fix memory corruption in old GCC

This typo introduced memory corruption when __builtin_add_overflow
is not available but uint128_t is.  GCC before 5 are one of such
situatins.

See also https://rubyci.org/logs/rubyci.s3.amazonaws.com/opensuseleap/ruby-master/log/20191009T120004Z.log.html.gz

1 files changed, 1 insertions, 1 deletions

diff --git a/gc.c b/gc.c
index 7cc3c53d1a..f4292370a0 100644
--- a/gc.c
+++ b/gc.c
@@ -135,7 +135,7 @@ size_add_overflow(size_t x, size_t y)
 
 #elif defined(DSIZE_T)
     RB_GNUC_EXTENSION DSIZE_T dx = x;
-    RB_GNUC_EXTENSION DSIZE_T dy = x;
+    RB_GNUC_EXTENSION DSIZE_T dy = y;
     RB_GNUC_EXTENSION DSIZE_T dz = dx + dy;
     p = dz > SIZE_MAX;
     z = (size_t)dz;