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

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

[hpa at zytor dot com]

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.

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

[pinskia at gcc dot gnu.org]

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
x86 (and I think MIPS) has an interrupt attribute, will that work instead?

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

[pinskia at gcc dot gnu.org]

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

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
RISCV does have the interrupt attribute too which should work here.

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

[pinskia at gcc dot gnu.org]

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

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
the only major target which does not have the interrupt attribute is aarch64.

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

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement

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

[hpa at zytor dot com]

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

--- Comment #4 from H. Peter Anvin <hpa at zytor dot com> ---
The interrupt attribute typically does two things:

1. It changes the return instruction;
2. It marks all registers as saved.

2 is exactly the *opposite* of what I want; I would like to improve performance
by the fact that the compiler-invisible entry flow has already saved all
registers, whether or not they are saved in the ABI. Thus, I would like it to
treat all (non-fixed) registers as *clobbered*, not *saved*.

Ideally, "nosaved" and "interrupt" should be possible to use together, to get
effect #1 of the interrupt attribute, but that is usually less important.

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

[hjl.tools at gmail dot com]

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

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2024-01-11

--- Comment #5 from H.J. Lu <hjl.tools at gmail dot com> ---
There is no_caller_saved_registers attribute.  We can add
no_callee_saved_registers attribute.

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

[cvs-commit at gcc dot gnu.org]

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

--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by H.J. Lu <hjl@gcc.gnu.org>:

https://gcc.gnu.org/g:a96549dce7636edfc693bf758ef27fcd8adc6161

commit r14-8469-ga96549dce7636edfc693bf758ef27fcd8adc6161
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Tue Jan 23 06:59:50 2024 -0800

    x86: Add no_callee_saved_registers function attribute

    When an interrupt handler is implemented by an assembly stub which does:

    1. Save all registers.
    2. Call a C function.
    3. Restore all registers.
    4. Return from interrupt.

    it is completely unnecessary to save and restore any registers in the C
    function called by the assembly stub, even if they would normally be
    callee-saved.

    Add no_callee_saved_registers function attribute, which is complementary
    to no_caller_saved_registers function attribute, to mark a function which
    doesn't have any callee-saved registers.  Such a function won't save and
    restore any registers.  Classify function call-saved register handling
    type with:

    1. Default call-saved registers.
    2. No caller-saved registers with no_caller_saved_registers attribute.
    3. No callee-saved registers with no_callee_saved_registers attribute.

    Disallow sibcall if callee is a no_callee_saved_registers function
    and caller isn't a no_callee_saved_registers function.  Otherwise,
    callee-saved registers won't be preserved.

    After a no_callee_saved_registers function is called, all registers may
    be clobbered.  If the calling function isn't a no_callee_saved_registers
    function, we need to preserve all registers which aren't used by function
    calls.

    gcc/

            PR target/103503
            PR target/113312
            * config/i386/i386-expand.cc (ix86_expand_call): Replace
            no_caller_saved_registers check with call_saved_registers check.
            Clobber all registers that are not used by the callee with
            no_callee_saved_registers attribute.
            * config/i386/i386-options.cc (ix86_set_func_type): Set
            call_saved_registers to TYPE_NO_CALLEE_SAVED_REGISTERS for
            noreturn function.  Disallow no_callee_saved_registers with
            interrupt or no_caller_saved_registers attributes together.
            (ix86_set_current_function): Replace no_caller_saved_registers
            check with call_saved_registers check.
            (ix86_handle_no_caller_saved_registers_attribute): Renamed to ...
            (ix86_handle_call_saved_registers_attribute): This.
            (ix86_gnu_attributes): Add
            ix86_handle_call_saved_registers_attribute.
            * config/i386/i386.cc (ix86_conditional_register_usage): Replace
            no_caller_saved_registers check with call_saved_registers check.
            (ix86_function_ok_for_sibcall): Don't allow callee with
            no_callee_saved_registers attribute when the calling function
            has callee-saved registers.
            (ix86_comp_type_attributes): Also check
            no_callee_saved_registers.
            (ix86_epilogue_uses): Replace no_caller_saved_registers check
            with call_saved_registers check.
            (ix86_hard_regno_scratch_ok): Likewise.
            (ix86_save_reg): Replace no_caller_saved_registers check with
            call_saved_registers check.  Don't save any registers for
            TYPE_NO_CALLEE_SAVED_REGISTERS.  Save all registers with
            TYPE_DEFAULT_CALL_SAVED_REGISTERS if function with
            no_callee_saved_registers attribute is called.
            (find_drap_reg): Replace no_caller_saved_registers check with
            call_saved_registers check.
            * config/i386/i386.h (call_saved_registers_type): New enum.
            (machine_function): Replace no_caller_saved_registers with
            call_saved_registers.
            * doc/extend.texi: Document no_callee_saved_registers attribute.

    gcc/testsuite/

            PR target/103503
            PR target/113312
            * gcc.dg/torture/no-callee-saved-run-1a.c: New file.
            * gcc.dg/torture/no-callee-saved-run-1b.c: Likewise.
            * gcc.target/i386/no-callee-saved-1.c: Likewise.
            * gcc.target/i386/no-callee-saved-2.c: Likewise.
            * gcc.target/i386/no-callee-saved-3.c: Likewise.
            * gcc.target/i386/no-callee-saved-4.c: Likewise.
            * gcc.target/i386/no-callee-saved-5.c: Likewise.
            * gcc.target/i386/no-callee-saved-6.c: Likewise.
            * gcc.target/i386/no-callee-saved-7.c: Likewise.
            * gcc.target/i386/no-callee-saved-8.c: Likewise.
            * gcc.target/i386/no-callee-saved-9.c: Likewise.
            * gcc.target/i386/no-callee-saved-10.c: Likewise.
            * gcc.target/i386/no-callee-saved-11.c: Likewise.
            * gcc.target/i386/no-callee-saved-12.c: Likewise.
            * gcc.target/i386/no-callee-saved-13.c: Likewise.
            * gcc.target/i386/no-callee-saved-14.c: Likewise.
            * gcc.target/i386/no-callee-saved-15.c: Likewise.
            * gcc.target/i386/no-callee-saved-16.c: Likewise.
            * gcc.target/i386/no-callee-saved-17.c: Likewise.
            * gcc.target/i386/no-callee-saved-18.c: Likewise.

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

[hpa at zytor dot com]

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

--- Comment #7 from H. Peter Anvin <hpa at zytor dot com> ---
Note: this is now implemented for x86, but it affects other targets as well.