I've looked into this myself. I don't think it's actually anything wrong in gmp-wrappers. It's because CmmBuildInfoTables expects all the info tables in the (CmmProc info_tbls lbl g) to be defined in the graph g. But Jan's new optimisation to the stack overflow check (Note [Always false stack check] in CmmLayoutStack) ends up dropping a conditional branch, so a call to gc is eliminated, and hence a proc point is eliminated, and we end up with an unused thing in info_tbls, which confuses CmmBuildInfoTables Sigh. I don't know why this only happens on Windows. I'm looking into a fix Simon From: Simon Peyton-Jones Sent: 15 November 2013 22:18 To: ghc-devs@haskell.org Subject: bundle panic Windows build is failing again in a new way. It was fine a couple of days ago. Does this ring any bells? ghc-stage1.exe: panic! (the 'impossible' happened) (GHC version 7.7.20131107 for i386-unknown-mingw32): bundle Simon "inplace/bin/ghc-stage1.exe" -static -H32m -O -Werror -Wall -H64m -O0 -package-name integer-gmp-0.5.1.0 -hide-all-packages -i -ilibraries/integer-gmp/. -ilibraries/integer-gmp/dist-install/build -ilibraries/integer-gmp/dist-install/build/autogen -Ilibraries/integer-gmp/dist-install/build -Ilibraries/integer-gmp/dist-install/build/autogen -Ilibraries/integer-gmp/. -optP-include -optPlibraries/integer-gmp/dist-install/build/autogen/cabal_macros.h -package ghc-prim-0.3.1.0 -Wall -package-name integer-gmp -XHaskell2010 -dcore-lint -no-user-package-db -rtsopts -Ilibraries/integer-gmp/mkGmpDerivedConstants/dist -c libraries/integer-gmp/cbits/gmp-wrappers.cmm -o libraries/integer-gmp/dist-install/build/cbits/gmp-wrappers.o ghc-stage1.exe: panic! (the 'impossible' happened) (GHC version 7.7.20131107 for i386-unknown-mingw32): bundle c8 integer_cmm_importIntegerFromByteArrayzh [(c8, label: block{v c8}_info rep:StackRep [False, True, True, True]), (cg, label: block{v cg}_info rep:StackRep [False, True, True, True])] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug HEAD (master)$