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[Issue 20838] on modern (x86_64) CPUs, dmd emit cmpxchg8b instead of CMPXCHG16B
May 17, 2020
mw
May 17, 2020
Mathias LANG
May 17, 2020
mw
May 17, 2020
Seb
May 18, 2020
mw
May 18, 2020
kinke
May 18, 2020
mw
May 18, 2020
mw
May 18, 2020
mw
May 18, 2020
kinke
May 18, 2020
mw
May 18, 2020
kinke
May 18, 2020
mw
May 19, 2020
kinke
May 17
mhh
May 17
mhh
May 18
mhh
May 17, 2020
https://issues.dlang.org/show_bug.cgi?id=20838

--- Comment #1 from mw <mingwu@gmail.com> ---
BTW, I only tested on x86_64 Linux, I think this bug on other platform too MacOS, Windows (on modern x86_64 CPUs with CX16 support).

--
May 17, 2020
https://issues.dlang.org/show_bug.cgi?id=20838

Mathias LANG <pro.mathias.lang@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pro.mathias.lang@gmail.com

--- Comment #2 from Mathias LANG <pro.mathias.lang@gmail.com> ---
Doesn't affect OSX (used objdump, grepped for cmpxchg).

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May 17, 2020
https://issues.dlang.org/show_bug.cgi?id=20838

--- Comment #3 from mw <mingwu@gmail.com> ---
Yes, verified, not on MacOS.

Thank you (at least there is a system I can use now).


$ objdump -disassemble-all c.o > c.o.asm
$ grep -i cmpxchg  c.o.asm
     80a:       49 0f c7 08     cmpxchg16b      (%r8)

$ uname -a
Darwin 19.4.0 Darwin Kernel Version 19.4.0: Wed Mar  4 22:28:40 PST 2020;
root:xnu-6153.101.6~15/RELEASE_X86_64 x86_64

--
May 17, 2020
https://issues.dlang.org/show_bug.cgi?id=20838

Seb <greeenify@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |greeenify@gmail.com

--- Comment #4 from Seb <greeenify@gmail.com> ---
> at least there is a system I can use now).

Well, you could just use LDC like almost everyone else ;-)

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May 18, 2020
https://issues.dlang.org/show_bug.cgi?id=20838

--- Comment #5 from mw <mingwu@gmail.com> ---
LDC ? did I miss sth?

--------------------------------------------------------------------------------
$ ldc2 -m64 -c c.d
$ obj2asm c.o > c.o.asm
$ grep -i xchg  c.o.asm
                cmpxchg8b       [RSI]
                cmpxchg8b       [RSI]
                cmpxchg8b       [RSI]
                cmpxchg8b       [RSI]
                cmpxchg8b       [RSI]
.data._D100TypeInfo_S3ldc10intrinsics__T13CmpXchgResultTS4core8internal6atomic__T11_AtomicTypeTS1c1NZ5UCentZQCq6__initZ
segment
_D100TypeInfo_S3ldc10intrinsics__T13CmpXchgResultTS4core8internal6atomic__T11_AtomicTypeTS1c1NZ5UCentZQCq6__initZ:
.data._D100TypeInfo_S3ldc10intrinsics__T13CmpXchgResultTS4core8internal6atomic__T11_AtomicTypeTS1c1NZ5UCentZQCq6__initZ
ends


$ ldc2 --version
LDC - the LLVM D compiler (1.21.0):
  based on DMD v2.091.1 and LLVM 10.0.0
  built with LDC - the LLVM D compiler (1.21.0)
  Default target: x86_64-unknown-linux-gnu
  Host CPU: skylake
  http://dlang.org - http://wiki.dlang.org/LDC
--------------------------------------------------------------------------------

Want to file a bug against LDC ?

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May 18, 2020
https://issues.dlang.org/show_bug.cgi?id=20838

kinke <kinke@gmx.net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |kinke@gmx.net

--- Comment #6 from kinke <kinke@gmx.net> ---
(In reply to mw from comment #5)
> Want to file a bug against LDC ?

No need, cmpxchg16 is used for all x86_64 CPUs: https://d.godbolt.org/z/HesA24

For the few old CPUs not supporting it, it can be disabled via `-mattr=-cx16` (but then it doesn't fall back to cmpxchg8 anway, so no idea how your results came about).

--
May 18, 2020
https://issues.dlang.org/show_bug.cgi?id=20838

--- Comment #7 from mw <mingwu@gmail.com> ---
Hi kinke,

> so no idea how your results came about

I downloaded directly from:

https://github.com/ldc-developers/ldc/releases/download/v1.21.0/ldc2-1.21.0-linux-x86_64.tar.xz


And I just downloaded 1.20, which is on the d.godbolt.org page you mentioned, but the result is the same:

--------------------------------------------------------------------------------
$ wget https://github.com/ldc-developers/ldc/releases/download/v1.20.0/ldc2-1.20.0-linux-x86_64.tar.xz

$ ldc2 -m64 -c c.d
$ obj2asm c.o > c.o.asm
$ grep -i xchg  c.o.asm
                cmpxchg8b       [RSI]
                cmpxchg8b       [RSI]
                cmpxchg8b       [RSI]
                cmpxchg8b       [RSI]
                cmpxchg8b       [RSI]
.data._D100TypeInfo_S3ldc10intrinsics__T13CmpXchgResultTS4core8internal6atomic__T11_AtomicTypeTS1c1NZ5UCentZQCq6__initZ
segment
_D100TypeInfo_S3ldc10intrinsics__T13CmpXchgResultTS4core8internal6atomic__T11_AtomicTypeTS1c1NZ5UCentZQCq6__initZ:
.data._D100TypeInfo_S3ldc10intrinsics__T13CmpXchgResultTS4core8internal6atomic__T11_AtomicTypeTS1c1NZ5UCentZQCq6__initZ
ends

$ ldc2 --version
LDC - the LLVM D compiler (1.20.0):
  based on DMD v2.090.1 and LLVM 9.0.1
  built with LDC - the LLVM D compiler (1.20.0)
  Default target: x86_64-unknown-linux-gnu
  Host CPU: skylake
  http://dlang.org - http://wiki.dlang.org/LDC

$ uname -a
Linux titan 4.15.0-99-generic #100-Ubuntu SMP Wed Apr 22 20:32:56 UTC 2020
x86_64 x86_64 x86_64 GNU/Linux
--------------------------------------------------------------------------------


What else should I check?

Or, can try my step on a Linux box?

--
May 18, 2020
https://issues.dlang.org/show_bug.cgi?id=20838

--- Comment #8 from mw <mingwu@gmail.com> ---
And for GDC:

--------------------------------------------------------------------------------
$ gdc-10 -m64 -c c.d
$ obj2asm c.o > c.o.asm
$ grep -i xch  c.o.asm
        extrn   __atomic_compare_exchange
                call      __atomic_compare_exchange@PLT32

--------------------------------------------------------------------------------

On this page:  https://d.godbolt.org/z/HesA24

I changed the compiler to "gdc 9.2.0", and searched the window, and search for 'xch':

        mov     rsi, rax
        mov     edi, 16
        call    __atomic_compare_exchange
        mov     BYTE PTR [rbp-1], al
        .loc 3 1413 9
        movzx   eax, BYTE PTR [rbp-1]


I'm not an asm guy, can someone help to read where is this __atomic_compare_exchange? and confirm it's calling the correct CMPXCHG16B?


Thanks.

--
May 18, 2020
https://issues.dlang.org/show_bug.cgi?id=20838

--- Comment #9 from mw <mingwu@gmail.com> ---
Oh,

$ gdc-10 --version
gdc-10 (Debian 10.1.0-1) 10.1.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

--
May 18, 2020
https://issues.dlang.org/show_bug.cgi?id=20838

--- Comment #10 from kinke <kinke@gmx.net> ---
(In reply to mw from comment #7)
> What else should I check?

Wrt. LDC, I'm almost certain this is solely an issue with your 'workflow' involving obj2asm. - Godbolt runs on Linux (but you can inspect the produced assembly for any target using LDC's -mtriple option).

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