add function for checked mrb_int addition

include/mruby/numeric.h

@@ -35,6 +35,16 @@ mrb_float mrb_to_flo(mrb_state *mrb, mrb_value x);
 # define MRB_INT_OVERFLOW_MASK ((mrb_uint)1 << (MRB_INT_BIT - 1))
 #endif
 
+static inline mrb_bool
+mrb_int_add_overflow(mrb_int augend, mrb_int addend, mrb_int *sum)
+{
+  mrb_uint x = (mrb_uint)augend;
+  mrb_uint y = (mrb_uint)addend;
+  mrb_uint z = (mrb_uint)(x + y);
+  *sum = (mrb_int)z;
+  return !!(((x ^ z) & (y ^ z)) & MRB_INT_OVERFLOW_MASK);
+}
+
 #undef MRB_INT_OVERFLOW_MASK
 #undef mrb_uint
 #undef MRB_UINT_MAKE

src/numeric.c

@@ -1109,9 +1109,7 @@ mrb_fixnum_plus(mrb_state *mrb, mrb_value x, mrb_value y)
 
     if (a == 0) return y;
     b = mrb_fixnum(y);
-    c = a + b;
-    if (((a < 0) ^ (b < 0)) == 0 && (a < 0) != (c < 0)) {
-      /* integer overflow */
+    if (mrb_int_add_overflow(a, b, &c)) {
       return mrb_float_value(mrb, (mrb_float)a + (mrb_float)b);
     }
     return mrb_fixnum_value(c);

src/vm.c

@@ -12,6 +12,7 @@
 #include "mruby/class.h"
 #include "mruby/hash.h"
 #include "mruby/irep.h"
+#include "mruby/numeric.h"
 #include "mruby/proc.h"
 #include "mruby/range.h"
 #include "mruby/string.h"
@@ -1610,12 +1611,7 @@ RETRY_TRY_BLOCK:
 
           x = mrb_fixnum(regs_a[0]);
           y = mrb_fixnum(regs_a[1]);
-          z = x + y;
-#ifdef MRB_WORD_BOXING
-          z = (z << MRB_FIXNUM_SHIFT) / (1 << MRB_FIXNUM_SHIFT);
-#endif
-          if ((x < 0) != (z < 0) && ((x < 0) ^ (y < 0)) == 0) {
-            /* integer overflow */
+          if (mrb_int_add_overflow(x, y, &z)) {
             SET_FLT_VALUE(mrb, regs_a[0], (mrb_float)x + (mrb_float)y);
             break;
           }
@@ -1842,10 +1838,9 @@ RETRY_TRY_BLOCK:
         {
           mrb_int x = regs[a].attr_i;
           mrb_int y = GETARG_C(i);
-          mrb_int z = x + y;
+          mrb_int z;
 
-          if (((x < 0) ^ (y < 0)) == 0 && (x < 0) != (z < 0)) {
-            /* integer overflow */
+          if (mrb_int_add_overflow(x, y, &z)) {
             SET_FLT_VALUE(mrb, regs[a], (mrb_float)x + (mrb_float)y);
             break;
           }