UniqSupply: make mkSplitUniqSupply thread-safe

unsafeInterleaveIO is used instead of unsafeDupableInterleaveIO because
a mk_supply thunk that is simultaneously entered by two threads should
evaluate to the same UniqSupply.

The UniqSupply counter is now incremented atomically using the RTS's
atomic_inc().

To mitigate the extra overhead of unsafeInterleaveIO in the
single-threaded compiler, noDuplicate# is changed to exit early when
n_capabilities == 1.

1 files changed, 5 insertions, 1 deletions

diff --git a/compiler/cbits/genSym.c b/compiler/cbits/genSym.c
index 2d9779b898..8614e97e75 100644
--- a/compiler/cbits/genSym.c
+++ b/compiler/cbits/genSym.c
@@ -4,6 +4,10 @@
 static HsInt GenSymCounter = 0;
 
 HsInt genSym(void) {
-    return GenSymCounter++;
+    if (n_capabilities == 1) {
+        return GenSymCounter++;
+    } else {
+        return atomic_inc((StgWord *)&GenSymCounter);
+    }
 }