By: EduardoS (no.delete@this.spam.com), February 25, 2013 1:06 pm
Room: Moderated Discussions
Wilco (Wilco.Dijkstra.delete@this.ntlworld.com) on February 25, 2013 7:36 am wrote:
> Well it is useful for add in the same way it is useful for any other
> operation. Programmers just tend to compare with zero a lot.
I almost never compare the result of an ADD with zero, by the semantics of the operation this isnt very common or meaningful.
> Note if you really believe in condition codes, even loads should set the Z flag since ldr;
> cmp #0; beq/bne is extremely common (of course I prefer Thumb-2 cbz/cbnz instead).
That's true.
> For those you'd need additional instructions, just like add with carry.
So...
1) Add and set carry;
2) Add and set overflow;
3) Add with carry and set carry;
4) Sub and set borrow;
5) Sub and set overflow;
6) Sub with borrow and set borrow;
Quite a lot opcode space, no?
> For floating point you always use a compare first.
No, when you care about not getting meaningless results you may check wheter each operation didn't resulted in overflow or NaN, it is not very pratical to compare the output of each result.
> As long as you stay with C/C++ then you would not see overflow checks even if they
> were used in the source. For Java/C# things are different of course, but even then
> the number of overflow checks is small compared to the number of additions.
It is not so much about the language but about the target market, when processing transactions it is ok if 2 billion + 3 billion = 5 billion, it is not ok if it results in an overflow exception, but at least your results still consistent, the transaction fail, the application don't, and it is completly inacceptable if the result is 705,032,704, that's what I am saying, some apps check for overflow way too often, it is a requirement for them, C/C++ just make it harder.
Finally, my opinion about flags is, if you are going to use them, use a full set of them, a single flag doesn't bring so much benefits.
> Well it is useful for add in the same way it is useful for any other
> operation. Programmers just tend to compare with zero a lot.
I almost never compare the result of an ADD with zero, by the semantics of the operation this isnt very common or meaningful.
> Note if you really believe in condition codes, even loads should set the Z flag since ldr;
> cmp #0; beq/bne is extremely common (of course I prefer Thumb-2 cbz/cbnz instead).
That's true.
> For those you'd need additional instructions, just like add with carry.
So...
1) Add and set carry;
2) Add and set overflow;
3) Add with carry and set carry;
4) Sub and set borrow;
5) Sub and set overflow;
6) Sub with borrow and set borrow;
Quite a lot opcode space, no?
> For floating point you always use a compare first.
No, when you care about not getting meaningless results you may check wheter each operation didn't resulted in overflow or NaN, it is not very pratical to compare the output of each result.
> As long as you stay with C/C++ then you would not see overflow checks even if they
> were used in the source. For Java/C# things are different of course, but even then
> the number of overflow checks is small compared to the number of additions.
It is not so much about the language but about the target market, when processing transactions it is ok if 2 billion + 3 billion = 5 billion, it is not ok if it results in an overflow exception, but at least your results still consistent, the transaction fail, the application don't, and it is completly inacceptable if the result is 705,032,704, that's what I am saying, some apps check for overflow way too often, it is a requirement for them, C/C++ just make it harder.
Finally, my opinion about flags is, if you are going to use them, use a full set of them, a single flag doesn't bring so much benefits.
