Re: Subtraction + comparison in one asm instruction?

    Gabriel Dos Reis <gdr@soliton.integrable-solutions.net> wrote:


> 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.


In all the cases I've tested, there was no support for underflow trap,
so this can't be an argument against this optimization in these cases
(gcc optimizes int f(int x) { return (x - 1) + 1; } to "return x;",
so why not other optimizations that can hide overflows under these
conditions?). If underflow/overflow traps are supported, there could
be a compiler switch to allow the user to say that he has proved that
there are no overflows in his program, so that such optimizations
could be performed for speed-critical programs.

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