[commit: ghc] master: Document -fllvm-fill-undef-with-garbage (45c6fbc)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/45c6fbc5284f83e1253ff9f3b49fe54a76c20ba7/ghc

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commit 45c6fbc5284f83e1253ff9f3b49fe54a76c20ba7
Author: Ben Gamari <ben at smart-cactus.org>
Date:   Wed Jan 27 12:24:48 2016 +0100

    Document -fllvm-fill-undef-with-garbage


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45c6fbc5284f83e1253ff9f3b49fe54a76c20ba7
 docs/users_guide/debugging.rst | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/docs/users_guide/debugging.rst b/docs/users_guide/debugging.rst
index 80c9f8d..a7bbbb5 100644
--- a/docs/users_guide/debugging.rst
+++ b/docs/users_guide/debugging.rst
@@ -333,6 +333,12 @@ Checking for consistency
 
     Ditto for C-- level.
 
+.. ghc-flag:: -fllvm-fill-undef-with-garbage
+
+    Instructs the LLVM code generator to fill dead STG registers with garbage
+    instead of ``undef`` in calls. This makes it easier to catch subtle
+    code generator and runtime system bugs (e.g. see :ghc-ticket:`11487`).
+
 .. _checking-determinism:
 
 Checking for determinism