Zoltán Kócsi wrote: > Is there a documentation of the various magic letters that you can > apply to an operand in inline assembly? What I mean is this: > > asm volatile ( > " some_insn %X[operand] \n" > : [operand] "=r" (expr) > ); > > What I look for is documentation of 'X'. In particular, when (expr) is > a multi-register object, such as long long or double (or even a short, > on a 8-bit chip) and you want to select a particular part of it. The > only place I found some information was going through the > gcc/config/<chip>/<chip>.c file and trying to find the meaning of such > letters in the xxx_print_operand() function. See 5.38.4, Constraints for Particular Machines > If that is the correct > approach, then I think there's a problem with the arm-elf (I know it is > dead, but still). > > According to the comments in that function, for DI and DF arguments the > Q and R qualifiers supposed to select the least significant and most > significant 32 bits, respectively, of the 64-bit datum. Indeed that's > what they do, for a long long. However, for a double they don't seem to > take into account that on arm-elf the word order of a double is always > big-endian, regardless of the endianness of the rest. Therefore, they > select the wrong half of the datum. On arm-eabi, where the endianness > of doubles matches the rest, they work fine. I can't see why that matters. Surely you just select the part you're interested in, based on the endianness of your target. Andrew.
