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It's different viewpoints...It's different viewpoints...Posted May 29, 2011 19:27 UTC (Sun) by iabervon (subscriber, #722)In reply to: It's different viewpoints... by anton Parent article: What Every C Programmer Should Know About Undefined Behavior #3/3
In the 2.xx days, at least, -O2 meant "use all optimizations that don't change the behavior of any programs, including invalid ones, except totally crazy stuff" (e.g., if you start poking around at your stack frames or use non-volatile pointers to do MMIO, all bets are off). -O3 and higher would give you optimizations that wouldn't work with programs that do things that are technically not permitted. It would be nice if they had an optimization level that would be suitable for what most people think C is, as well as one that tells the compiler that the programmer has carefully avoided any undefined behavior.
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It's different viewpoints... Posted May 30, 2011 1:01 UTC (Mon) by vonbrand (subscriber, #4458) [Link] AFAICS, nothing whatsoever has changed then, undefined behaviour is "totally crazy stuff, that nobody in their right mind would expevt to work as intended everywhere"...
It's different viewpoints... Posted May 30, 2011 1:56 UTC (Mon) by iabervon (subscriber, #722) [Link]
In the 2.xx days, the "totally crazy" stuff was what actual C programmers, who only had 3rd-hand knowledge of the spec, knew couldn't be defined. Pretty much all of the available processors used 2's complement, and everyone assumed that signed overflow used 2's complement and certainly produced some value or other. You couldn't be sure what you'd get from an uninitialized variable, but it would produce some value (and would continue to have that value until you wrote to it). On the other hand, people had no idea what the function call ABI was, or how the stack frame would be laid out, so they couldn't guess what would happen with undefined behavior there. It's gone from "it's hard to get it wrong" (you needed to know a lot about your platform to write code that breaks going from -O0 to -O3) to "it's hard to get it right" (you need to know a lot about the C language to avoid writing code that breaks going from -O0 to -O2).
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