Re: Subtraction + comparison in one asm instruction?

"Vincent Lefevre" <vincent+news@vinc17.org> writes:

[...]

| In my code, if c - 1 is not representable, this is an undefined
| behaviour. Thus the compiler could choose to do anything in this
| case and the optimization would be valid

Unless the implementation chooses to bind to other contraints where
the C standard leaves the issue implementation-defined or undefined.
It is quite reasonable to expect an underflow trap/exception, if such
a thing is supported by the executing environment (e.g. conforming to
"Language Independent Arithmetic, part 1"). In that case, I would not
like to see the compiler deliberately suppress that expectation in my
codes.

One of the purposes of the C standards leaving some behaviour undefined
is to give implementations the opportunities to satisfy other "bindings".

| (if there are problems
| with some codes, the compiler could still provide switches to
| disable the optimization for such broken codes).

Err, the codes aren't necessarily broken. It may just be that the
inputs are exercising boundary values.

-- Gaby

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