Andrew Haley wrote: > Ingo Rohloff wrote: > > >> I am sorry that I ask about this topic, >> even if it has been discussed so often, but I am really at a loss: >> >> First of all is there any website, link or docu (whatever) which lists some >> examples of what is legal (under strict aliasing) and what is not ? >> > > Yes. Any version of the C standard will tell you. > http://www.open-std.org/JTC1/SC22/WG14/www/standards > > >> Let me try to explain what I understood: >> >> With the following variable definitions, >> >> unsigned int *ptr1; >> unsigned short *ptr2; >> >> "strict aliasing" means that the compiler is allowed to assume that >> "ptr1" and "ptr2" will NOT acceess to the same address (object). >> > > Right. > > >> For optimization this means that if someone writes >> >> ptr2[0]=0x1111 >> ptr1[0]=0x22222222; >> >> then the compiler can assume that ptr2[0] still is "0x1111", because "ptr1" >> must not access to memory which is accessed through ptr2. >> >> Is this correct so far ? >> > > Yes. > > >> Now I am writing code for an embedded application (no OS). >> >> Because I know the architecture I want to do the following >> --- >> unsigned int value; >> unsigned char *ptr2; >> >> *((unsigned int *)ptr2) = value; >> --- >> >> The idea here is that you write a 32 bit value in big endian order to a >> byte (unsigned char) array. >> > > That's perfectly OK. Standard C has special rules for character types, which > alias everything. > > >> (The used architecture is big endian; I know that on some architectures >> that means that the pointer must point to an address which is aligned...) >> >> >> Now my understanding was that this code breaks strict aliasing rules, >> because I cast a "unsigned char *" to an "unsigned int *". >> > > No, because of the special rules for character types. > > >> BUT: The g++ 4.4.2 compiler does not complain with "-Wstrict-aliasing". >> >> Is the above statement OK or not ? >> >> Or is it, because "ptr2" is an "unsigned char *", that ptr2 is assumed to >> be able to alias anything ? (Because it is a char pointer). >> > > That's right. > > >> Does these "char pointer can alias anything" really only refer to "char *" >> or also to "unsigned char *". >> > > All character types. > > But don't believe me: go to http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1124.pdf > and read 6.3.2.3 Paragraph 7. > > Andrew. > > Hey Ingo, > First of all is there any website, link or docu (whatever) which lists some > examples of what is legal (under strict aliasing) and what is not ? > As the C standard is quite formal, here's a handy explanation about strict aliasing rules and how they could affect compiler optimizations: http://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html Andi
