ghc-prim: fix hs_cmpxchg64 function prototype

hs_cmpxchg64 must return a StgWord64, otherwise incorrect runtime
results of 64-bit MO_Cmpxchg will appear in 32-bit unregisterised
builds, which go unnoticed at compile-time due to C implicit casting
in .hc files.

(cherry picked from commit 9fa545722f9151781344446dd5501db38cb90dd1)

2 files changed, 3 insertions, 3 deletions

diff --git a/libraries/ghc-prim/cbits/atomic.c b/libraries/ghc-prim/cbits/atomic.c
index 2ac6d26e1f..c15c596ddd 100644
--- a/libraries/ghc-prim/cbits/atomic.c
+++ b/libraries/ghc-prim/cbits/atomic.c
@@ -308,8 +308,8 @@ hs_cmpxchg32(StgWord x, StgWord old, StgWord new)
   return __sync_val_compare_and_swap((volatile StgWord32 *) x, (StgWord32) old, (StgWord32) new);
 }
 
-extern StgWord hs_cmpxchg64(StgWord x, StgWord64 old, StgWord64 new);
-StgWord
+extern StgWord64 hs_cmpxchg64(StgWord x, StgWord64 old, StgWord64 new);
+StgWord64
 hs_cmpxchg64(StgWord x, StgWord64 old, StgWord64 new)
 {
   return __sync_val_compare_and_swap((volatile StgWord64 *) x, old, new);
diff --git a/rts/include/stg/Prim.h b/rts/include/stg/Prim.h
index 9a04a4eaba..9e5752584a 100644
--- a/rts/include/stg/Prim.h
+++ b/rts/include/stg/Prim.h
@@ -41,7 +41,7 @@ StgWord64 hs_atomic_xor64(StgWord x, StgWord64 val);
 StgWord hs_cmpxchg8(StgWord x, StgWord old, StgWord new_);
 StgWord hs_cmpxchg16(StgWord x, StgWord old, StgWord new_);
 StgWord hs_cmpxchg32(StgWord x, StgWord old, StgWord new_);
-StgWord hs_cmpxchg64(StgWord x, StgWord64 old, StgWord64 new_);
+StgWord64 hs_cmpxchg64(StgWord x, StgWord64 old, StgWord64 new_);
 StgWord hs_atomicread8(StgWord x);
 StgWord hs_atomicread16(StgWord x);
 StgWord hs_atomicread32(StgWord x);