David Brown wrote: > Georg-Johann Lay wrote: >> Thomas, George schrieb: >>> >>> I checked the patches of the fix in PR53448 >>> >>> Even when the requested alignment in greater than the >>> BIGGEST_ALIGNMENT (considering it to be 2 for Xmega and 1 for other >>> devices), we are not producing a warning to the user that he is >>> trying to align more than the required. Is this behavior right ? >> >> Yes. >> >>> Should the warning be there in this case ? >> >> No. If alignment is bigger than BIGGEST_ALIGNMENT, then code will still >> be all right; just a bit more memory consuming. >> >> However, some hardware might need a more restrictive alignment, you >> named an example. To arrange for that, the user can align data by hand. >> >> This align-by-hand will be an exception and is not commonly needed for >> code that runs on xmega. >> >> On a strict-alignment platform like ARM, you would set BIGGEST_ALIGNMENT >> to 32 or maybe even 64 for 64-bit ARMs. >> >> If alignments are less restrictive than BIGGEST_ALIGNMENT you will get >> wrong code, e.g. for variables located in memory. Because on AVR there >> are no restrictions, BIGGEST_ALIGNMENT is 8. >> >> In the case where the user wants to align, say, an I/O buffer, she can >> use the alignment attribute to arrange for that exception to the rule, >> and no diagnostic should be issued then because the user explicitly >> asked to align the object. >> >> Johann > > Forgive me for my ignorance here, but what use is "BIGGEST_ALIGNMENT"? > > Each type has a /minimum/ alignment and a /preferred/ alignment, > dependent on the type, its size, the target cpu, and perhaps other > details (such as memory buses or other internal structures). For > example, a 32-bit int might require 4-byte alignment on one target, but > be fine with 1-byte alignment on another, and on a third target it might > work okay with 1-byte alignment but be faster with 4-byte alignment. > > On the other hand, there are times when the user explicitly wants to > /increase/ the alignment of an item, using the "align" attribute. I > can't remember having used this on an AVR, but I've used it on other > processors, such as to align data with cache lines or to match DMA > buffers. And I could imagine uses on an AVR also - if you want a fast > circular buffer or lookup table, you might use 256-byte aligned 256-byte > arrays to allow you to do indexing using 8-bit calculations instead of > 16-bit. > > I /love/ compiler warnings, and enable almost all of them - but if I > explicitly use an "align" attribute to align data at 256 bytes, I don't > want a warning telling me it is unnecessary, or bigger than > "BIGGEST_ALIGNMENT". And I certainly don't want a warning telling me > the compiler is refusing to match my requirements just because it is > bigger than "BIGGEST_ALIGNMENT". > > So where does "BIGGEST_ALIGNMENT" fit in?
For BA, see the internals at [1]. To grasp the full meaning you want to have a look at the compiler sources. Johann [1] http://gcc.gnu.org/onlinedocs/gccint/Storage-Layout.html#index-BIGGEST_005fALIGNMENT-3881 _______________________________________________ AVR-GCC-list mailing list AVR-GCC-list@nongnu.org https://lists.nongnu.org/mailman/listinfo/avr-gcc-list
