https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106822
--- Comment #3 from Benjamin B. Meier <benjamin.meier70 at gmail dot com> --- > The reason for the difference is because in the case where there is an int in > the structure, the alignment is 4 bytes so a 2 or 4 byte read for a and b > will not cause an fault while in the first case where there is no int, the > alignment is 1 and might cause a fault. Gcc is conservative here on doing the > optimization. Okay, got it. The alignment actually makes a difference here. Thanks, for the explanation. > Also || is a short cutting or in c so I don't think this optimization can be > done for the original case in a well defined way. I see that as well, however reading the right hand side expression of the ||-operator has in this case no observable side-effect. Plus if we know that s->a can be read, we know as well that s cannot be a NULL pointer. Given that, I would assume the optimization should be valid (and applied). Another supporting argument for ignoring the short cutting behavior is the version of "MyStruct" with "int x;": In this case the compiler also always reads both operands (and therefore ignores the short cut). So if the compiler uses a nice optimization without jumps if it knows the alignment is at least 2 bytes, it probably could also use an optimization for the case that the alignment is only 1 byte. A possible optimization would then look like the code of f2. Finally, it's nothing really important, but I still do not yet completely see why optimizing the jump away in f is an issue:)
