[Bug target/103503] New: RFE: no save registers attribute

["hpa at zytor dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103503

            Bug ID: 103503
           Summary: RFE: no save registers attribute
           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: ---
            Target: multiple

When a common assembly interrupt entry code or an equivalent hardware engine is
used to handle register saves in an interrupt routine, it may be completely
unnecessary to save and restore registers in the interrupt handler itself, even
if they would normally be clobbered.

Unfortunately [-f]call-used-reg is not supported by either
__attribute__((optimize)) nor _Pragma("GCC optimize"); otherwise that would be
a very valid solution.

Putting all interrupt handlers in a separate compilation unit is awkward at the
very best.

AVR has __attribute__((OS_{task,main})) for this purpose, but being able to do
it in general would improve interrupt/trap latency.

See also bug 38534.

Note: not saving the registers in the assembly wrapper is generally not an
option; similarly, whether or not a hardware engine can do it practically
depends on both the hardware implementation and the ABI. The RISC-V ABI, for
example, scatters clobbered and saved registers all over the register map,
which makes doing such a thing in hardware difficult.