> Well, I think it's probably grammatical, but how about: > > If the register representation of a non-scalar mode has holes in it, > we expect the scalar units to be concatenated together, with the holes > distributed evenly among the scalar units. Each scalar unit must occupy > at least one register.
Fine with me, thanks. > This actually was one of the more important changes :-) In combination > with later patches, the idea is to move away from UNITS_PER_WORD tests > when endianness is regular (all big or all little) and only do them > when the distinction between bytes and words makes a real difference. > > The specific motivating examples were SVE predicate registers, which > occupy VL*2 bytes for some runtime VL. They are smaller than a word > when VL<4, word-sized when VL==4, and bigger than a word when VL>4. > We therefore can't calculate: > > GET_MODE_SIZE (ymode) > UNITS_PER_WORD > > at compile time. This is one of the patches that avoids forcing the > issue unless the answer really matters. So you plan to modify it again to remove the ysize > UNITS_PER_WORD test? Or you don't really care about the REG_WORDS_BIG_ENDIAN != WORDS_BIG_ENDIAN case? I understand that this formulation is intended to hide various combinations of WORDS_BIG_ENDIAN and BYTES_BIG_ENDIAN, but it's a net loss for most targets where the old code boils down to an unconditional assignment to info->offset. Would it be doable to use the same handling as in subreg_size_lowpart_offset? > For WORDS_BIG_ENDIAN != REG_WORDS_BIG_ENDIAN targets? In practice > the old code didn't handle the case in which a single word spans more > than one register: if xmode was bigger than a word, ymode was smaller > than a word, and the number of registers in a ymode was smaller than > the number of registers in a word, we would need to take "normal" > endianness into account to resolve the subword register offset while > using REG_WORDS_BIG_ENDIAN for the word component. Instead the old > code reversed the endianness relative the size of ymode, regardless of > whether ymode was bigger than a word or smaller than a word. In other > words, the assumption seems to have been that REG_WORDS_BIG_ENDIAN is > effectively "endianness across multiple registers" and there is no need > to subdivide register offsets into words and subwords. > > In practice that was OK, since AFAICT no target with WORDS_BIG_ENDIAN != > REG_WORDS_BIG_ENDIAN had subword-sized registers. This in turn means > that "block endianness" is always word endianness for these targets. Since you tested on c6x-elf, that's OK, but I think that a comment before the if (WORDS_BIG_ENDIAN != REG_WORDS_BIG_ENDIAN) test would be in order, stating the implicit assumption made at this point. -- Eric Botcazou
