[Bug tree-optimization/109407] New: instruction cmpxchgl run error when using with g++ -O

[Bug tree-optimization/109407] New: instruction cmpxchgl run error when using with g++ -O

[johgjc at yeah dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109407

            Bug ID: 109407
           Summary: instruction cmpxchgl run error when using with g++ -O
           Product: gcc
           Version: 9.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johgjc at yeah dot net
  Target Milestone: ---

Created attachment 54810
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54810&action=edit
a simple test code

the following code,
when using g++ test.cpp, the a.out is a endless loop as expected.
when using g++ -O2 test.cpp, the a.out returns and prints dwOriV=8

#include<cstdint>
#include<cstdio>
int main(){
    volatile int dwOriV = 32;
    int8_t dwRtnV = 0;
    int dwExpV = 16;
    int dwDstV = 8;
    while(true){
        __asm__ __volatile__(
        "lock;"
        "cmpxchgl %[NEW_VALUE], %[DST];"
        "sete %[RET];"
        : [RET] "=q"(dwRtnV),[DST] "+m"(dwOriV)
        :[NEW_VALUE] "r"(dwDstV), "a"(dwExpV)
        : "memory", "cc"
        );
        if(dwRtnV) {break;}
    }
    printf("%d",dwOriV);
    return 0;
}


my gcc version:
gcc (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
Copyright (C) 2019 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.

[Bug tree-optimization/109407] instruction cmpxchgl run error when using with g++ -O

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109407

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

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The inline-asm is incorrect as the EAX/RAX register gets clobbered by cmpxchgl
So you need something like (which works now):

int tmp;
__asm__ __volatile__(
        "lock;"
        "cmpxchgl %[NEW_VALUE], %[DST];"
        "sete %[RET];"
        : [RET] "=q"(dwRtnV),[DST] "+m"(dwOriV)
        , "=a"(tmp) // newly added
        :[NEW_VALUE] "r"(dwDstV), "a"(dwExpV)
        : "memory", "cc"
);

[Bug tree-optimization/109407] instruction cmpxchgl run error when using with g++ -O

[johgjc at yeah dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109407

--- Comment #2 from johgjc <johgjc at yeah dot net> ---
thank you very much.


At 2023-04-04 23:55:25, "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
wrote:
>https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109407
>
>Andrew Pinski <pinskia at gcc dot gnu.org> changed:
>
>           What    |Removed                     |Added
>----------------------------------------------------------------------------
>         Resolution|---                         |INVALID
>             Status|UNCONFIRMED                 |RESOLVED
>
>--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>The inline-asm is incorrect as the EAX/RAX register gets clobbered by cmpxchgl
>So you need something like (which works now):
>
>int tmp;
>__asm__ __volatile__(
>        "lock;"
>        "cmpxchgl %[NEW_VALUE], %[DST];"
>        "sete %[RET];"
>        : [RET] "=q"(dwRtnV),[DST] "+m"(dwOriV)
>        , "=a"(tmp) // newly added
>        :[NEW_VALUE] "r"(dwDstV), "a"(dwExpV)
>        : "memory", "cc"
>);
>
>-- 
>You are receiving this mail because:
>You reported the bug.