On 07/12/2018 06:19 PM, Michael Eager wrote: > Hi All -- > > I'm updating GCC on a dual-endianity processor. In little-endian mode, > memory is little-endian, but the core remains big-endian. Double-word > values are in even/odd register pairs, with the MSW in the > lower-numbered (even) register. Values in memory are stored with LSW in > the low-address word. How unpleasant. > > From info: > REG_WORDS_BIG_ENDIAN [Macro] > On some machines, the order of words in a multiword object differs > between registers in memory. In such a situation, define this macro > to describe the order of words in a register. The macro > WORDS_BIG_ENDIAN controls the order of words in memory. > > I set REG_WORDS_BIG_ENDIAN to 1 and WORDS_BIG_ENDIAN to the memory > endianity. > > None of the targets in the GCC repo define this. Does anyone know of an > out-of-repo target which sets REG_WORDS_BIG_ENDIAN? The closest in-tree target is probably c6x, allows selecting big/little for WORDS_BIG_ENDIAN, but REG_WORDS_BIG_ENDIAN is always 0. > > Most test cases pass, but I'm finding a few cases which fail to > calculate a subreg correctly. Subreg_lowpart_offset() is called while > emitting RTL, for example, to truncate a DImode integer to SImode. But > subreg_lowpart_offset() does not know whether it is computing the offset > of a memory value or register value. It returns the word offset > according to memory endianity, ignoring REG_WORDS_BIG_ENDIAN. (This is > also the case with subreg_size_lowpart_offset()). I think you'd probably want to avoid (subreg (mem)) if at all possible. That's probably good advice in general anyway. > > I'm working on a patch to pass the inner RTX to subreg_lowpart_offset(), > but this function is called in many places and it isn't always clear > what object the subreg applies to, or even if there is an object. For > example, maybe_mode_change() calls subreg_size_lowpart_offset() before > generating a REG rtx. This seems to be the wrong approach. > > There are other subreg functions which seem to be specifically for > registers, like subreg_get_info() and subreg_regno_offset(). It isn't > clear whether these can replace any of the calls to > subreg_lowpart_offset(). About the only advice I'd have would be to look at the c6x and see if it's handling any of this stuff correctly. BUt I suspect you're largely going to be on your own. Jeff
