https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111020
Bug ID: 111020
Summary: RFE: RISC-V: ability to cherry-pick additional
instructions
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: hpa at zytor dot com
Target Milestone: ---
For very deeply embedded use, it is sometimes highly desirable to control the
instruction set on a very fine grained basis. For example, the Zbb extension
contains a mixture of things that most likely requires separate functional
units. However, as an example, the ctz instruction is highly useful to speed up
interrupt latency in designs that do not have vectorized interrupt handling
(which is, in its most basic form, a dedicated ctz unit.) It would be massive
hardware bloat to require the full Zbb set to add this one instruction.
Once the instruction is added, though, one would like to be able to use it as
fully as possible.
This, obviously, creates binaries that are specifically tuned toward a single
processor implementation, but that is pretty much the essence of deeply
embedded, where in the normal case the entire software stack from the OS to
application is linked together in a single binary, or at the very least
compiled together, often from a single source tree.
As far as object code compatibility is concerned, this is very much a
"programmer beware" situation. There is no need for heroics in terms of tagging
objects with the exact instruction set, for example.
