https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109484
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> --- (In reply to 。 from comment #2) > (In reply to Richard Biener from comment #1) > > but you clobber 'temp' early and fail to indicate that so GCC allocates the > > same register as part of the "+m" output. > > The requirements you describe are not reflected in the documentation. The > document only says that `GCC assumpts that the assembler code consumes its > inputs before producing outputs`, and this code fits the assumption. First, > it reads the input from %1, then write the output to %0, then write the > output to %1. No outputs happend before inputs. You first write to 'temp' and then read from it. The wording applies to the assigned register / address, _not_ to the C variables mapped. Note I'm not an expert here and I wonder if an output operand is the appropriate way to create a scratch register for arbitrary use.
