On 04/09/2015 08:56 AM, Alain Ketterlin wrote:
Marko Rauhamaa <ma...@pacujo.net> writes:
Alain Ketterlin <al...@dpt-info.u-strasbg.fr>:
No, it would not work for signed integers (i.e., with lo and hi of
int64_t type), because overflow is undefined behavior for signed.
All architectures I've ever had dealings with have used 2's-complement
integers. Overflow is well-defined, well-behaved and sign-independent
wrt addition, subtraction and multiplication (but not division).
You are confused: 2's complement does not necessarily mean modular
arithmetic. See, e.g.,
http://stackoverflow.com/questions/16188263/is-signed-integer-overflow-still-undefined-behavior-in-c
So the C standard can specify such things as undefined. The
architecture still will do something specific, right or wrong, and
that's what Marko's claim was about. The C compiler has separate types
for unsigned and for signed, while the underlying architecture of every
twos complement machine I have used did add, subtract, and multiply as
though the numbers were unsigned (what you call modular arithmetic).
In my microcoding days, the ALU did only unsigned arithmetic, while the
various status bits had to be interpreted to decide whether a particular
result was overflow or had a carry. It was in interpreting those status
bits that you had to use the knowledge of whether a particular value was
signed or unsigned.
Then the microcode had to present those values up to the machine
language level. And at that level, we had no carry bits, or overflow,
or anything directly related.
--
DaveA
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