[Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR

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* [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR
@ 2021-08-12 19:47 kees at outflux dot net
  2021-08-12 20:08 ` [Bug target/101891] " pinskia at gcc dot gnu.org
                   ` (10 more replies)
  0 siblings, 11 replies; 12+ messages in thread
From: kees at outflux dot net @ 2021-08-12 19:47 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

            Bug ID: 101891
           Summary: Adjust -fzero-call-used-regs to always use XOR
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kees at outflux dot net
  Target Milestone: ---

Currently -fzero-call-used-regs will use a pattern of:

XOR regA,regA
MOV regA,regB
MOV regA,regC
...
RET

However, this introduces both a register ordering dependency (e.g. the CPU
cannot clear regB without clearing regA first), and while greatly reduces
available ROP gadgets, it does technically leave a set of "MOV" ROP gadgets at
the end of functions (e.g. "MOV regA,regC; RET").

Please switch to always using XOR:

XOR regA,regA
XOR regB,regB
XOR regC,regC
...
RET

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
@ 2021-08-12 20:08 ` pinskia at gcc dot gnu.org
  2022-01-28 20:29 ` qinzhao at gcc dot gnu.org
                   ` (9 subsequent siblings)
  10 siblings, 0 replies; 12+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-08-12 20:08 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|middle-end                  |target
             Target|                            |x86_64-linux-gnu

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The target emits the RTL this way.

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
  2021-08-12 20:08 ` [Bug target/101891] " pinskia at gcc dot gnu.org
@ 2022-01-28 20:29 ` qinzhao at gcc dot gnu.org
  2022-05-05 13:35 ` qinzhao at gcc dot gnu.org
                   ` (8 subsequent siblings)
  10 siblings, 0 replies; 12+ messages in thread
From: qinzhao at gcc dot gnu.org @ 2022-01-28 20:29 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

qinzhao at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-01-28
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
  2021-08-12 20:08 ` [Bug target/101891] " pinskia at gcc dot gnu.org
  2022-01-28 20:29 ` qinzhao at gcc dot gnu.org
@ 2022-05-05 13:35 ` qinzhao at gcc dot gnu.org
  2022-05-09 15:37 ` cvs-commit at gcc dot gnu.org
                   ` (7 subsequent siblings)
  10 siblings, 0 replies; 12+ messages in thread
From: qinzhao at gcc dot gnu.org @ 2022-05-05 13:35 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

qinzhao at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |13.0

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
                   ` (2 preceding siblings ...)
  2022-05-05 13:35 ` qinzhao at gcc dot gnu.org
@ 2022-05-09 15:37 ` cvs-commit at gcc dot gnu.org
  2022-05-09 15:43 ` qinzhao at gcc dot gnu.org
                   ` (6 subsequent siblings)
  10 siblings, 0 replies; 12+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2022-05-09 15:37 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Qing Zhao <qinzhao@gcc.gnu.org>:

https://gcc.gnu.org/g:0b86943aca51175968e40bbb6f2662dfe3fbfe59

commit r13-213-g0b86943aca51175968e40bbb6f2662dfe3fbfe59
Author: Qing Zhao <qing.zhao@oracle.com>
Date:   Mon May 9 15:34:34 2022 +0000

    i386: Adjust -fzero-call-used-regs to always use XOR [PR101891]

    Currently on i386, -fzero-call-used-regs uses a pattern of:

    XOR regA,regA
    MOV regA,regB
    MOV regA,regC
    ...
    RET

    However, this introduces both a register ordering dependency (e.g. the CPU
    cannot clear regB without clearing regA first), and while greatly reduces
    available ROP gadgets, it does technically leave a set of "MOV" ROP gadgets
    at the end of functions (e.g. "MOV regA,regC; RET").

    This patch will switch to always use XOR on i386:

    XOR regA,regA
    XOR regB,regB
    XOR regC,regC
    ...
    RET

    gcc/ChangeLog:

            PR target/101891
            * config/i386/i386.cc (zero_call_used_regno_mode): use V2SImode
            as a generic MMX mode instead of V4HImode.
            (zero_all_mm_registers): Use SET to zero instead of MOV for
            zeroing scratch registers.
            (ix86_zero_call_used_regs): Likewise.

    gcc/testsuite/ChangeLog:

            * gcc.target/i386/zero-scratch-regs-1.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-10.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-13.c: Add -msse.
            * gcc.target/i386/zero-scratch-regs-14.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-15.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-16.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-17.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-18.c: Add -fno-stack-protector
            -fno-PIC, adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-19.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-2.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-20.c: Add -msse.
            * gcc.target/i386/zero-scratch-regs-21.c: Add -fno-stack-protector
            -fno-PIC, Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-22.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-23.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-26.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-27.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-28.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-3.c: Add -fno-stack-protector.
            * gcc.target/i386/zero-scratch-regs-31.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-4.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-5.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-6.c: Add -fno-stack-protector.
            * gcc.target/i386/zero-scratch-regs-7.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-8.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-9.c: Add -fno-stack-protector.

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
                   ` (3 preceding siblings ...)
  2022-05-09 15:37 ` cvs-commit at gcc dot gnu.org
@ 2022-05-09 15:43 ` qinzhao at gcc dot gnu.org
  2022-05-24 15:04 ` cvs-commit at gcc dot gnu.org
                   ` (5 subsequent siblings)
  10 siblings, 0 replies; 12+ messages in thread
From: qinzhao at gcc dot gnu.org @ 2022-05-09 15:43 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

--- Comment #3 from qinzhao at gcc dot gnu.org ---
fixed in gcc13.

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
                   ` (4 preceding siblings ...)
  2022-05-09 15:43 ` qinzhao at gcc dot gnu.org
@ 2022-05-24 15:04 ` cvs-commit at gcc dot gnu.org
  2022-05-24 15:55 ` cvs-commit at gcc dot gnu.org
                   ` (4 subsequent siblings)
  10 siblings, 0 replies; 12+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2022-05-24 15:04 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Qing Zhao <qinzhao@gcc.gnu.org>:

https://gcc.gnu.org/g:79ae75cc252154cf4ad75d28c3c909ff90f0cc76

commit r12-8413-g79ae75cc252154cf4ad75d28c3c909ff90f0cc76
Author: Qing Zhao <qing.zhao@oracle.com>
Date:   Tue May 24 15:03:40 2022 +0000

    i386: Adjust -fzero-call-used-regs to always use XOR [PR101891]

    Currently on i386, -fzero-call-used-regs uses a pattern of:

    XOR regA,regA
    MOV regA,regB
    MOV regA,regC
    ...
    RET

    However, this introduces both a register ordering dependency (e.g. the CPU
    cannot clear regB without clearing regA first), and while greatly reduces
    available ROP gadgets, it does technically leave a set of "MOV" ROP gadgets
    at the end of functions (e.g. "MOV regA,regC; RET").

    This patch will switch to always use XOR on i386:

    XOR regA,regA
    XOR regB,regB
    XOR regC,regC
    ...
    RET

    gcc/ChangeLog:

            PR target/101891
            * config/i386/i386.cc (zero_call_used_regno_mode): use V2SImode
            as a generic MMX mode instead of V4HImode.
            (zero_all_mm_registers): Use SET to zero instead of MOV for
            zeroing scratch registers.
            (ix86_zero_call_used_regs): Likewise.

    gcc/testsuite/ChangeLog:

            * gcc.target/i386/zero-scratch-regs-1.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-10.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-13.c: Add -msse.
            * gcc.target/i386/zero-scratch-regs-14.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-15.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-16.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-17.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-18.c: Add -fno-stack-protector
            -fno-PIC, adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-19.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-2.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-20.c: Add -msse.
            * gcc.target/i386/zero-scratch-regs-21.c: Add -fno-stack-protector
            -fno-PIC, Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-22.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-23.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-26.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-27.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-28.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-3.c: Add -fno-stack-protector.
            * gcc.target/i386/zero-scratch-regs-31.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-4.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-5.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-6.c: Add -fno-stack-protector.
            * gcc.target/i386/zero-scratch-regs-7.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-8.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-9.c: Add -fno-stack-protector.

    (cherry picked from commit 0b86943aca51175968e40bbb6f2662dfe3fbfe59)

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
                   ` (5 preceding siblings ...)
  2022-05-24 15:04 ` cvs-commit at gcc dot gnu.org
@ 2022-05-24 15:55 ` cvs-commit at gcc dot gnu.org
  2022-05-24 15:55 ` qinzhao at gcc dot gnu.org
                   ` (3 subsequent siblings)
  10 siblings, 0 replies; 12+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2022-05-24 15:55 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Qing Zhao <qinzhao@gcc.gnu.org>:

https://gcc.gnu.org/g:c8d636cbe38b6a369528f58227c96b2b77b1fd3a

commit r11-10029-gc8d636cbe38b6a369528f58227c96b2b77b1fd3a
Author: Qing Zhao <qing.zhao@oracle.com>
Date:   Tue May 24 15:54:06 2022 +0000

    i386: Adjust -fzero-call-used-regs to always use XOR [PR101891]

    Currently on i386, -fzero-call-used-regs uses a pattern of:

    XOR regA,regA
    MOV regA,regB
    MOV regA,regC
    ...
    RET

    However, this introduces both a register ordering dependency (e.g. the CPU
    cannot clear regB without clearing regA first), and while greatly reduces
    available ROP gadgets, it does technically leave a set of "MOV" ROP gadgets
    at the end of functions (e.g. "MOV regA,regC; RET").

    This patch will switch to always use XOR on i386:

    XOR regA,regA
    XOR regB,regB
    XOR regC,regC
    ...
    RET

    gcc/ChangeLog:

            PR target/101891
            * config/i386/i386.c (zero_call_used_regno_mode): use V2SImode
            as a generic MMX mode instead of V4HImode.
            (zero_all_mm_registers): Use SET to zero instead of MOV for
            zeroing scratch registers.
            (ix86_zero_call_used_regs): Likewise.

    gcc/testsuite/ChangeLog:

            * gcc.target/i386/zero-scratch-regs-1.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-10.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-13.c: Add -msse.
            * gcc.target/i386/zero-scratch-regs-14.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-15.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-16.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-17.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-18.c: Add -fno-stack-protector
            -fno-PIC, adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-19.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-2.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-20.c: Add -msse.
            * gcc.target/i386/zero-scratch-regs-21.c: Add -fno-stack-protector
            -fno-PIC, Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-22.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-23.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-26.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-27.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-28.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-3.c: Add -fno-stack-protector.
            * gcc.target/i386/zero-scratch-regs-31.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-4.c: Add -fno-stack-protector
            -fno-PIC.
            * gcc.target/i386/zero-scratch-regs-5.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-6.c: Add -fno-stack-protector.
            * gcc.target/i386/zero-scratch-regs-7.c: Likewise.
            * gcc.target/i386/zero-scratch-regs-8.c: Adjust mov to xor.
            * gcc.target/i386/zero-scratch-regs-9.c: Add -fno-stack-protector.

    (cherry picked from commit 0b86943aca51175968e40bbb6f2662dfe3fbfe59)

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
                   ` (6 preceding siblings ...)
  2022-05-24 15:55 ` cvs-commit at gcc dot gnu.org
@ 2022-05-24 15:55 ` qinzhao at gcc dot gnu.org
  2022-05-24 16:42 ` arjan at linux dot intel.com
                   ` (2 subsequent siblings)
  10 siblings, 0 replies; 12+ messages in thread
From: qinzhao at gcc dot gnu.org @ 2022-05-24 15:55 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

qinzhao at gcc dot gnu.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #6 from qinzhao at gcc dot gnu.org ---
fixed in gcc11 and gcc12 too.

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
                   ` (7 preceding siblings ...)
  2022-05-24 15:55 ` qinzhao at gcc dot gnu.org
@ 2022-05-24 16:42 ` arjan at linux dot intel.com
  2022-05-24 18:47 ` qing.zhao at oracle dot com
  2022-05-24 18:55 ` arjan at linux dot intel.com
  10 siblings, 0 replies; 12+ messages in thread
From: arjan at linux dot intel.com @ 2022-05-24 16:42 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

Arjan van de Ven <arjan at linux dot intel.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |arjan at linux dot intel.com

--- Comment #7 from Arjan van de Ven <arjan at linux dot intel.com> ---
from a performance angle, the xor-only sequence is not so great at all; modern
CPUs are really good at eliminating mov's so that's why the code originally was
added to do a combo of xor and mov..

I can understand the security versus performance tradeoff.
(the original tuning was done to basically make it entirely free, so that it
could just be always enabled)

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
                   ` (8 preceding siblings ...)
  2022-05-24 16:42 ` arjan at linux dot intel.com
@ 2022-05-24 18:47 ` qing.zhao at oracle dot com
  2022-05-24 18:55 ` arjan at linux dot intel.com
  10 siblings, 0 replies; 12+ messages in thread
From: qing.zhao at oracle dot com @ 2022-05-24 18:47 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

--- Comment #8 from Qing Zhao <qing.zhao at oracle dot com> ---
> On May 24, 2022, at 11:42 AM, arjan at linux dot intel.com <gcc-bugzilla@gcc.gnu.org> wrote:
> 
> --- Comment #7 from Arjan van de Ven <arjan at linux dot intel.com> ---
> from a performance angle, the xor-only sequence is not so great at all; modern
> CPUs are really good at eliminating mov's so that's why the code originally was
> added to do a combo of xor and mov..

Are you saying that the Xor-only sequence is slower than the previous XOR + MOV
sequence?
If so, can you explain a little bit more on why? And do you have any data for
this claim?

Thanks.

^ permalink raw reply	[flat|nested] 12+ messages in thread

* [Bug target/101891] Adjust -fzero-call-used-regs to always use XOR
  2021-08-12 19:47 [Bug middle-end/101891] New: Adjust -fzero-call-used-regs to always use XOR kees at outflux dot net
                   ` (9 preceding siblings ...)
  2022-05-24 18:47 ` qing.zhao at oracle dot com
@ 2022-05-24 18:55 ` arjan at linux dot intel.com
  10 siblings, 0 replies; 12+ messages in thread
From: arjan at linux dot intel.com @ 2022-05-24 18:55 UTC (permalink / raw)
  To: gcc-bugs

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101891

--- Comment #9 from Arjan van de Ven <arjan at linux dot intel.com> ---
I don't have recent measurements since we did this work quite some time ago.

basically on the CPU level (speaking for Intel style cpus at least), a CPU can
eliminate (meaning: no execution resources used) 1 to 3 (depending on
generation) register to register per clock cycle.. There's ALSO a path in the
hardware for optimizing XOR <reg><reg> sequences to avoid execution
resources... when we did both we maximized the total number of these
eliminations...
while only XOR you can get bottlenecked on execution if you have too many.
(all the mov's should have no other instructions depending on them, so even
though they depend on the XOR, they're still fully 'orphan' for the out of
order engine)

^ permalink raw reply	[flat|nested] 12+ messages in thread

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2022-01-28 20:29 ` qinzhao at gcc dot gnu.org
2022-05-05 13:35 ` qinzhao at gcc dot gnu.org
2022-05-09 15:37 ` cvs-commit at gcc dot gnu.org
2022-05-09 15:43 ` qinzhao at gcc dot gnu.org
2022-05-24 15:04 ` cvs-commit at gcc dot gnu.org
2022-05-24 15:55 ` cvs-commit at gcc dot gnu.org
2022-05-24 15:55 ` qinzhao at gcc dot gnu.org
2022-05-24 16:42 ` arjan at linux dot intel.com
2022-05-24 18:47 ` qing.zhao at oracle dot com
2022-05-24 18:55 ` arjan at linux dot intel.com

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