Attached a simple test case. It should print 7457, but the C function is called with 0 as the third argument.

If I compile with -O0 or omit the unsafe keyword in the FFI import it works as it should.

In gdb disassembly looks to me as edx (the place for third argument on 64-bit) is set to 7457, then the opaquify is inlined, but it doesn't preserve edx and then third_arg is called with the zeroed edx.

Specs

64-bit Archlinux with arch-haskell repo

gcc -v:

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/5.3.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /build/gcc-multilib/src/gcc-5-20160209/configure --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared --enable-threads=posix --enable-libmpx --with-system-zlib --with-isl --enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu --disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object --enable-linker-build-id --enable-lto --enable-plugin --enable-install-libiberty --with-linker-hash-style=gnu --enable-gnu-indirect-function --enable-multilib --disable-werror --enable-checking=release
Thread model: posix
gcc version 5.3.0 (GCC)

ghc compile output:

Glasgow Haskell Compiler, Version 7.10.3, stage 2 booted by GHC version 7.10.3
Using binary package database: /usr/lib/ghc-7.10.3/package.conf.d/package.cache
wired-in package ghc-prim mapped to ghc-prim-0.4.0.0-6cdc86811872333585fa98756aa7c51e
wired-in package integer-gmp mapped to integer-gmp-1.0.0.0-3c8c40657a9870f5c33be17496806d8d
wired-in package base mapped to base-4.8.2.0-0d6d1084fbc041e1cded9228e80e264d
wired-in package rts mapped to builtin_rts
wired-in package template-haskell mapped to template-haskell-2.10.0.0-3c4cb52230f347282af9b2817f013181
wired-in package ghc mapped to ghc-7.10.3-3a39f8f970ff545623196002970730d1
wired-in package dph-seq not found.
wired-in package dph-par not found.
Hsc static flags: 
wired-in package ghc-prim mapped to ghc-prim-0.4.0.0-6cdc86811872333585fa98756aa7c51e
wired-in package integer-gmp mapped to integer-gmp-1.0.0.0-3c8c40657a9870f5c33be17496806d8d
wired-in package base mapped to base-4.8.2.0-0d6d1084fbc041e1cded9228e80e264d
wired-in package rts mapped to builtin_rts
wired-in package template-haskell mapped to template-haskell-2.10.0.0-3c4cb52230f347282af9b2817f013181
wired-in package ghc mapped to ghc-7.10.3-3a39f8f970ff545623196002970730d1
wired-in package dph-seq not found.
wired-in package dph-par not found.
*** Chasing dependencies:
Chasing modules from: *Main.hs
Stable obj: []
Stable BCO: []
Ready for upsweep
  [NONREC
      ModSummary {
         ms_hs_date = 2016-04-05 14:24:20.801997492 UTC
         ms_mod = Main,
         ms_textual_imps = [import (implicit) Prelude, import Data.Word]
         ms_srcimps = []
      }]
*** Deleting temp files:
Deleting: 
compile: input file Main.hs
Created temporary directory: /tmp/ghc1541_0
*** Checking old interface for Main:
[1 of 1] Compiling Main             ( Main.hs, Main.o )
*** Parser:
*** Renamer/typechecker:
*** Desugar:
Result size of Desugar (after optimization)
  = {terms: 317, types: 387, coercions: 3}
*** Core Linted result of Desugar (after optimization):
*** Simplifier:
Result size of Simplifier iteration=1
  = {terms: 261, types: 290, coercions: 14}
*** Core Linted result of Simplifier:
Result size of Simplifier iteration=2
  = {terms: 216, types: 262, coercions: 18}
*** Core Linted result of Simplifier:
Result size of Simplifier = {terms: 216, types: 262, coercions: 18}
*** Core Linted result of Simplifier:
*** Specialise:
Result size of Specialise = {terms: 216, types: 262, coercions: 18}
*** Core Linted result of Specialise:
*** Float out(FOS {Lam = Just 0, Consts = True, OverSatApps = False}):
Result size of Float out(FOS {Lam = Just 0,
                              Consts = True,
                              OverSatApps = False})
  = {terms: 274, types: 305, coercions: 18}
*** Core Linted result of Float out(FOS {Lam = Just 0, Consts = True, OverSatApps = False}):
*** Simplifier:
Result size of Simplifier iteration=1
  = {terms: 407, types: 388, coercions: 70}
*** Core Linted result of Simplifier:
Result size of Simplifier iteration=2
  = {terms: 463, types: 375, coercions: 25}
*** Core Linted result of Simplifier:
Result size of Simplifier = {terms: 430, types: 362, coercions: 25}
*** Core Linted result of Simplifier:
*** Simplifier:
Result size of Simplifier iteration=1
  = {terms: 426, types: 363, coercions: 25}
*** Core Linted result of Simplifier:
Result size of Simplifier = {terms: 426, types: 363, coercions: 25}
*** Core Linted result of Simplifier:
*** Simplifier:
Result size of Simplifier iteration=1
  = {terms: 310, types: 291, coercions: 25}
*** Core Linted result of Simplifier:
Result size of Simplifier iteration=2
  = {terms: 248, types: 217, coercions: 25}
*** Core Linted result of Simplifier:
Result size of Simplifier iteration=3
  = {terms: 336, types: 242, coercions: 25}
*** Core Linted result of Simplifier:
Result size of Simplifier = {terms: 336, types: 242, coercions: 25}
*** Core Linted result of Simplifier:
*** Float inwards:
Result size of Float inwards
  = {terms: 336, types: 242, coercions: 25}
*** Core Linted result of Float inwards:
*** Called arity analysis:
Result size of Called arity analysis
  = {terms: 336, types: 242, coercions: 25}
*** Core Linted result of Called arity analysis:
*** Simplifier:
Result size of Simplifier = {terms: 336, types: 242, coercions: 25}
*** Core Linted result of Simplifier:
*** Demand analysis:
Result size of Demand analysis
  = {terms: 336, types: 242, coercions: 25}
*** Core Linted result of Demand analysis:
*** Worker Wrapper binds:
Result size of Worker Wrapper binds
  = {terms: 369, types: 283, coercions: 25}
*** Core Linted result of Worker Wrapper binds:
*** Simplifier:
Result size of Simplifier iteration=1
  = {terms: 354, types: 266, coercions: 25}
*** Core Linted result of Simplifier:
Result size of Simplifier = {terms: 354, types: 266, coercions: 25}
*** Core Linted result of Simplifier:
*** Float out(FOS {Lam = Just 0, Consts = True, OverSatApps = True}):
Result size of Float out(FOS {Lam = Just 0,
                              Consts = True,
                              OverSatApps = True})
  = {terms: 356, types: 267, coercions: 25}
*** Core Linted result of Float out(FOS {Lam = Just 0, Consts = True, OverSatApps = True}):
*** Common sub-expression:
Result size of Common sub-expression
  = {terms: 356, types: 267, coercions: 25}
*** Core Linted result of Common sub-expression:
*** Float inwards:
Result size of Float inwards
  = {terms: 356, types: 267, coercions: 25}
*** Core Linted result of Float inwards:
*** Simplifier:
Result size of Simplifier = {terms: 356, types: 267, coercions: 25}
*** Core Linted result of Simplifier:
*** Tidy Core:
Result size of Tidy Core = {terms: 356, types: 267, coercions: 25}
*** Core Linted result of Tidy Core:
writeBinIface: 18 Names
writeBinIface: 81 dict entries
*** CorePrep:
Result size of CorePrep = {terms: 654, types: 379, coercions: 25}
*** Core Linted result of CorePrep:
*** Stg2Stg:
*** CodeGen:
*** Assembler:
/usr/bin/gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -I. -x assembler -c /tmp/ghc1541_0/ghc_2.s -o Main.o
Upsweep completely successful.
*** Deleting temp files:
Deleting: /tmp/ghc1541_0/ghc_3.c /tmp/ghc1541_0/ghc_2.s /tmp/ghc1541_0/ghc_1.s
Warning: deleting non-existent /tmp/ghc1541_0/ghc_3.c
Warning: deleting non-existent /tmp/ghc1541_0/ghc_1.s
link: linkables are ...
LinkableM (2016-04-05 15:42:11.288210053 UTC) Main
   [DotO Main.o]
Linking Main ...
*** C Compiler:
/usr/bin/gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -c /tmp/ghc1541_0/ghc_4.c -o /tmp/ghc1541_0/ghc_5.o -I/usr/lib/ghc-7.10.3/include
*** C Compiler:
/usr/bin/gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE -c /tmp/ghc1541_0/ghc_7.s -o /tmp/ghc1541_0/ghc_8.o -I/usr/lib/ghc-7.10.3/include
*** Linker:
/usr/bin/gcc -fno-stack-protector -DTABLES_NEXT_TO_CODE '-Wl,--hash-size=31' -Wl,--reduce-memory-overheads -Wl,--no-as-needed -o Main Main.o Test.o -L/usr/lib/ghc-7.10.3/base_HQfYBxpPvuw8OunzQu6JGM -L/usr/lib/ghc-7.10.3/integ_2aU3IZNMF9a7mQ0OzsZ0dS -L/usr/lib/ghc-7.10.3/ghcpr_8TmvWUcS1U1IKHT0levwg3 -L/usr/lib/ghc-7.10.3/rts /tmp/ghc1541_0/ghc_5.o /tmp/ghc1541_0/ghc_8.o -Wl,-u,ghczmprim_GHCziTypes_Izh_static_info -Wl,-u,ghczmprim_GHCziTypes_Czh_static_info -Wl,-u,ghczmprim_GHCziTypes_Fzh_static_info -Wl,-u,ghczmprim_GHCziTypes_Dzh_static_info -Wl,-u,base_GHCziPtr_Ptr_static_info -Wl,-u,ghczmprim_GHCziTypes_Wzh_static_info -Wl,-u,base_GHCziInt_I8zh_static_info -Wl,-u,base_GHCziInt_I16zh_static_info -Wl,-u,base_GHCziInt_I32zh_static_info -Wl,-u,base_GHCziInt_I64zh_static_info -Wl,-u,base_GHCziWord_W8zh_static_info -Wl,-u,base_GHCziWord_W16zh_static_info -Wl,-u,base_GHCziWord_W32zh_static_info -Wl,-u,base_GHCziWord_W64zh_static_info -Wl,-u,base_GHCziStable_StablePtr_static_info -Wl,-u,ghczmprim_GHCziTypes_Izh_con_info -Wl,-u,ghczmprim_GHCziTypes_Czh_con_info -Wl,-u,ghczmprim_GHCziTypes_Fzh_con_info -Wl,-u,ghczmprim_GHCziTypes_Dzh_con_info -Wl,-u,base_GHCziPtr_Ptr_con_info -Wl,-u,base_GHCziPtr_FunPtr_con_info -Wl,-u,base_GHCziStable_StablePtr_con_info -Wl,-u,ghczmprim_GHCziTypes_False_closure -Wl,-u,ghczmprim_GHCziTypes_True_closure -Wl,-u,base_GHCziPack_unpackCString_closure -Wl,-u,base_GHCziIOziException_stackOverflow_closure -Wl,-u,base_GHCziIOziException_heapOverflow_closure -Wl,-u,base_ControlziExceptionziBase_nonTermination_closure -Wl,-u,base_GHCziIOziException_blockedIndefinitelyOnMVar_closure -Wl,-u,base_GHCziIOziException_blockedIndefinitelyOnSTM_closure -Wl,-u,base_GHCziIOziException_allocationLimitExceeded_closure -Wl,-u,base_ControlziExceptionziBase_nestedAtomically_closure -Wl,-u,base_GHCziEventziThread_blockedOnBadFD_closure -Wl,-u,base_GHCziWeak_runFinalizzerBatch_closure -Wl,-u,base_GHCziTopHandler_flushStdHandles_closure -Wl,-u,base_GHCziTopHandler_runIO_closure -Wl,-u,base_GHCziTopHandler_runNonIO_closure -Wl,-u,base_GHCziConcziIO_ensureIOManagerIsRunning_closure -Wl,-u,base_GHCziConcziIO_ioManagerCapabilitiesChanged_closure -Wl,-u,base_GHCziConcziSync_runSparks_closure -Wl,-u,base_GHCziConcziSignal_runHandlersPtr_closure -lHSbase-4.8.2.0-HQfYBxpPvuw8OunzQu6JGM -lHSinteger-gmp-1.0.0.0-2aU3IZNMF9a7mQ0OzsZ0dS -lHSghc-prim-0.4.0.0-8TmvWUcS1U1IKHT0levwg3 -lHSrts -lCffi -lgmp -lm -lrt -ldl
link: done
*** Deleting temp files:
Deleting: /tmp/ghc1541_0/ghc_10.rsp /tmp/ghc1541_0/ghc_9.rsp /tmp/ghc1541_0/ghc_8.o /tmp/ghc1541_0/ghc_7.s /tmp/ghc1541_0/ghc_6.rsp /tmp/ghc1541_0/ghc_5.o /tmp/ghc1541_0/ghc_4.c
*** Deleting temp dirs:
Deleting: /tmp/ghc1541_0