[commit: ghc] master: add a comment (6c06db1)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/6c06db13a670f156a79301c89b6bf2669d91af88/ghc

>---------------------------------------------------------------

commit 6c06db13a670f156a79301c89b6bf2669d91af88
Author: Simon Marlow <marlowsd at gmail.com>
Date:   Thu Jul 31 11:47:44 2014 +0100

    add a comment


>---------------------------------------------------------------

6c06db13a670f156a79301c89b6bf2669d91af88
 compiler/cmm/PprCmm.hs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/compiler/cmm/PprCmm.hs b/compiler/cmm/PprCmm.hs
index b5beb07..cc31240 100644
--- a/compiler/cmm/PprCmm.hs
+++ b/compiler/cmm/PprCmm.hs
@@ -138,6 +138,9 @@ pprCmmGraph g
   $$ nest 2 (vcat $ map ppr blocks)
   $$ text "}"
   where blocks = postorderDfs g
+    -- postorderDfs has the side-effect of discarding unreachable code,
+    -- so pretty-printed Cmm will omit any unreachable blocks.  This can
+    -- sometimes be confusing.
 
 ---------------------------------------------
 -- Outputting CmmNode and types which it contains