On Thu, Apr 22, 2021 at 12:10 PM Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> wrote: > On Thu, 2021-04-22 at 11:28 -0500, Peng Yu wrote: > > On 4/22/21, Xi Ruoyao via Gcc-help <gcc-help@xxxxxxxxxxx> wrote: > > > On Wed, 2021-04-21 at 21:40 +0200, Stefan Ring wrote: > > > > On Wed, Apr 21, 2021 at 8:41 PM Xi Ruoyao via Gcc-help > > > > <gcc-help@xxxxxxxxxxx> wrote: > > > > > > > > > > I sincerely don't like to see someone try to "learn" by running > > > > > some > > > > > program containing "i = i++ + i++" and "inspect" the output. > > > > > > > > Nothing wrong with that! I did almost exactly this in the early > > > > nineties on DOS, with 16-bit Borland C++ ;) > > > > > > > > But now I see what you're getting at. I understand the > > > > assembly/machine code as output. But you were probably talking about > > > > the result of the calculation. > > > > > > Well, even the asm/machine code can't be relied on. As it is an UB > > > the > > > compiler can output some really strange thing, maybe: > > > > What original C code cause the compile to generate these different > > assembly code snippets? They seem to come from different C code unless > > you are taking about optimization for the same C code. If it is the > > latter, then it seems the optimization is not done properly in some of > > the cases. I don't understand what point you want to prove here. > > "i = i++ + i++" invokes undefined behavior. The standard has no > requirement. The compiler can do anything. > > I mean you can't learn to program (at least in C) just by doing > experiments here and there and trying to parse some result out of it. > And your reaction proved my point, again: after so many strange > experiments you still don't even know undefined behavior. I don’t think this statement make much sense. Nobody just do experiments. > > Maybe the only "meaningful output" is: > > > > > > > test.c:6:4: warning: operation on 'i' may be undefined [-Wsequence- > > > > point] > > > > > > (I think you absolutely understand what I posted above, but I'll still > > > post it to warn new learners :) > > Stefan: you see that? The post is really necessary :(. > -- > Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> > School of Aerospace Science and Technology, Xidian University > > -- Regards, Peng
