[cfe-users] unsequenced modification and access

[Javier Múgica]

I get the warning Wunsequenced if I write code like

for(k=n;k;) k--, aux+=*p**p++;

or

for(i=n;i--;) *x++=*(x+1);


Clang behabour here is right. The access to *p or *x, to the one without the ++, is unsequenced with respect to the increment of the pointer indicated by the other p++ or x++. This means that it is undefined which may take place before. But I suppose that the compiler (any compiler) will just push the ++ after the whole instruction, thereby achieving the programmer's desired effect. May I rely on that?

The first example is of a kind I use often in some innermost loops that need to be optimized. I *suppose* that the right change in order to get pedantic-good code and not decreasing the performance of the emitted code is

for(k=n;k;){k--; Double auxb=*p++; aux+=auxb*auxb;}

for the compiler will optimize away the variable auxb when optimizing. I preferred this solutions instead of

for(k=n;k;){k--; Double aux+=*p**p; p++;}

But maybe nowadays compilers are clever enough when optimizing, and in particular llvm, that the three different codings would compile to the same, and there is no need to worry about helping the compiler that much.

Regards