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Have you seen this page?Have you seen this page?Posted May 25, 2011 5:12 UTC (Wed) by khim (subscriber, #9252)In reply to: Sure, but then you deserve what you get... by iabervon Parent article: What Every C Programmer Should Know About Undefined Behavior #3/3
How would gcc, a C program, compute the result of a signed overflow without risking crashing? Have you seen this page? Specifically the libraries part? Do you know why GMP, MPFR and MPC are requirements, not options? Actually I think this particular optimization does not use multiprecision arithmetic, but if it's needed in some passes it is available to GCC despite the fact that it's C program. P.S. Note that is you really want "overflow", not "undefined behavior" you can do that and the fact that GCC is a C program does not stop you.
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Have you seen this page? Posted May 25, 2011 5:44 UTC (Wed) by iabervon (subscriber, #722) [Link] Those libraries compute the well-defined results of calculations with large numbers; they don't give the result of signed overflow. They aren't going to help for figuring out the results of running (unoptimized, on the target machine):
if (MAXINT + 1 == -MAXINT) printf("Wow, 1's complement!\n");
because that's a matter of processor architecture, not mathematics. And certainly gcc isn't going to try eliciting the undefined behavior itself and replicating it, because the undefined behavior might be "your compiler crashes processing unreachable code", which the C specification doesn't allow.
Have you seen this page? Posted May 26, 2011 8:27 UTC (Thu) by marcH (subscriber, #57642) [Link]
> Those libraries compute the well-defined results of calculations with large numbers; they don't give the result of signed overflow.
Of course they can do this; the latter is just one modulus operation away from the former.
> because that's a matter of processor architecture, not mathematics.
Surprise: processors architectures are rooted in arithmetics. All of them, even though they differ with each other.
Have you seen this page? Posted May 27, 2011 9:45 UTC (Fri) by dgm (subscriber, #49227) [Link]
> Surprise: processors architectures are rooted in arithmetics. All of them, even though they differ with each other.
Still not a matter of mathematics: both 1's and 2's compliment are equally correct. The matter is about the choice made by the processor designers, and thus, a matter of processor architecture.
Have you seen this page? Posted May 27, 2011 14:03 UTC (Fri) by marcH (subscriber, #57642) [Link]
It's a matter of both: 1. Choose the architecture, 2. Apply the maths specific to this architecture.
GMP or else can help for the latter (of course not for the former).
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