On 2012-01-07, Peter Bigot <big...@acm.org> wrote: > On Sat, Jan 7, 2012 at 9:00 AM, Grant Edwards <grant.b.edwa...@gmail.com> > wrote: > >> It's quite possible that the 4.x compiler is more aggressive about >> moving stuff around. ?When working on low level stuff like an MSP430, >> I pretty much always have my Makefiles set up to generate mixed >> C/assmebly language listings so that it's easy to keep an eye on what >> the compiler is doing. ?One advantage of that is that you quickly >> learn what C constructs are handled efficiently by the compiler wand >> what aren't -- you learn that the smallest, fastest way to do things >> in C on an MSP430 is not the same as on an AVR. > > The 4.x compilers (especially 4.6.x) do optimize much more > aggressively, but if you use the defined peripheral registers like > P2OUT mspgcc had better not re-order them across sequence points, > since those are marked volatile. If you do see something like this, > it's almost certainly a bug and I'd appreciate it being reported on > the SF tracker.
However the "test code" might be non-volatile, and as such GCC is still free to move it outside the set/clear pair (AFAICT). The GCC documentaion I've found appears to indicate that the restriction on moving code across sequence points only affects volatile accesses. Quoting from http://gcc.gnu.org/onlinedocs/gcc-4.5.3/gcc/Volatiles.html#Volatiles The minimum either standard specifies is that at a sequence point all previous accesses to volatile objects have stabilized and no subsequent accesses have occurred. Note that the GCC docs only say that _volatile_ accesses can't be moved across sequence points. More searches reveal similar statements along with a few seemingly well-informed postings where it is explicitly stated that non-volatile accesses may be moved across sequence points. For example from http://blog.regehr.org/archives/28 (which appears to be a rather well-informed article): Summary: Most compilers can and will move accesses to non-volatile objects around accesses to volatile objects, so don't rely on the program ordering being respected. I don't know if it's still an issue, but at one point he mentions a "volatile" bug in MSP430: Compilers are not totally reliable in their translation of accesses to volatile-qualified objects. I've written extensively about this subject elsewhere, but here's a quick example: volatile int x; void foo (void) { x = x; } The proper behavior of this code on the actual machine is unambiguous: there should be a load from x, then a store to it. However, the port of GCC to the MSP430 processor behaves differently: $ msp430-gcc -O vol.c -S -o - foo: ret [I don't know which version of mspgcc that refers to, and it's not really related to our topic but it does lead me to believe that the author knows what he's talking about.] Anyhow.... I can't find anywhere in GCC docs that say non-volatile accesses can't be moved across sequence points or volatile accesses. If sequence points and volatile accesses only constrain ordering of accesses to volatile objects, and what you're timing doesn't involve volatile objects, then adding sequence points doesn't matter. However, adding a memory barrier ought to do the trick. I think. Pointers to GCC docs actually stating what to expect would be most welcome... -- Grant Edwards grant.b.edwards Yow! I KAISER ROLL?! at What good is a Kaiser Roll gmail.com without a little COLE SLAW on the SIDE? ------------------------------------------------------------------------------ Ridiculously easy VDI. With Citrix VDI-in-a-Box, you don't need a complex infrastructure or vast IT resources to deliver seamless, secure access to virtual desktops. With this all-in-one solution, easily deploy virtual desktops for less than the cost of PCs and save 60% on VDI infrastructure costs. Try it free! http://p.sf.net/sfu/Citrix-VDIinabox _______________________________________________ Mspgcc-users mailing list Mspgcc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/mspgcc-users
