[Bug c/105048] New: [enhancement] Allow specific register constraints

[Bug c/105048] New: [enhancement] Allow specific register constraints

[ehem+gccbugs at m5p dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105048

            Bug ID: 105048
           Summary: [enhancement] Allow specific register constraints
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: inline-asm
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ehem+gccbugs at m5p dot com
  Target Milestone: ---

Using local register variables to constrain assembly-language inputs and
outputs to particular registers is really ugly.  Better to have explicit syntax
for this.  I would suggesting using "R<register>" for this.

Using "R" conflicts with x86, but x86 effectively already has this since "a",
"b", "c", "d", "S", "D" covers most of the registers used for calling
conventions.  I though also note Clang doesn't appear to have implemented "R"
which suggests this has rarely, if ever, been used. (another letter could be
used on x86, or it might be possible to repurpose)

This is particularly valuable for interfacing with systems using foreign
calling conventions.

The example is how one might interface with a hypervisor on ARM64.  This seems
valuable as the register specification is a feature of the
inline-assembly-language, not a feature of the variable.

This could result in superior output, by avoiding copies.  In particular
`hypercall_inline()` might well end up with 5 extra `mov` instructions (plus a
call and return) which are never optimized out, whereas `hypercall_macro()` is
rather likely to produce a single instruction.

[Bug c/105048] [enhancement] Allow specific register constraints

[ehem+gccbugs at m5p dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105048

--- Comment #1 from Elliott M <ehem+gccbugs at m5p dot com> ---
Created attachment 52684
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=52684&action=edit
Sample of with and without this feature

Hmm, Bugzilla attachment during initial submission failed.

[Bug c/105048] [enhancement] Allow specific register constraints

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105048

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Dup of bug 90181.

*** This bug has been marked as a duplicate of bug 90181 ***