py: Add MICROPY_USE_GCC_MUL_OVERFLOW_INTRINSIC.

Most MCUs apart from Cortex-M0 with Thumb 1 have an instruction
for computing the "high part" of a multiplication (e.g., the upper
32 bits of a 32x32 multiply).

When they do, gcc uses this to implement a small and fast
overflow check using the __builtin_mul_overflow intrinsic, which
is preferable to the guard division method previously used in smallint.c.

However, in contrast to the previous mp_small_int_mul_overflow
routine, which checks that the result fits not only within mp_int_t
but is SMALL_INT_FITS(), __builtin_mul_overflow only checks for
overflow of the C type. As a result, a slight change in the code
flow is needed for MP_BINARY_OP_MULTIPLY.

Other sites using mp_small_int_mul_overflow already had the
result value flow through to a SMALL_INT_FITS check so they didn't
need any additional changes.

Do similarly for the _ll and _ull multiply overflows checks.

Signed-off-by: Jeff Epler <jepler@gmail.com>

1 files changed, 60 insertions, 0 deletions

diff --git a/py/runtime_utils.c b/py/runtime_utils.c
index b92c6bd76..50526b210 100644
--- a/py/runtime_utils.c
+++ b/py/runtime_utils.c
@@ -50,3 +50,63 @@ mp_obj_t mp_call_function_2_protected(mp_obj_t fun, mp_obj_t arg1, mp_obj_t arg2
         return MP_OBJ_NULL;
     }
 }
+
+#if !MICROPY_USE_GCC_MUL_OVERFLOW_INTRINSIC
+bool mp_mul_ll_overflow(long long int x, long long int y, long long int *res) {
+    bool overflow;
+
+    // Check for multiply overflow; see CERT INT32-C
+    if (x > 0) { // x is positive
+        if (y > 0) { // x and y are positive
+            overflow = (x > (LLONG_MAX / y));
+        } else { // x positive, y nonpositive
+            overflow = (y < (LLONG_MIN / x));
+        } // x positive, y nonpositive
+    } else { // x is nonpositive
+        if (y > 0) { // x is nonpositive, y is positive
+            overflow = (x < (LLONG_MIN / y));
+        } else { // x and y are nonpositive
+            overflow = (x != 0 && y < (LLONG_MAX / x));
+        } // End if x and y are nonpositive
+    } // End if x is nonpositive
+
+    if (!overflow) {
+        *res = x * y;
+    }
+
+    return overflow;
+}
+
+#define MP_UINT_MAX (~(mp_uint_t)0)
+#define MP_INT_MAX ((mp_int_t)(MP_UINT_MAX >> 1))
+#define MP_INT_MIN (-MP_INT_MAX - 1)
+
+bool mp_mul_mp_int_t_overflow(mp_int_t x, mp_int_t y, mp_int_t *res) {
+    // Check for multiply overflow; see CERT INT32-C
+    if (x > 0) { // x is positive
+        if (y > 0) { // x and y are positive
+            if (x > (MP_INT_MAX / y)) {
+                return true;
+            }
+        } else { // x positive, y nonpositive
+            if (y < (MP_INT_MIN / x)) {
+                return true;
+            }
+        } // x positive, y nonpositive
+    } else { // x is nonpositive
+        if (y > 0) { // x is nonpositive, y is positive
+            if (x < (MP_INT_MIN / y)) {
+                return true;
+            }
+        } else { // x and y are nonpositive
+            if (x != 0 && y < (MP_INT_MAX / x)) {
+                return true;
+            }
+        } // End if x and y are nonpositive
+    } // End if x is nonpositive
+
+    // Result doesn't overflow
+    *res = x * y;
+    return false;
+}
+#endif