[Bug target/110899] New: RFE: Attributes preserve_most and preserve_all

["elver at google dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110899

            Bug ID: 110899
           Summary: RFE: Attributes preserve_most and preserve_all
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: elver at google dot com
  Target Milestone: ---

Clang/LLVM implements the function attributes "preserve_most" and
"preserve_all":

[1] preserve_most: "On X86-64 and AArch64 targets, this attribute changes the
calling convention of a function. The preserve_most calling convention attempts
to make the code in the caller as unintrusive as possible. This convention
behaves identically to the C calling convention on how arguments and return
values are passed, but it uses a different set of caller/callee-saved
registers. This alleviates the burden of saving and recovering a large register
set before and after the call in the caller. If the arguments are passed in
callee-saved registers, then they will be preserved by the callee across the
call. This doesn’t apply for values returned in callee-saved registers.

- On X86-64 the callee preserves all general purpose registers, except for R11.
R11 can be used as a scratch register. Floating-point registers (XMMs/YMMs) are
not preserved and need to be saved by the caller.

- On AArch64 the callee preserve all general purpose registers, except X0-X8
and X16-X18."

[2] preserve_all: "On X86-64 and AArch64 targets, this attribute changes the
calling convention of a function. The preserve_all calling convention attempts
to make the code in the caller even less intrusive than the preserve_most
calling convention. This calling convention also behaves identical to the C
calling convention on how arguments and return values are passed, but it uses a
different set of caller/callee-saved registers. This removes the burden of
saving and recovering a large register set before and after the call in the
caller. If the arguments are passed in callee-saved registers, then they will
be preserved by the callee across the call. This doesn’t apply for values
returned in callee-saved registers.

- On X86-64 the callee preserves all general purpose registers, except for R11.
R11 can be used as a scratch register. Furthermore it also preserves all
floating-point registers (XMMs/YMMs).

- On AArch64 the callee preserve all general purpose registers, except X0-X8
and X16-X18. Furthermore it also preserves lower 128 bits of V8-V31 SIMD -
floating point registers."

[1] https://clang.llvm.org/docs/AttributeReference.html#preserve-most
[2] https://clang.llvm.org/docs/AttributeReference.html#preserve-all


These attributes, esp. preserve_most, provides a convenient way to optimize the
generated code for calls to rarely taken slow paths, such as error-reporting
functions. Recently, we're looking to make use of this in the Linux kernel [3],
with potentially additional usecases being discussed.

[3] https://lkml.kernel.org/r/20230804090621.400-1-elver@google.com