .debug.frames section of
+your object files.
+
+Currently we only bother to produce unwinding information for registers which
+are necessary to reconstruct flow-of-execution. On x86_64 this includes $rbp
+(which is the STG stack pointer) and $rsp (the C stack pointer).
+
+Let's consider how GHC would annotate a C-- program with unwinding information
+with a typical C-- procedure as would come from the STG-to-Cmm code generator,
+
+  entry()
+     { c2fe:
+           v :: P64 = R2;
+           if ((Sp + 8) - 32 < SpLim) (likely: False) goto c2ff; else goto c2fg;
+       c2ff:
+           R2 = v :: P64;
+           R1 = test_closure;
+           call (stg_gc_fun)(R2, R1) args: 8, res: 0, upd: 8;
+       c2fg:
+           I64[Sp - 8] = c2dD;
+           R1 = v :: P64;
+           Sp = Sp - 8;          // Sp updated here
+           if (R1 & 7 != 0) goto c2dD; else goto c2dE;
+       c2dE:
+           call (I64[R1])(R1) returns to c2dD, args: 8, res: 8, upd: 8;
+       c2dD:
+           w :: P64 = R1;
+           Hp = Hp + 48;
+           if (Hp > HpLim) (likely: False) goto c2fj; else goto c2fi;
+       ...
+  },
+
+Let's consider how this procedure will be decorated with unwind information
+(largely by CmmLayoutStack). Naturally, when we enter the procedure `entry` the
+value of Sp is no different from what it was at its call site. Therefore we will
+add an `unwind` statement saying this at the beginning of its unwind-annotated
+code,
+
+  entry()
+     { c2fe:
+           unwind Sp = Just Sp + 0;
+           v :: P64 = R2;
+           if ((Sp + 8) - 32 < SpLim) (likely: False) goto c2ff; else goto c2fg;
+
+After c2fe we we may pass to either c2ff or c2fg; let's first consider the
+former. In this case there is nothing in particular that we need to do other
+than reiterate what we already know about Sp,
+
+       c2ff:
+           unwind Sp = Just Sp + 0;
+           R2 = v :: P64;
+           R1 = test_closure;
+           call (stg_gc_fun)(R2, R1) args: 8, res: 0, upd: 8;
+
+In contrast, c2fg updates Sp midway through its body. To ensure that unwinding
+can happen correctly after this point we must include an unwind statement there,
+in addition to the usual beginning-of-block statement,
+
+       c2fg:
+           unwind Sp = Just Sp + 0;
+           I64[Sp - 8] = c2dD;
+           R1 = v :: P64;
+           unwind Sp = Just Sp + 8;
+           Sp = Sp - 8;
+           if (R1 & 7 != 0) goto c2dD; else goto c2dE;
+
+The remaining blocks are simple,
+
+       c2dE:
+           unwind Sp = Just Sp + 8;
+           call (I64[R1])(R1) returns to c2dD, args: 8, res: 8, upd: 8;
+       c2dD:
+           unwind Sp = Just Sp + 8;
+           w :: P64 = R1;
+           Hp = Hp + 48;
+           if (Hp > HpLim) (likely: False) goto c2fj; else goto c2fi;
+       ...
+  },
+
+
+The flow of unwinding information through the compiler is a bit convoluted:
+
+ * C-- begins life in StgCmm without any unwind information. This is because we
+   haven't actually done any register assignment or stack layout yet, so there
+   is no need for unwind information.
+
+ * CmmLayoutStack figures out how to layout each procedure's stack, and produces
+   appropriate unwinding nodes for each adjustment of the STG Sp register.
+
+ * The unwind nodes are carried through the sinking pass. Currently this is
+   guaranteed not to invalidate unwind information since it won't touch stores
+   to Sp, but this will need revisiting if CmmSink gets smarter in the future.
+
+ * Eventually we make it to the native code generator backend which can then
+   preserve the unwind nodes in its machine-specific instructions. In so doing
+   the backend can also modify or add unwinding information; this is necessary,
+   for instance, in the case of x86-64, where adjustment of $rsp may be
+   necessary during calls to native foreign code due to the native calling
+   convention.
+
+ * The NCG then retrieves the final unwinding table for each block from the
+   backend with extractUnwindPoints.
+
+ * This unwind information is converted to DebugBlocks by Debug.cmmDebugGen
+
+ * These DebugBlcosk are then converted to, e.g., DWARF unwinding tables
+   (by the Dwarf module) and emitted in the final object.
+
+See also: Note [Unwinding information in the NCG] in AsmCodeGen.
+-}
+
+-- | A label associated with an 'UnwindTable'
+data UnwindPoint = UnwindPoint !Label !UnwindTable
+
+instance Outputable UnwindPoint where
+  ppr (UnwindPoint lbl uws) =
+      braces $ ppr lbl<>colon
+      <+> hsep (punctuate comma $ map pprUw $ Map.toList uws)
+    where
+      pprUw (g, expr) = ppr g <> char '=' <> ppr expr
+
 -- | Maps registers to expressions that yield their "old" values
 -- further up the stack. Most interesting for the stack pointer Sp,
 -- but might be useful to document saved registers, too.
 type UnwindTable = Map.Map GlobalReg UnwindExpr
 
 -- | Expressions, used for unwind information
-data UnwindExpr = UwConst Int                   -- ^ literal value
-                | UwReg GlobalReg Int           -- ^ register plus offset
+data UnwindExpr = UwConst !Int                  -- ^ literal value
+                | UwReg !GlobalReg !Int         -- ^ register plus offset
                 | UwDeref UnwindExpr            -- ^ pointer dereferencing
                 | UwLabel CLabel
                 | UwPlus UnwindExpr UnwindExpr
@@ -278,17 +431,6 @@ instance Outputable UnwindExpr where
                             = pprPrec 2 e0 <> char '*' <> pprPrec 2 e1
   pprPrec _ other           = parens (pprPrec 0 other)
 
-extractUnwind :: CmmBlock -> UnwindTable
-extractUnwind b = go $ blockToList mid
-  where (_, mid, _) = blockSplit b
-        go :: [CmmNode O O] -> UnwindTable
-        go []       = Map.empty
-        go (x : xs) = case x of
-          CmmUnwind g so -> Map.insert g (toUnwindExpr so) $! go xs
-          CmmTick {}     -> go xs
-          _other         -> Map.empty
-                            -- TODO: Unwind statements after actual instructions
-
 -- | Conversion of Cmm expressions to unwind expressions. We check for
 -- unsupported operator usages and simplify the expression as far as
 -- possible.
diff --git a/compiler/cmm/MkGraph.hs b/compiler/cmm/MkGraph.hs
index ae7c5097af..ed795a1d3e 100644
--- a/compiler/cmm/MkGraph.hs
+++ b/compiler/cmm/MkGraph.hs
@@ -14,6 +14,7 @@ module MkGraph
   , mkRawJump
   , mkCbranch, mkSwitch
   , mkReturn, mkComment, mkCallEntry, mkBranch
+  , mkUnwind
   , copyInOflow, copyOutOflow
   , noExtraStack
   , toCall, Transfer(..)
@@ -270,6 +271,8 @@ mkJumpReturnsTo dflags f callConv actuals ret_lbl ret_off updfr_off  = do
 mkUnsafeCall  :: ForeignTarget -> [CmmFormal] -> [CmmActual] -> CmmAGraph
 mkUnsafeCall t fs as = mkMiddle $ CmmUnsafeForeignCall t fs as
 
+mkUnwind     :: GlobalReg -> CmmExpr -> CmmAGraph
+mkUnwind r e  = mkMiddle $ CmmUnwind [(r, e)]
 
 --------------------------------------------------------------------------
 
diff --git a/compiler/cmm/PprCmm.hs b/compiler/cmm/PprCmm.hs
index 9517ea3c09..089066a1ab 100644
--- a/compiler/cmm/PprCmm.hs
+++ b/compiler/cmm/PprCmm.hs
@@ -197,7 +197,9 @@ pprNode node = pp_node <+> pp_debug
                    else empty
 
       -- unwind reg = expr;
-      CmmUnwind r e -> text "unwind " <> ppr r <+> char '=' <+> ppr e
+      CmmUnwind regs ->
+          text "unwind "
+          <> commafy (map (\(r,e) -> ppr r <+> char '=' <+> ppr e) regs) <> semi
 
       -- reg = expr;
       CmmAssign reg expr -> ppr reg <+> equals <+> ppr expr <> semi