jepler/circuitpython
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity

py/objint_longlong: Fix left shift of negative values.

Browse source 
Previous comment was wrong, left shifting a negative value is UB in C. Use
the same approach as small int shifts (from runtime.c).

Signed-off-by: Angus Gratton <angus@redyak.com.au>
This commit is contained in:
Angus Gratton 2025-07-22 10:48:16 +10:00 committed by Damien George
parent 3185bb5827
commit 5d9ef6bfb6
1 changed files with 5 additions and 5 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

10
py/objint_longlong.c
View file

@ -208,14 +208,14 @@ mp_obj_t mp_obj_int_binary_op(mp_binary_op_t op, mp_obj_t lhs_in, mp_obj_t rhs_i
case MP_BINARY_OP_LSHIFT:
case MP_BINARY_OP_INPLACE_LSHIFT:
if ((int)rhs_val < 0) {
if (rhs_val < 0) {
// negative shift not allowed
mp_raise_ValueError(MP_ERROR_TEXT("negative shift count"));
}
result = lhs_val << (int)rhs_val;
// Left-shifting of negative values is implementation defined in C, but assume compiler
// will give us typical 2s complement behaviour unless the value overflows
overflow = rhs_val > 0 && ((lhs_val >= 0 && result < lhs_val) || (lhs_val < 0 && result > lhs_val));
overflow = rhs_val >= (sizeof(long long) * MP_BITS_PER_BYTE)
|| lhs_val > (LLONG_MAX >> rhs_val)
|| lhs_val < (LLONG_MIN >> rhs_val);
result = (unsigned long long)lhs_val << rhs_val;
break;
case MP_BINARY_OP_RSHIFT:
case MP_BINARY_OP_INPLACE_RSHIFT: