On Mon, Jan 29, 2018 at 09:39:36PM +0100, Georg-Johann Lay wrote: > My bad. If GCC uses post-increment, then the value in the > post-incremented register no more represents src. But when src+1 is used > in the remainder, gcc detects that this value has already been computed > and reuses the post-incremented reg instead of recomputing src+1. > > Hence src does /not/ change, whereas the register used do address *src > /does/ if post-increment is used. As src does not change, there's no > need to express it in terms of constraints. Yes exactly (messages crossed, whoops). > >Well, the problem I'm originally set out to solve is writing a decent > >memcpy() function for unaligned source/destination pairs. I should have > >mentioned it sooner (wild XY problem draws near). > > Some libc implementation already perform such pre- and post-alignment, > e.g. Newlib provided it is compiled for speed (and the machine part > doesn't deviates from that default). Things like a good memcpy I wouldn't even try to write in inline assembler, not even for a very simple CPU; memcpy is so important that writing ~100 lines of assembler code is well worth it. Inline assembler is more suitable for access to, say, instructions writing special registers that GCC does not know about. Segher
