[Bug inline-asm/100953] New: Add memory clobbers just for reads or just for writes

[Bug inline-asm/100953] New: Add memory clobbers just for reads or just for writes

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100953

            Bug ID: 100953
           Summary: Add memory clobbers just for reads or just for writes
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: inline-asm
          Assignee: unassigned at gcc dot gnu.org
          Reporter: segher at gcc dot gnu.org
  Target Milestone: ---

See
<https://lore.kernel.org/linux-toolchains/20210604182357.GA1688170@rowland.harvard.edu/T/#mb0e293105ae45974af7ed435847e2a6f72158a0a>
.

[Bug inline-asm/100953] Add memory clobbers just for reads or just for writes

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100953

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
                 CC|                            |rguenth at gcc dot gnu.org
   Last reconfirmed|                            |2021-06-08
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Obviously "just for writes" is nonsense, but what's missing is the ability
to add a (use "memory"), which maybe just means allowing to specify
"memory" as use?  That would prevent moving stores across such asm() but
it would not cause "pointless reloading of globals".

[Bug inline-asm/100953] Add memory clobbers just for reads or just for writes

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100953

--- Comment #2 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Sure :-)  But syntactically it probably is best put amongst the clobbers,
all code parsing that already knows about handling various special cases
of syntax (well, just "memory" and "cc", and the various ways of naming
registers).

[Bug inline-asm/100953] Add memory clobbers just for reads or just for writes

[foom at fuhm dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100953

James Y Knight <foom at fuhm dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |foom at fuhm dot net

--- Comment #3 from James Y Knight <foom at fuhm dot net> ---
For the compiler-barrier use-cases, you can use the __atomic_signal_fence
builtin instead of an empty inline-asm statement.

E.g. if you want to make sure that all writes apperaing after a barrier are
actually emitted after all reads/writes appearing before the barrier, use
__atomic_signal_fence(__ATOMIC_RELEASE).