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. > incl %eax > incl %eax > addl %eax, %eax > > or > > incl %eax > addl %eax, %eax > incl %eax > > or > > addl %eax, %eax > addl %eax, $2 > > or > > ud2 # you are invoking UB and breaking world peace! - by gcc > > 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 :) > -- > Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> > School of Aerospace Science and Technology, Xidian University > > -- Regards, Peng
