On 2012-01-07, Peter Bigot <big...@acm.org> wrote: > On Sat, Jan 7, 2012 at 10:28 AM, Grant Edwards ><grant.b.edwa...@gmail.com> wrote:
>> However the "test code" might be non-volatile, and as such GCC is >> still free to move it outside the set/clear pair (AFAICT). [...] >> 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. > > Yes, today I think you're right. I've always been a bit fuzzy on this stuff, so don't bet the farm on my opinions. It seems to be a common belief in embedded programmers that compilers aren't allowed to (and won't) move anything past a volatile access, but I've never been able to find any convincing basis for that belief. > In fact, that's what I thought back in early December when I added > cycle timing to test430 and found a situation where the compiler > helpfully moved the test code out from between the start/stop counter > reads. I did something that stopped that from happening, though I > don't remember what; it wasn't a memory barrier. > > Later in December, the topic arose again in the context of the thread > at http://gcc.gnu.org/ml/gcc/2011-12/msg00008.html, during which > discussion I became convinced any such move was a compiler error, > apparently because I mis-read Richard Guenther's response in > http://gcc.gnu.org/ml/gcc/2011-12/msg00026.html. The discussion > there ultimately revealed an underlying gcc bug that will eventually > be fixed in a 4.7.x release, so confusion about the issue is common > even among those who are expected to know better. I don't have a complete copy of the C standard, but some of the excerpts I have read certainly didn't make anything any clearer in my mind. I think reading standards like that is a skillset that takes a lot more work/practice than I've put in. >> Pointers to GCC docs actually stating what to expect would be >> most welcome... > > The current version of what you cited > (http://gcc.gnu.org/onlinedocs/gcc/Volatiles.html) is the best I know > of, That is clearer than the older versions. Now, if only I wasn't perpetually confused over what's a sequence point and what isn't... [I know it's well defined, but that doesn't mean I can remember it correctly.] -- Grant Edwards grant.b.edwards Yow! I'm thinking about at DIGITAL READ-OUT systems gmail.com and computer-generated IMAGE FORMATIONS ... ------------------------------------------------------------------------------ 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
