On Mon, Apr 13, 2020 at 8:34 AM Bryce Cherry via Gcc <gcc@gcc.gnu.org> wrote:
> Hello all, > > I'm just curious about this, but what is the percentage of (and what are > the) unused instructions for each supported architecture under GCC? > I would bet that this info doesn't exist. And you would have to clarify user vs supervisor instructions. GCC doesn't generate any supervisor instructions. My guess is that on a very simple RISC CPU, the percentage of user mode instructions used is pretty high simply because there are not that many instructions. But the more complicated CISC architectures with lots of instructions are going to be lower because someone would have had to write a pattern specifically to use a special instruction. Maybe someone with broad architecture experience can comment on which simple architecture has the most used. Intuition says a CISC architecture like the x86, m68k, or VAX has the most unused instructions but those have had a lot of work so we might be surprised. Interesting question but it would take a large survey and the number of instructions per cpu model within an architecture will vary. A productive alternative for GCC developers is the question "which instructions on which architectures would it be nice if GCC could take advantage of but currently doesn't?" This one might get an answer. --joel RTEMS
